ANATOMY & PHYSIOLOGY II THE BODY SYSTEMS
Anatomy & Physiology II The Body Systems Michelle Cochrane 2014 All rights reserved. This material is subject to copyright and may not be reprinted or reproduced in any manner without prior consent from the author. Reproduced by Om Shanti College with permission from the author for educational purposes only. 2
LESSON 3 CARDIOVASCULAR SYSTEM 3.1 Circulation 3.1.1 Coronary Circulation 3.1.2 Pulmonary Circulation 3.1.3 Systemic Circulation 3.2 Blood Vessels 3.2.1 Arteries 3.2.2 Veins 3.2.3 Capillaries 3.3 The Heart 3.4 Blood Pressure 3.5 Blood 3.5.1 Plasma 3.5.2 Red Blood Cells 3.5.3 White Blood Cells 3.5.4 Platelets 3.5.5 Blood Types 3
Our circulatory system is what supplies the cells and tissues of our body with all the vital supplies that they need such as oxygen and nutrients. It is also responsible for carrying away all the waste products and toxins that we don t want. There are two parts to the circulatory system: Cardiovascular system the heart and blood vessels Lymphatic system the lymph nodes and lymph vessels For this lesson we will be concentrating on the cardiovascular system. Often, when you hear or read the term circulatory system, this is what is being referred to. The lymphatic system is usually considered separately. The cardiovascular system is made up of: The heart The blood vessels The blood 3.1 CIRCULATION Circulation refers to the system of blood vessels which carry the blood around to all the different parts of our body. The circulation is divided into three separate systems, all with their own job to do: Coronary circulation Pulmonary circulation Systemic circulation 4
3.1.1 CORONARY CIRCULATION Coronary circulation is what transports blood to the heart muscles themselves, providing them with oxygen and nutrients. The word coronary is used to refer to things to do with the heart. Our heart muscles do great work in pumping blood around to the rest of the body, but they also need nourishing themselves in order to remain in good, healthy working order. Ever wondered what causes a heart attack? That is when one of the coronary arteries or veins supplying the heart muscle becomes blocked and the muscle gets starved of oxygen. This causes some of the cells in the heart muscle to die. Not good news. 3.1.2 PULMONARY CIRCULATION This is the circulation between the heart and lungs. The word pulmonary refers to the lungs. As with the other parts of the circulatory system, the arteries carry blood from the heart to the lungs and the veins carry blood from the lungs back to the heart again. Except this time there is one very big difference. The pulmonary arteries carry deoxygenated blood (blood without oxygen). The pulmonary veins carry oxygenated blood. So why the difference? Well, the lungs are where the blood picks up its oxygen. So blood going to the lungs will be deoxygenated and blood leaving the lungs will be oxygenated. So while it may seem backwards at first, when we stop and think about it starts to make sense. 3.1.3 SYSTEMIC CIRCULATION Systemic circulation is the part of the circulation which transports blood between the heart and the rest of the body. It includes everything that is not part of the coronary or pulmonary circulation. All living cells in our body require oxygen and nutrients for survival as well as the removal of waste products. In this case, the arteries carry oxygenated blood from the heart to the body, while the veins bring the deoxygenated blood back again. 5
3.2 BLOOD VESSELS The blood vessels are a series of hollow tubes which transport the blood around the body. There are three main types of blood vessels: Arteries carry blood away from the heart. Veins carry blood towards the heart. Capillaries are small vessels within the body tissues, linking the veins and arteries. Notice we do not say that the arteries carry oxygen. Look back to the section above on pulmonary circulation. The terms arteries and veins refer to the direction in which the blood is travelling, not whether it contains oxygen. 3.2.1 ARTERIES Our arteries are the largest blood vessels in the body. They are responsible for carrying blood away from the heart to the rest of the body. Pulmonary arteries carry deoxygenated blood from the heart to the lungs. Systemic arteries carry oxygenated blood from the heart to the rest of the body. The wall of the arteries is formed from an impermeable membrane consisting of three layers: The tunica intima - the inner layer, made of thin epithelial tissue The tunica media the middle layer, made of smooth muscle The tunica adventitia the outer layer made of strong connective tissue. The walls of the arteries have to be strong, because blood is pumped out of the heart under high pressure. They also have to be impermeable so that the blood does not leak out. The smooth muscle tissue allows the arteries to expand and contract which helps to control out blood pressure. As the arteries spread through the body they divide and become smaller. The smallest arteries are called arterioles, and these provide the connection between the arteries and capillaries. 6
There are many arteries through the body. Some of the more important ones are: Aorta the main artery leaving the heart. Pulmonary arteries - between the heart and the lungs. Vertebral and carotid arteries lead to the head and the brain. Brachial arteries lead to the arms. Iliac and femoral arteries lead to the legs. There are other, smaller, arteries that lead to the extremities and the internal organs. 3.2.2 VEINS Our veins are responsible for carrying blood back to the heart from the rest of the body. Pulmonary veins carry oxygenated blood from the lungs to the heart. Systemic veins carry deoxygenated blood from the rest of the body to the heart. Like the arteries, the wall of the veins is formed from an impermeable membrane consisting of three layers. In the veins, the muscle layer is not as thick. The blood is not under so much pressure here, so the veins do not need to be as strong. In the veins, the blood is not being forced through at high pressure by the heart. This means that it has to rely on other ways to keep moving - otherwise the effect of gravity means that all of our blood would end up in our feet. Skeletal muscle movement helps to improve venous blood flow because the action of the muscles contracting and relaxing acts as a pump. Yes, exercise really does improve the circulation. The other way is by having a series of one way valves all the way along the veins which stops the blood from flowing in the wrong direction. Failure of one of these valves can result in pooling of the blood, a condition known as varicose veins. 7
Again like the arteries, as the veins spread through the body they divide and become smaller. The smallest veins are called venules, and these provide the connection between the veins and capillaries. There are many veins through the body. Some of the more important ones are: Vena cava the main veins entering the heart. Pulmonary veins - between the heart and the lungs. Vertebral and jugular veins from the head and the brain. Brachial and cephalic veins from the arms. Iliac, femoral and saphenous veins from the legs. There are more veins than there are arteries. Other, smaller, veins lead from the extremities and the internal organs to the heart. 3.2.3 CAPILLARIES The capillaries are the smallest blood vessels in our cardiovascular system and act as a link between the arteries and the veins. These tiny vessels are typically just 1mm long and 8-10 µm (0.008 0.01mm) wide just wide enough to allow red blood cells to pass through. Capillaries have a very thin wall consisting of just one layer, the tunica intima, which can be as thin as one row of cells. The capillary wall is permeable, allowing substances such as gases, electrolytes, nutrients and hormones to pass through. However red blood cells are too big to pass through the walls and remain in the capillaries. Most of the tissues in the body have capillaries but there are a few exceptions. Tendons and ligaments are poorly vascularised that is they have a very low number of capillaries. This is why they are slow to heal when injured. Cartilage and epithelium do not have any capillaries but receive their nutrients from blood vessels in the nearby connective tissue. In the eye, the cornea and lens receive nutrients from the fluid inside the eye. 8
3.3 THE HEART The heart is a muscular organ situated in the centre of the chest, slightly to the left side. It has an important job to do, pumping blood around the circulatory system, providing oxygen and nutrients and carrying away wastes. 3.3.1 HEART STRUCTURE Tissues The heart tissue is made of 3 main layers: Epicardium the outside layer of the heart is made from connective tissue. Myocardium makes up most of the heart tissue. This is made from cardiac muscle which contracts to pump blood. Endocardium the inner layer made of epithelial tissue. The whole heart is surrounded by the pericardium, a fluid filled sac which provides support, shock absorption and lubrication so that the heart muscles can move without causing friction. The inner layer of the pericardium is formed from the epicardium. Chambers The heart comprises of four chambers the left and right atria (singular: atrium) and left and right ventricles. The atria are on top and the ventricles are on the bottom. Blood flows into the atria and out of the ventricles. Deoxygenated blood returning from the body flows into the right atrium, then out of the right ventricle to the lungs. Oxygenated blood from the lungs flows into the left atrium, then out of the left ventricle to the rest of the body. Confusing? Here is one way to remember. Think of the letters L for left and R for right: L from the lungs to the left atrium R from the rest of the body to the right atrium. 9
Blood vessels As we learned earlier, arteries lead from the heart while veins lead to the heart. There are just a few that actually enter or leave the heart itself: Pulmonary artery carries deoxygenated blood from the heart to the lungs. Pulmonary veins carry oxygenated blood from the lungs to the heart. Aorta carries oxygenated blood from the heart to the body. Superior and inferior vena cava carry deoxygenated blood from the body to the heart. There are a couple of things to pay attention to here: Remember, the terms artery and vein refer to the direction of blood flow, not whether it carries oxygen. There are more veins than arteries. The arteries are bigger and stronger and can handle a much higher level of pressure. All other veins and arteries branch off the ones mentioned above. Valves The heart also contains a number of valves, all of which function to make sure that the blood flows in the right direction. Our circulatory system is designed for one-way traffic only, and these valves help to keep it that way. Aortic valve between the left ventricle and the aorta. Mitral valve between the left atrium and the left ventricle. Pulmonary valve between the right ventricle and the pulmonary artery. Tricuspid valve between the right atrium and the right ventricle. 3.3.2 HEART FUNCTION The job of the heart is to pump the blood around the circulatory system. It does by contracting and relaxing the muscles of the different chambers in turn. Contraction of the heart muscles is called systole. Relaxation of the muscles is called diastole. These two terms are used when talking about blood pressure, which we will come to later. 10
The atria and the ventricles contract separately in order to allow blood to be pumped from one to the other, with the valves mentioned above stopping the blood from going in the wrong direction. The average adult heart beat is 60 100 beats per minute. Relaxation period All chambers relax (diastole). Blood flows passively into atria. Systole: contraction of heart muscles Diastole: relaxation of heart muscles Ventricular Systole Atrial diastole. Ventricular systole forces blood into arteries Ventricular filling Ventricular diastole. Atrial systole forces blood into ventricles. 3.4 BLOOD PRESSURE Blood pressure is the amount of pressure exerted on the walls of the blood vessels by the blood. The pressure is highest in the aorta and the other major arteries. Blood pressure is higher during systole (contraction of the heart muscles) and lower during diastole (relaxation of heart muscles). 11
Blood pressure is usually expressed as two measurements one for systolic pressure and one for diastolic, with the systolic given first. The average blood pressure for a healthy adult is 120/80. Systolic Diastolic Normal <120 <80 Prehypertensive 120-139 80-89 Stage 1 hypertension 140-159 90-99 Stage 2 hypertension >160 >100 Hypertension is defined as blood pressure at or above 140/90. The diastolic pressure is more important since this indicates how much pressure the system is under when the heart is relaxed. Factors which influence blood pressure are: Body mass index Sodium intake Stress Family history Alcohol consumption Fluid consumption Exercise Metabolism You will study hypertension further in your pathology course. 3.5 BLOOD Our blood flows all round our body, carrying vital oxygen and nutrients to where they are needed and carrying away waste products. Blood is classified as a connective tissue. Although it does not provide physical support and structure like the other connective tissues, it does support the body through its supply of nutrients and also provides a connection between different parts of the body. The average adult has about 4-6 litres of blood, with males having more than females. It sounds like a lot, but it s only a small proportion of the total body fluids. Blood is usually slightly alkaline (ph 7.35-7.45), with an average temperature of 37 o C. 12
Blood has a lot of important functions to perform in the body. These are split into distribution functions, regulation functions and protection functions: Distribution functions o Delivering oxygen from the lungs o Delivering nutrients from the digestive system o Removing waste products from the body o Transporting hormones to the target cells and organs Regulation functions o Maintaining body temperature by absorbing and distributing heat o Maintaining normal ph in body tissues o Maintaining adequate fluid in the circulatory system Protection functions o Preventing blood loss by clotting o Preventing and fighting infection The key constituents of the blood are: Plasma 55% Red blood cells 45% White blood cells <1% Platelets <1% We will now have a look at all of these different components. 3.5.1 PLASMA Plasma is the fluid component of the blood. It makes up over half of the blood volume. Most of that is water, but there are also lots of other substances in the plasma: Water (90%) Proteins Glucose Electrolytes Hormones Clotting factors Carbon dioxide Waste products Others 13
Plasma carries out many of the functions of the blood including: Maintaining blood pressure by providing adequate fluid Transporting nutrients from the digestive system to the cells and tissues Providing proteins which are used in the clotting and immune processes Exchange of vitamins and electrolytes needed for cells to function properly Removal of toxins and waste products Plasma itself is actually a pale yellow coloured fluid. The red colour of blood is provided from the red blood cells which are suspended in it. Plasma is often given in hospital to people who have lost large volumes of blood through trauma in order to increase their blood pressure. 3.5.2 RED BLOOD CELLS Red blood cells are also known as erythrocytes (erythro = red, cyte = cell). There are 1 billion RBCs in every drop of blood. Red blood cells only live for about 120 days, which means we always have to make new ones. Our bodies produce about 2.4 million new RBCs every second. That s quite impressive. RBCs are unique among the cells of our body. They are the only ones that don t have a nucleus. This helps them to squash down and squeeze through the very narrow capillaries. Unfortunately it also means that they are not able to reproduce themselves. Cells use the DNA contained in their nucleus in order to reproduce. So no nucleus, no reproduction. This means that we need to get new RBCs in a different way. New RBCs are made in the bone marrow. The main purpose of the red blood cells is to transport oxygen around the body, delivering it to where it is needed. The oxygen is attached to an iron-containing protein molecule called haemoglobin. (Haem = iron, globin = protein). The oxygen is attracted to the iron in the haemoglobin and binds to it. Think about how quickly iron rusts. That is because it is so attractive to oxygen which forms a chemical reaction turning it into rust. Just like iron turns a reddish brown when it rusts, so haemoglobin turns red when it combines with oxygen. This is what gives blood its colour. 14
It is very important to get enough iron in your diet. Lack of iron can result in anaemia (literally translated as lack of iron ). If there is not enough iron in the blood, there will also not be enough oxygen. 3.5.4 WHITE BLOOD CELLS White blood cells (WBCs) only make up a small proportion of the total blood but they have a very important job to do. WBCs form an important part of our immune system, defending us from disease from bacteria, viruses, fungal infections, parasites and toxins. There are five types of WBC, each with their own job to do. Monocytes Lymphocytes Eosinophils Basophils Neutrophils Let s have a closer look at each of these. Monocytes Monocytes are the largest type of WBC, and make up about 7-8% of the total number of WBCs. They have three main functions: Phagocytosis of bacteria and pathogens Promote immune response Produce anti-inflammatory chemicals Phagocytosis is the process by which the monocytes destroy the bacteria. There are 3 stages: 1. The monocyte binds to the bacteria. 2. The monocyte surrounds the bacteria and engulfs it. 3. The chemicals within the cell act on the bacteria to break it down and destroy it. Think of it like Pacman swallowing up a ghost. Lymphocytes Lymphocytes are the second most common type of WBCs, making up about 15-40% of the total. Their main functions are: Destroy viruses and cancer cells Regulate the immune system 15
Secrete antibodies Eosinophils Eosinophils make up a small proportion of the WBCs, just 1-6% of the total. The main purpose of eosinophils is to fight parasites. They also have a role to play in allergic reactions, causing the inflammation part of the allergic response. Basophils Basophils are the least common type of WBC. They only account for 0.1-0.3% of WBCs. Basophils form a key part of the allergic response. They trigger the release of histamine, the main chemical involved in allergies, as well as vasodilation (expansion of blood vessels) which causes a drop in blood pressure. Neutrophils Neutrophils are the most common type of WBCs, accounting for 60-70% of the total number of WBCs. Neutrophils have a key role to play in fighting bacterial and fungicidal infections. They do this through the process of phagocytosis engulfing the bacteria and fungi and destroying them. Neutrophils are what is present in pus. It actually consists of dead neutrophils which die off after they have finished performing their task of killing the toxins. 3.5.4 PLATELETS Platelets are one of the things that make mammals unique. We are the only members of the animal kingdom that actually have them. Platelets have a very important role to play by stopping bleeding that occurs when blood vessels are ruptured or injured. They do this by sticking to the edges of wounds (adhesion) and also by sticking to each other (aggregation). The process of stopping bleeding is known as haemostasis (not to be confused with homeostasis which is the process of maintaining balance in the body). 16
There are three stages to haemostasis: 1. Vascular spasm the injured tissues release chemicals. The smooth muscle in the blood vessel contracts. 2. Platelet plug formation platelets are attracted to the area and stick to the exposed collagen fibres at the injury site. They then release chemicals which attract more platelets. 3. Coagulation the platelets are reinforced with a mesh of fibrin threads. Clotting factors also help to turn the blood plasma around the clot from a liquid to a gel. Clotting factors are plasma proteins made in the liver. They circulate in the blood in an inactive form until they are required. They are activated during stage 3 of haemostasis. Vitamin K is required to produce clotting factors, which is why an injection is often given to newborn babies who have not yet received enough vitamin K. BLOOD TYPES Although all blood is made up of the same basic components, it is not all quite the same. What blood type we are is determined by the blood type of our parents, and also determines what types of blood we are able to receive if we ever need a blood transfusion. The ABO Blood Group System There are four major blood types, which are identified by the presence or absence of type A or B antigens (markers) on the outside of red blood cells. Antigens are substances that can trigger an immune response if they are foreign to the body. Some antigens can trigger the patient s immune system to attack the transfused blood so it is important to make sure that the right type of blood is used. Group A has A antigens on the RBCs and B antibodies in the plasma Group B has B antigens on the RBCs and A antibodies in the plasma Group AB has both A and B antigens on the RBCs but no antibodies in the plasma Group O has no antigens on the RBCs, but both A and B antibodies in the plasma Group O is known as the universal donor because there are no antigens which would cause it to be rejected by recipients. 17
Group AB is the universal receiver because it has no antibodies which would cause it to reject other blood types. The ways in which the blood types can be matched is shown below: Blood type O 49% of Australians, it is the most common type and is also the universal donor so it is always in demand for donations. Blood type A 38% of Australians, it can be given to people with blood types A and AB Blood type B 10% of Australians, it can be given to people with blood types B and AB Blood type AB 3% of Australians. The rarest type, it can only be given to people with AB blood, but AB people can receive all other blood types. Positive and Negative As well as the A and B antigens, there is also a third type of antigen called the rhesus (Rh) factor. The Rh antigen can be either present (positive +) or absent (negative -). People who are Rh- cannot accept Rh+ blood because they do not recognise the Rh antigen. The Rh antigen can affect pregnancy. If a mother carries a baby that is Rh+ (from the father), then she may develop antibodies against the Rh antigen. This will not affect the first pregnancy, but if she gets pregnant again her immune system may reject another Rh+ baby. For this reason, Rh- women who become pregnant are given an injection to stop them developing the antibodies. 18