THE CIRCULATORY SYSTEM

Transcription

1 Biology 30S THE CIRCULATORY SYSTEM Name: This module adapted from bblearn.merlin.mb.ca 1

2 Introduction to Circulation The first organ to form, and the last organ to die. The heart is the pump of life. The main functions of the circulatory system: Types of circulatory system in animals: Cardiovascular disease accounts for the death of more Canadians than any other disease. In 1999, cardiovascular disease accounted for 78,942 Canadian deaths. 35% of all male deaths in Canada in 1999 were due to heart diseases, diseases of the blood vessels and stroke. For women, the toll was even higher 37% of all female deaths in 1999 were due to cardiovascular disease. Why do you think Cardiovascular related deaths are so high in Canada? The circulatory or cardiovascular system plays a vital role of maintaining homeostasis in the human body. This homeostasis depends on the continuous and controlled movement of blood through the thousands of kilometres of blood vessels that ultimately reach every cell in the body. It is in the microscopic blood vessels that blood performs its ultimate transport function. Nutrients and other essential materials pass from blood into fluids surrounding the cells and waste products are removed. In this module, you will study the structure and function of the human circulatory system including the heart and the lymphatic system. You will also study and research some of the disorders and diseases that affect this system. 2

3 Introduction to Lesson 1 All living things require nutrients to grow and reproduce. Single celled organisms and very simple multicellular organisms are able to diffuse these nutrients from the environment into their cells. As organisms become more sophisticated (more than two cell layers thick), a transport mechanism is required to transport nutrients to the cells and transport wastes away from cells. In humans, the circulatory system performs this transport function. Lesson 1 Outcomes By the end of this lesson, you should be able to: -List six ways in which the circulatory system maintains homeostasis in the human body. -Explain how the structure of the five different types of blood vessels (arteries, arterioles, veins, venules, capillaries) is related to their function. -Describe how the structure of the heart is related to its function, i.e. double pump. -Identify and trace blood flow through the following structures of the heart from a specimen, model, or diagram: left and right atria left and right ventricle left and right pulmonary arteries left and right pulmonary veins superior/inferior venae cavae septum aorta left and right semilunar valves left and right atrioventricular valves 3

4 -Describe the difference between the systemic and pulmonary circulatory system. -Identify the following systemic blood vessels from a specimen, model, or diagram: carotid arteries jugular veins subclavian artery and vein superior/inferior venae cavae coronary artery and vein renal artery and vein iliac artery and vein hepatic portal vein Lesson 1 Overview Following is a list of topics covered in this lesson. The Circulatory System and Homeostasis Blood Vessels The Heart Major Systemic Blood Vessels Fetal Circulation The Circulatory System and Homeostasis The human circulatory system (also known as the cardiovascular system) consists of the heart, which is a muscular pumping device, and a closed system of vessels that are known as arteries, arterioles, veins, venules and capillaries. Blood contained in the circulatory system is pumped by the heart around a closed circle or circuit of vessels as it passes again and again through the various "circulations" of the body. The circulatory system is vital to the maintenance of homeostasis in the body. Maintaining homeostasis depends on the continuous and controlled movement of blood through the thousands of kilometres of capillaries that permeate every tissue and reach every cell in the body. It is in the microscopic capillaries that blood performs its transport function. 4

5 The circulatory system performs a number of important homeostatic functions in the human body. These include: Blood Vessels - Arteries Blood Vessels Blood vessels are the channels through which blood is distributed to body tissues. The vessels make up two closed systems of tubes that begin and end at the heart. These two systems are: Arteries Characteristics: The aorta, the largest artery in the human body, is about 25 mm in diameter. Arterioles on the other hand are only about 0.2 mm in diameter. 5

6 Artery walls are thick, strong and muscular and made of three tissue layers. In large arteries the middle layer is made mostly of elastic fibers. These fibers allow the large arteries to accommodate surges of blood pumped by the heart. The thick elastic walls stretch with the changing blood flow. This property of elasticity means that they can expand to accept a volume of blood, and then contract and squeeze back to their original size after the pressure is released. A good way to think of them is like a balloon. When you blow into the balloon, it inflates to hold the air. When you release the opening, the balloon squeezes the air back out. As the arteries get smaller, the blood pressure gets less and the need for elastic fibers in the middle section diminishes. This layer becomes mostly muscle fibers that contract, changing the size of the arterial channel, regulating the pressure and amount of blood that enters the capillaries. The artery's outer layer is made of fibrous, connective tissue, nerves, and tiny blood vessels that nourish the artery's walls. The diagram below illustrates the structure of arteries. Figure Structure of Arteries ( n.html 6

7 Capillaries and Veins Capillaries Characteristics: The average diameter of a capillary is 7/1000 mm (7 µm), just wide enough to let red blood cells pass through single file. Capillaries are really more like a web than a branched tube. It is in the capillaries that the exchange between the blood and the cells of the body takes place. Here the blood gives up its oxygen and takes on carbon dioxide. In the special capillaries of the kidneys, the blood gives up many waste products in the formation of urine. Capillary beds are also the sites where white blood cells are able to leave the blood and defend the body against harmful invaders. The diagram below illustrates a capillary bed. Figure Capillary Bed ( n.html) There are approximately km of capillaries in an adult. Because these minute capillaries are so numerous, (about 10 billion) they present a huge surface area to the tissue. More than 800 square meters of surface area allows for a great deal of exchange between the blood and the tissues. Tissues are so permeated with capillaries that rarely are any cells more than one cell layer away from a capillary. 7

8 This is necessary to allow for the diffusion of materials between the body cells and the walls of the capillaries. The walls of capillaries are only one cell thick. Substances in the blood and substances in body tissue are exchanged only across the capillary endothelium. Capillary networks serve nearly all of the living tissue of the body, their concentration in the tissue depending on the local need for exchange of materials. Muscles, which are called upon frequently to move the body, and the kidneys, which must remove waste products constantly, require great quantities of food and oxygen and are well supplied with capillaries. On the other hand, the cornea of the eye, a very inactive tissue, has none. Veins Characteristics: Because the walls of the veins are thinner and less rigid than arteries, veins can hold more blood. Almost 70 percent of the total blood volume is in the veins at any given time. Although the blood is forced into the arteries under pressure, by the time it reaches the veins, this pressure is very low. Blood pressure in the veins is less than 1/10 of the pressure in the aorta. Therefore, another mechanism must be present for getting blood back to the heart. How do veins function in moving the blood despite the low pressure? 8

9 The diagram below illustrates the structure of veins. Veins, like other blood vessels are subject to problems. What is varicose veins? The Heart One of the first important events of your life took place about three and one-half weeks into your embryonic development. Your heart began to beat. You may be excused for not remembering as you were only about 2.5 mm in size at the time. Your heart now is about the size of a large fist and has a mass of approximately 300 grams. It is a tough muscular organ which beats about 70 times and pumps 5 liters of blood every minute. Pumping over 7000 liters of blood each day, it has pumped about 35 million liters in your life time as a grade eleven student. While most of the hollow organs of the body do have muscular layers, the heart is almost entirely muscle. Unlike most of the other hollow organs, whose muscle layers are composed of smooth muscle, the heart is composed of cardiac muscle called the myocardium. The heart is surrounded by a fluid-filled membrane called the pericardium.the pericardial fluid bathes the heart, preventing friction between its outer wall and the membrane. 9

10 The human heart is really two pumps working side by side. A thick wall of muscle, called the septum, separates the heart's right and left sides. Each side is divided into two chambers: the atrium and the ventricle. The upper chambers, the left atrium and right atrium, collect blood returning to the heart through veins. The thin muscles of their walls push blood a short distance into the lower chambers, the left ventricle and right ventricle. The thick, muscular walls of the ventricles contract forcefully, pushing blood out of the heart to the lungs and body through arteries. The heart is responsible for pumping the blood to every cell in the body. It is also responsible for pumping blood to the lungs, where the blood gives up carbon dioxide and takes on oxygen. The heart is able to pump blood to both regions efficiently because there are really two separate circulatory circuits with the heart as the common link. Some even refer to the heart as two separate hearts, a right heart (pulmonary system) and left heart (systemic system). In the pulmonary system, blood leaves the heart through the pulmonary trunk which branches into the left and right pulmonary arteries, goes to the lungs, and returns to the heart through the left and right pulmonary veins. In the systemic system, blood leaves the heart through the aorta, goes to all the organs of the body through the systemic arteries, and then returns to the heart through the systemic veins. Arteries carry blood away from the heart and veins carry blood toward the heart. Most of the time, arteries carry oxygenated blood and veins carry deoxygenated blood. However, there is an exception. The pulmonary arteries leaving the right ventricle for the lungs carry deoxygenated blood and the pulmonary veins carry oxygenated blood. The diagram below illustrates this relationship. Figure Pulmonary Circulation( pathways.html) 10

11 Blood from any body tissue other than the lungs returns to the heart through either of two veins: superior vena cava and inferior vena cava. Refer to the diagram below. Figure Blood Flow though the Heart ( 11

12 A simplified diagram of the blood flow: Two valves regulate the flow of blood between the atria and ventricles. These valves, commonly called the atrioventricular (AV) valves, consist of three flaps of tissue that together form a more or less funnel-shaped arrangement, the narrow end extending into the ventricle. The pressure of the blood in the atrium forces the valve open, but when pressure develops in the ventricle, the pressure pushes the flaps against each other, effectively closing the opening. These two valves that regulate blood flow between the atria and ventricles are - Tricuspid valve: Mitral valve: Two valves regulate the flow of blood between the ventricles and the major vessels leaving those ventricles. These valves are commonly known as semilunar valves. The two semilunar valves are Pulmonary valve: 12

13 Aortic valve: The branches of the aorta carry oxygenated blood to all parts of the body except the lungs. In the brain, a muscle, a gland, or some other organ, the oxygenated blood becomes deoxygenated blood as it releases its oxygen and accepts carbon dioxide from the tissues. Systemic and Coronary Circulation Major Systemic Arteries and Veins All systemic arteries are branches, either directly or indirectly, from the aorta. The aorta ascends from the left ventricle, curves to the left, and descends through the thorax and abdomen. This geography divides the aorta into three portions: After blood delivers oxygen to the tissues and picks up carbon dioxide, it returns to the heart through a system of veins. The capillaries, where gas exchange occurs, merge into venules and these converge to form larger and larger veins until the blood reaches either the superior vena cava or inferior vena cava, which drain into the right atrium. The diagram below illustrates the major blood vessels in the human body. 13

14 Figure Major Blood Vessels ( Coronary Circulation Function: There are two main coronary arteries, the right and left coronary arteries with two major branches each. They arise from the aorta right after it leaves the heart. The coronary arteries eventually branch into capillary beds that are embedded throughout the heart walls and supply the heart muscle with oxygenated blood. The coronary veins return blood from the heart muscle, but instead of emptying into another larger vein, they empty directly into the right atrium. 14

15 Figure Coronary Arteries ( Disease in coronary arteries prevents the heart from receiving enough oxygen. These diseases will be discussed in lesson 5. Fetal Circulation In the human fetus, the lungs are not functional; the placenta substitutes for the lungs as the organ of gas exchange. How does the fetus gas exchange work? Oxygenated blood is delivered to the fetus from the placenta by the umbilical vein. This highly oxygenated blood flows into the inferior vena cava, which enters the right atrium. However, deoxygenated blood being returned from the internal organs contaminates the pure placental blood flowing in the inferior vena cava. Fortunately, the volume of placental blood is large, so that the mixture entering the right atrium is relatively well oxygenated. Ordinarily, blood would flow directly from the right atrium into the right ventricle, and, in turn, would leave the heart through the pulmonary trunk to the lungs. This would be a useless course in the fetus since the lungs are inactive. 15

16 The main volume of the relatively pure blood in the right atrium crosses through a special opening, known as the foramen ovale, into the left atrium. From the left atrium the blood reaches the left ventricle, which pumps the blood into the aorta to be delivered through the systemic system. Thus, the foramen ovale is an important device to ensure that a considerable portion of the oxygenated blood passes directly from the right atrium into the left atrium. Blood that passes from the right atrium to the right ventricle will be directed through a second shunt, called the ductus arteriosus that leads to the aorta. Some of the blood will reach the lungs through the pulmonary trunk, but a greater part arising from the right ventricle will continue through the ductus arteriosus to the aorta. Fetal circulation ceases at birth. When the lungs of the newborn expand with air, pulmonary circulation begins so that there will be an adequate supply of oxygen to the body. Constriction of the ductus arteriosus occurs shortly after birth with the result that the blood leaving the right ventricle no longer bypasses the lungs. Also, the foramen ovale is gradually sealed. What is a Blue Baby? 16

17 Lesson 1 Exercise 1. List 5 ways that the human circulatory system contributes to homeostasis. 2. Compare and contrast the structure and function of arteries, capillaries and veins. 3. Describe main differences between arteries and veins in terms of structures, functions and locations. 4. Why is it necessary to have km of capillaries in your body? 5. Differentiate between the pulmonary and systemic circulatory systems. 6. Name and give the function of the four valves found in the heart. 7. Using the following diagram of the heart on the next page, label the major structures and trace the flow of blood using arrows. Use the colors blue and red to distinguish between deoxygenated and oxygenated blood. 17

18 8. How does the heart receive its supply of blood? 9. What two modifications to the circulatory system are found in developing embryos? 18

19 Lesson 1 Summary In this lesson, I learned: Introduction to Lesson 2 - Heartbeat Activity: Locate your pulse at rest. Count how many times it beats in 15 seconds (look at a clock), then multiply this number by 4. This is your pulse rate Approximately how many heartbeats have you had since you were born? Since your heart has been beating since you were 2 weeks old in utero (inside the uterus), how many heartbeats have you had since your heart was formed? The human heart beats an average of 70 times per minute, 24 hours a day, and 365 days a year. For those who have never had heart problems, the regular rhythmic beating of the heart is usually taken for granted. Did you know that the average heart beats or contracts over 3 billion times during a normal lifetime? 19

20 Lesson 2 Outcomes By the end of this lesson, you should be able to: 1. Differentiate between systole and diastole and relate these to heart sounds. 2. Describe the intrinsic control of heartbeat, i.e. nervous (SA Node, AV Node, Perkinje Fibers, Bundle of HIS), and chemical (adrenaline, noradrenaline). 3. Explain the role of pacemakers in regulating heartbeat. 4. Describe the effects of adrenaline and noradrenaline on heart rate. 5. Measure your own heart rate. 6. Explain the effect of physical activity on heart rate. 7. Calculate cardiac output given heart rate and stroke volume. 8. Relate cardiac output to fitness levels. Lesson 2 Overview Following is a list of topics covered in this lesson. Heartbeat Control of Heartbeat Artificial Pacemakers Cardiac Output Heart Rate, Stroke Volume and Fitness 20

21 Heartbeat A heartbeat is a two-part pumping action that takes approximately one second. The contraction of the heart and its anatomy cause the distinctive sounds heard when listening to the heart with a stethoscope. Systole and Diastole and The Lubb Dubb sound: See the diagram below. Figure Systole and Diastole ( After blood moves into the pulmonary artery and the aorta, the ventricles relax, and the pulmonary and aortic valves close. The lower pressure in the ventricles causes the tricuspid and mitral valves to open, and the cycle begins again. This series of contractions is repeated over and over again, increasing during times of exertion and decreasing while you are at rest. Your heart does not work alone, though. Your brain tracks the conditions around you climate, stress, and your level of physical activity and adjusts your cardiovascular system to meet those needs. 21

22 Control of Heartbeat The sinoatrial Node (SA Node): Why is the SA Node called the natural pacemaker? The atrioventricular Node (AV Node): Bundle of His: The impulse started in the SA node and picked up by the AV node reaches the muscles of the ventricles and causes them to contract. Purkinje fibers : 22

23 Series of events in one heartbeat: The following diagrams illustrate the sequence of events involved in a heart contraction. 23

24 Figure â The Contraction of the Heart (Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates, and A physician listening carefully to the heart with a stethoscope can detect if the valves are closing completely or not. Instead of a distinctive valve sound, the physician may hear a swishing sound if they are letting blood flow backward. When the swishing is heard tells the physician where the leaky valve is located. This condition is known as a heart murmur. Electrocardiograph: The EKG shows three slow, negative changes, known as P, R, and T. Positive deflections are the Q and S waves. The P wave represents the contraction impulse of the atria, the T wave the ventricular contraction. EKGs are useful in diagnosing heart abnormalities. 24

25 Figure Electrocardiogram (Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates, and The SA node sends electrical impulses at a certain rate, but your heart rate may still change depending on physical demands, stress, or hormonal factors. For example, when you run to catch a bus, the increased activity in your muscles produces a faster rate of cellular respiration. This leads to an increase in the amount of carbon dioxide in your blood. The medulla oblongata detects this increase and sends impulses along the nervous system causing the release of a hormone called noradrenaline. When noradrenaline reaches the SA node, it makes the node fire more rapidly. Once you have boarded the bus, your heart gradually slows to a resting rate due to an increase in blood pressure. This response is detected by special blood pressure receptors located in the walls of the aorta and carotid arteries that send messages to the medulla oblongata. Physical activity is not the only trigger for an increased heart rate. Your nervous system releases another hormone called adrenaline when you are nervous, angry, excited or after a sudden shock or sharp pain. All of these conditions produce what is called the "fight or flight" response a physiological change that prepares the body for anticipated activity. This response increases heart rate, increasing blood flow to the muscles. 25

26 Artificial Pacemakers and Cardiac Output Artificial Pacemakers Arrythmia: When the natural pacemaker fails to work properly, doctors can implant a small, batteryoperated device called an artificial pacemaker to help the heart beat in a regular rhythm. Artificial pacemakers can be permanently implanted into a person's chest or they may be temporary and located outside of the body. Both types use batteries to send electrical impulses to the heart. A wire or electrode is placed next to the heart and transmits small electrical charges to the heart. Most current pacemakers are demand pacemakers which have sensing devices to turn the pacemaker on when the heartbeat falls below a certain level. Cardiac Output The amount of blood pumped by the heart is called cardiac output. This is a measure of the volume of blood pumped from each ventricle per unit of time. It is also a measure of the level of oxygen delivery to the body. Two factors affect cardiac output: 26

27 Cardiac output can be calculated by multiplying stroke volume and heart rate. The average person has a stroke volume of about 70 ml and a resting heart rate of about 70 beats/minute. Therefore, the average person has a cardiac output of 70 x 70 = ml/min. Since the average person has about 5 L of blood, your total blood volume circulates through your body approximately once every minute. Heart Rate, Stroke Volume and Fitness Maximum heart rate (also known as Target heart rate) is the highest heart rate you can attain during strenuous physical activity. This rate will diminish as you get older although maximum heart rate does not appear to be related to fitness. The more important indicator of fitness is the length of time it takes for your heart to return to its resting level following physical activity. This is called recovery time, and this amount of time will diminish as you become more fit. Two factors affect stroke volume Regular cardiovascular exercise will enlarge the ventricular chambers and increase their distensibility (stretchiness). Athletes who are very fit have high stroke volumes. This means they can maintain high oxygen delivery to tissues at low heart rates. Some elite endurance trained athletes like Olympic cross country skiers have resting heart rates of as low as 30 beats/minute. 27

28 Lesson 2 Exercise 1. What causes the characteristic "lubb-dubb" heart sounds? 2. Describe the structures involved and the sequence of events involved in a heart contraction. 3. What is an EKG or ECG? Why are they useful? 4. Why do some people require an artificial pacemaker? 5. Describe the conditions that would cause the release of noradrenaline and adrenaline and their resulting effects of on heart rate. 6. How are heart rate and stroke volume related to fitness? 7. Explain the changes in pulse rate during exercise. 8. Extension: How does an AED (Automated External Defibrillator) device work? 28

29 Heart Rate Activity Effects of Exercise on Heart Rate Complete the following activity. Once you have performed the lab, prepare a report and submit it in the Assignments Tool M4 L2 Heart Rate. Your report should have the following categories: 1. Purpose 2. Observations/Data 3. Analysis/Conclusions Lesson 2 Lab Activity Effects of Exercise on Heart Rate Purpose: To measure your heart rate, recovery time and cardiac output before, during and after vigorous exercise. If you have a physical condition that makes it unadvisable for you to exercise vigorously, do not participate in the exercise portion of this activity. Procedure: 1. Find your resting heart rate by measuring your pulse. A pulse is a change in the diameter of arteries following a heart contraction. The easiest locations for measuring your pulse are directly under the back of your jawbone toward the neck or on the underside of your wrist directly beneath the thumb area. 2. While in a sitting position, record your pulse rate for 15 seconds. Multiply this number by four to determine your resting heart rate per minute. Record this value in the data table below. Repeat three times and determine the average. 3. Perform jumping jacks or running on the spot for one minute. Immediately after completing the exercise, measure your pulse rate for 15 seconds and calculate your heart rate after vigorous exercise. 29

30 4. Continue to measure your pulse rate in one minute intervals until your heart rate reaches its resting rate. The time it takes for your heart to reach its resting rate is your recovery time. Record this time in the data table. 5. Repeat steps 3 and 4 two more times and calculate the average of the three trials. Data Table Trial Average Resting Vigorous Exercise Recovery Time Analysis 1. What do you notice about the change in your heart rate from resting to vigorous exercise? 2. Compare your recovery time with other students. How does it compare? 3. Calculate your cardiac output for resting and vigorous exercise by using your averages and a stroke volume of 70 ml. 4. Why did you perform three trials for each heart rate measurement? Lesson 2 Summary In this lesson, you studied the physiology of heartbeat and the factors that influence heart rate. You also had an opportunity to measure your own heart rate and recovery rate. In the next lesson, you will study blood pressure and fluid exchange. 30

31 Introduction to Blood Pressure and Fluid Exchange Uncontrolled high blood pressure can lead to stroke, heart attack, heart failure or kidney failure. However, because there are no symptoms, nearly one-third of people with high blood pressure don't even know they have it. This is why high blood pressure is often called the "silent killer". The only way to tell if you have high blood pressure is to have your blood pressure checked. Do you know your blood pressure? Lesson 3 Outcomes By the end of this lesson, you should be able to: Identify systolic and diastolic blood pressure using a sphygmomanometer List and describe extrinsic factors (e.g., exercise, caffeine, nicotine) which affect transient blood pressure Differentiate between vasodilation and vasoconstriction Describe the control of blood pressure by the autonomic nervous system List and describe factors which affect arteriolar resistance Explain how changes in blood pressure help to maintain homeostasis in the body Explain how blood pressure and osmotic pressure contribute to fluid exchange at the capillary level Describe the term hypertension and discuss its causes, effects and treatment. 31

32 Lesson 3 Overview Following is a list of topics covered in this lesson. Blood pressure Measuring Blood pressure Hypertension Exchanges between Blood and Cells Regulation of Blood Pressure Blood Pressure Blood pressure is defined as The highest pressure occurs in the aorta, the large vessel that carries oxygenated blood away from the heart. As the blood passes into smaller vessels and the distance from the heart becomes greater, the pressure becomes reduced. The pressure in any artery varies as a result of two major factors. 1. Cardiac Output o Volume of blood. o Heart rate. 32

33 2. Arteriolar Resistance o Size. o Elasticity. o Measuring Blood Pressure Two different pressures are measured and compared in a blood pressure reading. Systolic pressure Diastolic pressure The pressure of the blood pressing against the arterial walls can be measured using a device called a sphygmomanometer. To measure blood pressure, an inflatable rubber cuff is wrapped around the upper arm. As air is pumped into the cuff, the cuff presses on the arteries of the arm. When the pressure in the cuff is high enough, the blood flow through the arteries ceases. A stethoscope is placed over one of the arteries in the elbow, and the air in the cuff is gradually released. At first, the person listening through the stethoscope hears no sound. Then, a sharp tapping sound is heard. This sound is made by the blood spurting 33

34 through a narrow opening in the compressed artery. The pressure reading just as this sound is heard is the systolic pressure. As more air is released from the cuff, the sound becomes muffled and then stops as the cuff ceases to press on the artery. The pressure reading just as the sound stops is the diastolic pressure. Blood pressure is measured in millimeters of mercury (mm Hg). It is expressed as a ratio of systolic pressure to diastolic pressure. A reading of 120/70 means that the person's systolic pressure is 120 mm Hg and the diastolic pressure is 70 mm Hg. It is expressed verbally as "120 over 70." Normal blood pressure is less than 130 mm Hg systolic and less than 85 mm Hg diastolic. Optimal blood pressure is less than 120 mm Hg systolic and less than 80 mm Hg diastolic. A typical reading for a healthy adult is 120/70. Readings for children and adolescents may be slightly higher. A physician can infer much about a person's health by taking a blood pressure reading. Hypertension High blood pressure or hypertension is defined in an adult as a blood pressure greater than or equal to 140 mm Hg systolic pressure or greater than or equal to 90 mm Hg diastolic pressure. High blood pressure directly increases the risk of coronary heart disease (which can lead to heart attack) and stroke, especially along with other risk factors. High blood pressure can occur in children or adults, but it's more common among people over age 35. It's particularly prevalent in middle-aged and elderly people, obese people, heavy drinkers and women who are taking birth control pills. It may run in families, but many people with a strong family history of high blood pressure never have it. People with diabetes mellitus, gout or kidney disease are more likely to have hypertension. Recall: Systolic Pressure: Diastolic Pressure: 34

35 Which is more dangerous to be higher than normal, systolic or diastolic pressure? Why? Blood pressure is normally controlled by nerves that have their center in the brain. If the blood pressure in certain vessels increases, the brain sends nerve impulses to the heart and to the blood vessels, causing the heart rate to slow and the blood vessels to widen. As a result the blood pressure decreases. If the blood pressure becomes too low, the brain sends impulses that cause the heart rate to increase and the blood vessels to narrow, increasing the blood pressure. This is another case of homeostasis maintaining a constant internal environment. If this regulatory mechanism cannot bring the blood pressure to normal levels, a condition known as hypertension is evident and medical assistance is required. The factors causing hypertension are not well understood. In 90% of the cases of hypertension, the cause is unknown. What do you think can cause hypertension? Risk Factors You Can Control Smoking Physical Inactivity Obesity Diet (Salt Intake) Diabetes Stress Risk Factors You Can t Control Age Ethnicity (South Asians, First Nations/Aboriginal Peoples or Inuit and Blacks are at increased risk) Family history The goal of treatment is to reduce the diastolic blood pressure to less than 90 mm Hg. Treatment consists of a combination of no-added-salt diet, weight loss if the person is over-weight, and drug medication. The excess fluid that sodium (salt) holds in the body may also put an added burden on the heart and "waterlog" the blood vessels, causing them to contract or narrow more easily. The blood vessels then take less diluted blood to the organs of the body than the quantity of normal blood that is required, depriving 35

36 the cells of some oxygen and nutrients that they need. For this reason, low-sodium diets are used in treating mild to moderately severe hypertension. However, in individuals with severe hypertension, salt restriction must be severe. When the demand for blood in various parts of the body is high (e.g. during exercise), the heart must pump faster, increasing the blood pressure in the vessels. Fatty tissue requires a lot of blood to feed it. Therefore, another way to reduce blood pressure and the stress on the heart is to lose weight. In addition to possibly lowering blood pressure and reducing weight, a low-fat, low-cholesterol diet may also help delay the beginning of arteriosclerosis. Medications called diuretics, which help rid the body of excess salt and therefore, of excess water, are often prescribed by the doctor in the treatment of hypertension. Blood Pressure in Capillaries and in Veins Blood pressure in the Capillaries The pressure of arterial blood is significantly reduced when the blood enters the capillaries. Capillaries are tiny vessels with a diameter just about that of a red blood cell (7 µm). Although the diameter of a single capillary is quite small, the number of capillaries supplied by a single arteriole is so great that the total area available for the flow of blood is increased. Therefore, the pressure of the blood as it enters the capillaries decreases. Blood pressure in the veins When blood leaves the capillaries and enters the venules and veins, little pressure remains to force it along. Blood in the veins below the heart is helped back up to the heart by the muscle pump. This is simply the squeezing effect of contracting muscles on the veins running through them. One-way flow to the heart is achieved by valves within the veins. 36

37 Exchanges between Blood and Cells Our blood does not come into direct contact with the cells it nourishes. However, the distribution of capillaries is so extensive that cells are never farther away than 1 cell layer from a capillary. When blood enters the arteriole end of a capillary, it is still under pressure produced by the contraction of the ventricle. As a result of this pressure, a substantial amount of water and some plasma proteins filter through the walls of the capillaries into the tissue space. This fluid, called interstitial fluid, is blood plasma minus most of the proteins. Interstitial fluid bathes the cells in the tissue space. Substances in the fluid can enter the cells by diffusion or active transport. Substances, like carbon dioxide, can diffuse out of cells and into the interstitial fluid. Near the venous end of a capillary, the blood pressure is greatly reduced. Here, another force comes into play. Although the composition of interstitial fluid is similar to that of blood plasma, it contains a smaller concentration of proteins than plasma and a greater concentration of water. This difference sets up an osmotic pressure. Although the osmotic pressure is small, it is greater than the blood pressure at the venous end of the capillary. Consequently, the fluid re-enters the capillary at the venous end. Regulation of Blood Pressure An adult human has been estimated to have some km of capillaries with a total surface area of some m 2 (an area greater than three tennis courts). The total volume of this system is roughly 5 liters, the same as the total volume of blood. However, if the heart and major vessels are to be kept filled, all the capillaries cannot be filled at once. Therefore, a continual redirection of blood from organ to organ takes place in response to the changing needs of the body. For example, during vigorous exercise, capillary beds in the skeletal muscles open at the expense of those in the abdomen. The reverse occurs after a heavy meal. 37

38 The table below shows the distribution of blood in the human body at rest and during vigorous exercise. Note the increase in blood supply to the working organs (skeletal muscles and heart). The increased blood supply to the skin aids in the dissipation of the heat produced by the muscles. Note also that the blood supply to the brain remains constant. The total blood flow during exercise increases because of a more rapid heartbeat and also a greater volume of blood pumped at each beat. Blood Flow ml/min At Rest Heart Kidneys 1, Skeletal Muscles During Strenuous Exercise 1,000 12,5000 Skin 400 1,900 Viscera 1, Brain Other Total 5,600 17,500 The walls of arterioles are encased in smooth muscle. Vasoconstriction (reduction in diameter of blood vessels) of arterioles decreases blood flow into the capillary beds they supply while vasodilation (increase in diameter of blood vessels) has the opposite effect. In time of danger or other stress, the arterioles supplying the skeletal muscles will be dilated while those supplying the digestive organs will decrease. These actions are controlled by the autonomic nervous system (ANS). The autonomic nervous system consists of sensory neurons and motor neurons that run between the central nervous system (especially the hypothalamus and medulla oblongata) and various internal organs such as the heart, lungs, viscera, and glands (both exocrine and endocrine). The ANS is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. For example, the contraction of both smooth muscle and cardiac muscle is controlled by motor neurons of the autonomic system. The ANS has two subsystems; the Sympathetic Nervous System which is involved in the fight or flight response and the Parasympathetic Nervous System which is involved in relaxation. Each of these subsystems operates in the 38

39 reverse of the other (antagonism). Both systems affect the same organs and act in opposition to maintain homeostasis. For example: when you are scared, the sympathetic system causes your heart to beat faster; the parasympathetic system reverses this effect. Baroreceptors When blood pressure exceeds acceptable levels, the receptors send nerve impulse messages to the medulla oblongata which causes the sympathetic (flight or fight) nerve impulses to decrease. This results in arteriole dilation and increased outflow of blood from the artery. The parasympathetic (relaxation) nerve impulses are increased, causing heart rate and stroke volume to decrease. The decreased cardiac output slows the movement of blood into the arteries, lowering blood pressure. Low blood pressure is also adjusted by the sympathetic nerve. Without nerve information from the pressure receptors of the carotid artery and aorta, the sympathetic nerve will continue to be stimulated, causing cardiac output to increase and arterioles to constrict. The increased flow of blood into the artery, accompanied by a decreased outflow raises blood pressure to acceptable levels. Control of Blood Pressure Regulation of Blood Pressure by Hormones The role of the kidney is Local Control of Blood Pressure in the Capillary Beds Cells where infection or other damage is occurring release substances like histamine that dilate the arterioles and increase blood flow in the area. Nitric oxide (NO) is also a potent dilator of blood vessels. When the endothelial cells that line blood vessels are stimulated, they synthesize nitric oxide. It quickly diffuses into the muscular walls of the vessels causing them to relax. 39

40 Nitroglycerine is often prescribed to reduce the pain of angina (heart pain). It does so by generating nitric oxide, which relaxes the walls of the arteries and arterioles. The prescription drug sildenafil citrate ("Viagra ") does the same for vessels supplying blood to the penis. The effects of these two drugs are additive and using them together could precipitate a dangerous drop in blood pressure. Shock Trauma to the body or severe bleeding may cause shock which may result in capillary beds opening without others closing in compensation. Although the volume of blood remains unchanged, blood pressure declines abruptly as blood pools in the capillary beds. The heart can only pump as much blood as it receives. If insufficient blood gets back to the heart, its output - and hence blood pressure - drops. The tissues fail to receive enough oxygen. This is especially critical for the brain and the heart itself. If untreated, shock is usually fatal. To cope with the problem, arterioles constrict and shut down the capillary beds - except those in the brain and heart. This reduces the volume of the system and helps maintain normal blood pressure. Air-breathing vertebrates that spend long periods under water (e.g., seals, penguins, turtles, and alligators) employ a similar mechanism to ensure that the oxygen supply of the heart and brain is not seriously diminished. When the animal dives, the blood supply to the rest of the body is sharply reduced so that what oxygen remains will be available for those organs needing it most: the brain and heart. 40

41 Lesson 3 Exercise 1. a) What is blood pressure? b) List and describe two major factors that affect blood pressure. 2. a) Differentiate between systolic and diastolic blood pressure. b) How is blood pressure measured? c) What are "normal" blood pressure measurements? d) What effect does exercise have on blood pressure? 3. a) Define hypertension. b) What are some of the possible side effects of hypertension? c) Discuss the causes, risk factors and treatment of hypertension. 41

42 4. a) Explain how blood pressure contributes to fluid exchange in the tissues b) What other factor contributes to fluid exchange at the tissue level. 5. Differentiate between vasoconstriction and vasodilation. 6. How is blood flow to the muscles, kidneys and brain affected by vigorous exercise? 7. Describe how the Autonomic Nervous System contributes to homeostasis by controlling blood pressure. Include the role of blood pressure receptors in your answer. 8. Describe the role of the kidney in controlling blood pressure. 9. Why is nitroglycerin used as a treatment for angina? 10. Why is shock sometimes fatal if untreated? 42

43 Lesson 3 Summary In this lesson, you have learned about blood pressure, how it is measured, and some problems associated with high blood pressure. You have also studied how blood pressure helps to maintain homeostasis and fluid exchange at the cell level. In the next lesson, you will study the Lymphatic System. Introduction to Lesson 4 - The Lymphatic System Your circulatory system is not your body s only vascular transport system. Closely associated with the blood vessels of the circulatory system is the lymphatic system. The lymphatic system is a network of glands and vessels that extend throughout your body. Lesson 4 Outcomes By the end of this lesson, you should be able to: Describe the function of the lymphatic system in the human body. List the components of lymph in the human body, i.e., fat, protein, water, white blood cells. Identify the following structures of the lymphatic system from a specimen, model, or diagram: adenoids tonsil lymph nodes spleen thoracic duct Differentiate between lymph vessels and blood vessels. 43

44 Lesson 4 Overview Following is a list of topics covered in this lesson. Function of the Lymphatic System Lymph Lymphatic Organs The Lymphatic System The lymphatic system consists of fluid called lymph, vessels that transport the lymph and organs that contain lymphoid tissue. The lymphatic system has three primary functions. First of all, it returns excess interstitial fluid (the fluid that surrounds the cells) to the blood. Interstitial fluid is very abundant, making up about 15% of the body mass. It is the clear, colourless liquid that appears when the skin is grazed or a blister is broken. All of the cells and tissues of the body must be continuously bathed by fluids. These fluids enable nutrients to pass from the capillaries to the cell membranes and waste products to return to the capillaries. Some of the fluid in blood is constantly passing through the capillary walls and entering the spaces between the cells. The walls of the capillaries prevent most of the blood cells and some of the proteins of the plasma from leaving the blood stream. However, some white blood cells leave the capillaries by forcing their way out between the cells that make up the capillary walls. The composition of intercellular fluid is much the same as the plasma of the blood. Interstitial fluid bridges the gap between capillaries and the isolated cells which are not in direct contact with a capillary. It is laden with substances needed by the cells and also takes away waste substances from the cells. Of the fluid that leaves the capillaries, about 90 percent is returned. The 10 percent that does not return becomes part of the interstitial fluid that surrounds the tissue cells. Lymph capillaries pick up the excess interstitial fluid and proteins and return them to the venous blood. 44

45 The second and probably most well known function of the lymphatic system is defense against invading microorganisms and disease. Lymph nodes and other lymphatic organs filter the lymph to remove microorganisms and other foreign particles. Lymphatic organs also contain lymphocytes that destroy invading organisms. Lymphatic organs will be discussed later in the lesson. The third function of the lymphatic system is the absorption of fats and fatsoluble vitamins from the digestive system and the subsequent transport of these substances to the venous circulation. The lining of the small intestine is covered with fingerlike projections called villi. There are blood capillaries and special lymph capillaries, called lacteals, in the center of each villus. The blood capillaries absorb most nutrients, but the fats and fat-soluble vitamins are absorbed by the lacteals. The absorbed fat droplets are transported to adipose (fat) tissue where they are stored. Figure Lacteal ( 45

46 Lymph Lymph is a fluid similar in composition to blood plasma (90% water, salts, proteins, hormones, nutrients, waste products, gases). As discussed above, it is derived from blood plasma as fluids pass through capillary walls at the arterial end. As the interstitial fluid begins to accumulate, it is picked up and removed by tiny lymphatic vessels and returned to the blood. As soon as the interstitial fluid enters the lymph capillaries, it is called lymph. Returning the fluid to the blood prevents a condition known as edema (swelling of the tissue) and helps to maintain normal blood volume and pressure. The lymphatic system consists of capillaries and lymph vessels that correspond to the capillaries and veins of the blood circulatory system. Lymphatic vessels, unlike blood vessels, only carry fluid away from the tissues. The smallest lymphatic vessels are the lymph capillaries, which begin in the tissue spaces as blind-ended sacs. Lymph capillaries are found in all regions of the body except the bone marrow, central nervous system, and tissues, such as the epidermis, that lack blood vessels. The wall of the lymph capillary is composed of endothelium in which the simple squamous cells overlap to form a simple one-way valve. This arrangement permits fluid to enter the capillary but prevents lymph from leaving the vessel. The lymph capillaries do not form a net like the blood capillaries but they resemble microscopic fingers that connect with the lymph vessels. The smaller lymph vessels unite, forming larger ones. See the diagram below. 46

47 Figure Lymph Capillaries ( Small lymphatic vessels join to form larger tributaries, called lymphatic trunks, which drain large regions. Lymphatic trunks merge until the lymph enters the two lymphatic ducts. The right lymphatic duct drains lymph from the upper right quadrant of the body. The thoracic duct drains all the rest. The diagram below illustrates the major lymphatic vessels and organs. Figure Lymphatic Vessels and Organs ( Like veins, the lymphatic tributaries have thin walls and have valves to prevent backflow of blood. There is no pump in the lymphatic system like the heart in the cardiovascular system. The pressure gradients to move lymph through the vessels come from the skeletal muscle action, respiratory movement, and contraction of smooth muscle in vessel walls. 47

48 Lymphatic Organs Lymphatic organs are characterized by clusters of lymphocytes and other white blood cells, such as macrophages in a framework of short, branching connective tissue fibers. The lymphocytes originate in the red bone marrow with other types of blood cells and are carried in the blood from the bone marrow to the lymphatic organs. When the body is exposed to microorganisms and other foreign substances, the lymphocytes multiply within the lymphatic organs and are sent in the blood to the site of the invasion. This is part of the immune response that attempts to destroy the invading agent. Lymph nodes are small bean-shaped structures that are usually less than 2.5 cm in length. They are widely distributed throughout the body along the lymphatic pathways where they filter the lymph before it is returned to the blood. There are three regions on each side of the body where lymph nodes tend to cluster. These areas are the groin, the armpit, and the neck. The typical lymph node is surrounded by a connective tissue capsule and divided into compartments called lymph nodules. The lymph nodules are dense masses of lymphocytes and macrophages and are separated by spaces called lymph sinuses. Several afferent lymphatic vessels, which carry lymph into the node, enter the node on the convex side. The lymph moves through the lymph sinuses and enters an efferent lymphatic vessel, which carries the lymph away from the node. Because there are more afferent vessels than efferent vessels, the passage of lymph through the sinuses is slowed down, which allow time for the cleansing process. See the diagram below. 48