Unit 7 Transport in Humans

Transcription

1 Unit 7 Transport in Humans 7.1 Circulatory System SUFEATIN SURHAN BIOLOGY MSPSBS 2010 Candidates should be able to: SYLLABUS CHECKLIST a) describe the circulatory system as a system of tubes with a pump and valves to ensure one-way flow of blood; b) describe the double circulation in terms of a low pressure circulation to the lungs and a high pressure circulation to the body tissues and relate these differences to the different functions of the two circuits; c) name the main blood vessels that carry blood to and from the heart, lungs, liver and kidney; d) describe the structure and function of the heart in terms of muscular contraction and the working of valves; e) compare the structure and function of arteries, veins and capillaries; f) investigate and state the effect of physical activity on pulse rate; g) describe coronary heart disease in terms of the occlusion of coronary arteries and state the possible causes (diet, stress and smoking) and preventive measures; h) identify red and white blood cells as seen under the light microscope on prepared slides, and in diagrams and photomicrographs; i) list the components of blood as red blood cells, white blood cells, platelets and plasma; j) state the functions of blood: red blood cells haemoglobin and oxygen transport; white blood cells phagocytosis, antibody formation and tissue rejection; platelets fibrinogen to fibrin, causing clotting; plasma transport of blood cells, ions, soluble food substances, hormones, carbon dioxide, urea, vitamins and plasma proteins; k) describe the transfer of materials between capillaries and tissue fluid.

2 The need for a transport system Throughout an organism s life, materials are constantly being moved to and from all parts of the body. In smaller animals, internal transport is by diffusion and active transport from cell to cell. A complex internal transport system has evolved in many animals, especially larger organisms. The human body has a transport system that consists of the heart that pumps blood around a complex system of blood vessels. The main function of the system is to pick up oxygen from the lungs and transport it to every cell in the body and to remove waste products from the cells and transport them to organs, which remove them from the blood and excrete them from the body. The pulmonary circulation is responsible for transporting: Deoxygenated blood from the right side of the heart to the lungs via the pulmonary artery and Oxygenated blood from the lungs to the left side of the heart again via the pulmonary vein. The systemic circulation is responsible for transporting: Oxygenated blood from the heart to all parts of the body via the aorta. Deoxygenated blood back to the right side of the heart again via the inferior and superior vena cava. Transport to body cells Oxygen (from lungs) Nutrients (Digested food products from the ileum) Transport away from body cells (metabolic products) for excretion Carbon dioxide (removed via lungs) Urea (removed in urine via kidney) The Circulatory system The circulatory system is a system which transports nutrients and other useful substances to the cells of the body and removes the waste these cells produce. The circulatory system is made up of three components: 1. Medium; which is the blood. 2. Blood vessels (artery, vein and capillary); 3. Pump; which is the heart. Note: There is a second transport system known as the lymphatic system (not included in the syllabus). This system is different because it has different vessels (lacteals / lymph capillaries), different transport medium i.e. lymph and no pump. Double circulation Blood travels through the heart twice in one complete circulation. Mammals have a closed double circulatory system. Their blood circulated through two separate systems joined only at the heart. The two systems are the Pulmonary Circulation and the Systemic Circulation. Comparison between the systemic and pulmonary circulation: Circulation SYSTEMIC PULMONARY Blood leaves the heart by Left ventricle Right Ventricle Artery from Pulmonary Aorta the heart arteries Blood pressure Blood flows Blood in arteries is Blood in veins is Blood returns to heart by Blood enters the heart by Higher due to the longer distance in which the blood has to be transported to all parts of the body In all blood vessels of the body except those of the lungs High in oxygen, low in carbon dioxide Low in oxygen, high in carbon dioxide Superior and inferior Vena cava Right atrium Lower due to short distance from the lungs to the heart Only in blood vessels of the lungs Low in oxygen, high in carbon dioxide High in oxygen, low in carbon dioxide. Pulmonary vein Left atrium 2 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

3 Advantages of Pulmonary circulation: Little force required as blood flow is not against gravity. Blood travels slowly in the lungs. This ensures that the blood is well oxygenated before it is returned to the heart. Prevents damage to the lung capillaries. Advantages of Systemic circulation: To ensure that the oxygenated blood and the nutrients are distributed to the body tissues at a fast rate. This helps to maintain the high metabolic rate in mammals. Provides high pressure for filtration process in the kidneys in urine formation. The importance of a double circulation Repeated circulation through the heart is necessary to maintain the blood pressure that is required to keep the circulation of the blood around the body. A double circulation is an efficient transport system as: Oxygenated blood is kept separate from deoxygenated blood. The septum in the heart ensures this complete separation. Oxygenated blood flows through the left side of the heart while deoxygenated blood flows through the right. The blood pressure in the systemic circulation is kept higher than that in the pulmonary circulation. The left ventricle, with thicker wall, pumps blood under higher pressure to the body and delivers oxygenated blood effectively to all parts of the body. The right ventricle has thinner wall and pumps blood to the lungs under lower pressure, thereby avoiding any lung damage. Coronary circulation The blood inside the heart does not serve the cardiac or heart muscle. Heart muscle receives its blood supply from two coronary arteries which branch from the aorta. Several small veins return venous blood from the heart muscle to the right atrium. Heart muscle differs from other muscles in at least three important ways: 1. It is made up of branching muscle fibres connected to each other in the form of a network. 2. Heart muscle contracts and relaxes rhythmically in what is termed beats. Blood 3. Heart muscle does not fatigue despite continuous rapid contractions over many years. Blood is a tissue and is made up of three major components: 1. Plasma liquid part of the blood 2. Cells Red blood cells (erythrocytes) and white blood cells (leucocytes) 3. Platelets The liquid and solid parts of the blood can be separated by centrifugation (spinning of a sample of blood in a test tube at high speed). The cells, which are heavy, will settle at the bottom of the test tube and the supernatant is the blood plasma. Plasma Supernatant: Plasma Solid part of the blood: RBCs, WBCs, Platelets A pale yellow watery fluid, made up of 90% water with the following substances in solution: 1. Soluble digested foods (amino acids, glucose). 2. Fat droplets (in suspension). 3. Vitamins. 4. Ions (salts) (e.g. Ca 2+, Na +, Cl - ). 5. Excretory materials (urea, carbon dioxide in the form of hydrogen carbonate / bicarbonate ion HCO 3- ). 6. Hormones (e.g. insulin, adrenaline). 7. Plasma proteins (e.g. prothrombin and fibrinogen for blood clotting, antibodies). 3 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

4 Functions: As a transport medium - 1. To transport blood cells throughout the body i.e. oxygen in red blood cells (oxyhaemoglobin) and white blood cells. 2. To transport nutrients (soluble food substances, ions, vitamins) to all body cells. 3. To transport hormones to target body organs 4. To transport plasma proteins to damaged sites in the body for clotting. 5. To transport carbon dioxide (in the form of HCO 3- ) from all body cells to the lungs to be excreted. 6. To transport urea (nitrogenous waste product) from all body cells to the kidneys to be excreted in the urine. 7. To distribute heat uniformly through the body, maintaining a constant body temperature (~37 C in humans). Plasma can be forced through blood vessel walls under high pressure, carrying with it food and oxygen from the blood stream. Once it is out of the blood vessel walls, the plasma forms a liquid called tissue fluid, which bathes every cell in the body. A large quantity of plasma in the form of tissue fluid is constantly circulating among body cells supplying food and oxygen and removing waste products. The red blood cells are adapted to perform their function by the following ways: The cytoplasm in red blood cells contains the pigment haemoglobin. Haemoglobin combines with oxygen in the lungs to become oxyhaemoglobin. This is a reversible process. The absence of nucleus (enucleated) means space for more haemoglobin enabling more oxygen to be transported increasing their efficiency. The cells are small (0.0072mm in diameter) and there are many of them, so they have a very large surface area for oxygen absorption. They have a flat, biconcave shape making their surface area for absorption (by diffusion) even larger. They are flexible, allowing them to be pushed easily through the small blood vessels i.e. capillaries whereby they become bell-shaped. The Red Blood Cells / Erythrocytes Function: to carry oxygen around the body in the form of oxyhaemoglobin. Haemoglobin is an iron compound which readily combines with oxygen in places where oxygen concentration is high (air sacs in lungs) to form oxyhaemoglobin. Oxyhaemoglobin is unstable and easily breaks down to release oxygen in places where oxygen concentration is low (respiring body cells). Hb + 4O 2 HbO 8 Haemoglobin (purplish red) High O2 concentration Low O2 concentration Oxyaemoglobin (bright red) The White Blood Cells / Leucocytes These cells are much bigger and less numerous (about in 1mm 3 blood) than red blood cells. Each is colourless, contains a nucleus, can move about and change its shape. White blood cells are also able to move in and out of capillaries. White blood cells can only live for a few days. There are several types of white blood cells, but the two main kinds are: 1. Lymphocytes produced in lymph glands and nodes. 2. Phagocytes produced in bone marrow. 4 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

5 Phagocytes Structure: Multi-lobed nucleus. Made in the bone marrow. Capable of movement and can squeeze out of capillaries (formation of pseudopodia into spaces among body cells). Function: They carry out phagocytosis i.e. engulf and ingest potential harmful bacteria to prevent or to overcome infection. In the process of fighting with the bacteria at the site of wound, some of the phagocytes are killed. These dead cells, together with the dead bacteria, form pus. Lymphocyte bacteria and clump the bacteria together (agglutination) for ingestion by phagocytes. Antibodies are specific to the organism against which they are produced. They may also directly kill the bacteria. Some antibodies are in the form of antitoxins i.e. neutralise poisons (toxins) in the blood which have been released by invading bacteria. Plays an important role in tissue rejection during organ transplant. Organ transplant and tissue rejection Organ or tissue transplant is when a person s damaged or diseased organ or tissue is replaced by a healthy one. The organ or tissue to be transplanted must not be rejected by the recipient s immune system (must be compatible). Any incompatible organ or tissue will be identified as a foreign body by the recipient s immune system. His lymphocytes may respond by producing antibodies to destroy the transplanted organ. Prevention of Tissue Rejection: Before transplantation can take place, the organ or tissue must be checked for compatibility, a procedure known as tissue matching. The tissue of both donor and recipient must be as genetically similar as possible, as in the case of sisters, brothers, parents and close relatives. This problem will not arise if the organ or tissue comes from the recipient s body itself. Use of immunosuppressive drugs to inhibit the activity of the recipient s immune system. Plateletes Structure: A large round nucleus occupying almost the whole cell Made in the lymph nodes Function: They produce antibodies which bind to the antigens (large, organic molecules made of protein/polysaccharides) on the surface of the Structure: Fragments of some cells in the bone marrow. They are about 3µm in diameter. Consists of cytoplasm surrounded by a cell membrane. No nucleus. Lifespan of only ~ 6 days 5 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

6 Function: Platelets play a part in blood clotting, forming a temporary plug at wound or cut. Clotting is important because: o It prevents excessive loss of blood from the body through a wound or cut. o It enables tissue repair to take place under the dried clot. o It prevents the entry of bacteria into the body. Clotting Damaged cells and platelets release an enzyme called thrombokinase into the blood. Thrombokinase acts on prothrombin (a plasma protein) which is converted into thrombin, an activated enzyme. Thrombin acts on another plasma protein, fibrinogen, which is converted into an insoluble stringy protein called fibrin, in the presence of calcium ions and vitamin K. Fibrin forms a mesh which traps blood cells and becomes a clot to prevent the entry of bacteria. The clot dries and hardens to form a scab. 6 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

7 Haemophilia Haemophilia is a genetic disease that is inherited and only affects the males in a family. The females may have the haemophilia allele but will not be affected, acting only as a carrier passing on this allele to their male children. A haemophiliac does not have blood clotting ability (absence of a clotting factor) so a minor wound may result in fatal bleeding. Body s line of defence against pathogens (disease-causing organisms) In general, the skin provides sufficient protection against the entry of bacteria into the body as it has adaptations for this purpose (solid physical barrier separating the internal environment from the external and the slightly acidic nature of sweat. However, if the skin is damaged such as by a wound, blood clotting prevents the bacteria from entering the body. If the first two defences are not sufficient and the pathogen manages to enter the body, white blood cells act as the last line of defence. Phagocytes remove the pathogen via phagocytosis, helped by the antibodies produced by lymphocytes. Once the infection has been cleared, some of the antibodies against this particular pathogen will remain within the circulatory system providing immunity against this pathogen in the future. Summary Main functions of Blood A. Blood as a transport system: o Transport of oxygen from lungs to body tissues in the form of oxyhaemoglobin. o Transport of carbon dioxide from body tissues to lungs. o To transport digested food from the villi to the liver and the body tissues. o To transport nitrogenous waste or urine from the cells to the kidneys for removal. o To transport vitamins and hormones, which are, required by the body for growth and other cell activities. B. Blood as a means to prevent infection: o To protect or defend the body against harmful bacteria and germs by phagocytosis. o To protect the body against harmful toxins discharged by bacteria by the production of antitoxins and antibodies by leucocytes. o To prevent the body from losing too much blood through a wound. o To prevent the entry of bacteria and other harmful substances into the body. D. Maintaining a constant body temperature: o To maintain a constant body temperature (about 37 0 C in humans). o To distribute heat uniformly throughout the body. Summary Transport function of blood SUBSTANCES Oxygen Carbon dioxide (as hydrogen carbonate ions) Urea (Nitrogenous waste) TRANSPORTED BY Red blood cells Plasma lungs FROM All body cells TO All body cells Lungs Plasma liver Kidneys Digested food Plasma intestine Body cells Hormones Heat Blood Vessels Plasma Plasma Endocrine glands Liver and muscles There are three types of blood vessels: 1. Arteries 2. Veins 3. Capillaries Target cells All body cells (excess to skin) *The flow of blood: Arteries Arterioles Capillaries Venules Veins Arteries carry blood away from the heart. Arteries will form smaller branches called arterioles. Capillaries are made up of microscopic vessels linking an arteriole and a venule. Capillaries take nutrients, oxygen and other useful substances to the cells and remove waste products produced in these cells. Veins carry blood towards the heart. Veins will form smaller branches called venules. The following are simplified diagrams showing a capillary network: C. Formation of clots: 7 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

8 The table below shows the differences between an artery, a vein and a capillary: Feature Arteries Veins Capillaries Function Wall Lumen size Valves Blood flow To transport oxygenated blood away from the heart except pulmonary artery Thick muscular walls (impermeable) Smaller than veins but bigger than capillaries Absent In pulses (rapid flow, in high pressure). Flow is maintained by the contraction of heart muscle. To transport deoxygenated blood towards the heart except pulmonary vein Thin muscular walls (impermeable) Largest Present (Semilunar valves: to prevent backflow of blood as blood is flowing at low pressure) No pulse (slow flow but faster than in capillaries). Blood flow is maintained by contraction of skeletal muscles, which squeeze the veins between them. To transport blood from arteries to veins *To allow exchange of substances (nutrients, waste products, gases) between blood and tissue fluid (fluid bathing cells) One cell thick walls (endothelium); Fenestrated (have gaps between the endothelial cells of the walls) Smallest Just enough for the diameter of one red blood cell Red blood cells travel in a single file Absent Pulse gradually disappears (slow flow). Flow is along the gradient of blood pressure. 8 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

9 The diagram below shows the major blood vessels in humans: ARTERIES Transport blood away from the heart to body tissues. Have strong thick muscular elastic walls. Transport oxygenated blood except the pulmonary artery. Valves are not present. Blood is generally under higher pressure. Lumen is small. Usually is deep seated except the artery in the neck. VEINS Transport blood from body tissues towards the heart. Have thin muscular walls. Transport deoxygenated blood except the pulmonary vein. Valves are present to prevent the back flow of blood. Blood is generally under lower pressure. Lumen is large. Usually superficial (found close to the surface of the skin) How valves work to prevent the backflow of blood Transfer of materials between capillaries, tissue fluids and body cells Differences between arteries and veins: Capillaries are adapted to allow the exchange of materials between the blood and the cells. Adaptations of capillaries include: 1. Capillaries have very thin, permeable walls, only one-cell thick to ensure rapid diffusion of materials. 2. Capillaries are abundant which provides a huge surface area for: Nutrients and oxygen to diffuse from blood to tissues. Waste materials and carbon dioxide diffuse from tissues into blood. 3. Blood flows through them slowly. 4. Body cells are never far from a capillary. The capillary walls act as a filter. Substances such as water with dissolved oxygen, glucose, fatty acids, glycerol, amino acids, vitamins, minerals and hormones are very tiny and so are able to pass 9 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

10 through the capillary walls and enter the tissue fluid. Larger ones such as red blood cells, most of the white blood cells and large protein molecules are held back. Tissue fluid Body cells are bathed in tissue or intercellular fluid. This fluid is actually dilute plasma (usually without plasma proteins). The fluid is formed continuously as plasma filters out between the cells in the capillary walls into the intercellular spaces. This occurs at the arteriole end of the capillary network due to the high pressure here. Tissue fluid seeps back into capillaries at the venule end of the capillary network. Some tissue fluid drains into the lymph vessels. This in turn, empty into the blood vessels. Blood capillary 10 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

11 EXTENSION MATERIAL What is lymph? A colourless, alkaline fluid similar in composition to blood except it contains less protein and lacks red blood cells. It contains fats absorbed by the lacteals of the small intestines. How is lymph formed? Not all tissue fluid returns to the blood capillaries. One-tenth enters a separate system of capillaries called lymph capillaries. Lymph capillaries are part of the lymph system. What is the difference between tissue fluid and lymph? They have similar composition but tissue fluid surrounds the tissues. Lymph is found in the lymph system. What are functions of the lymph? Transport digested fat into the circulatory system. Oxygen from red blood cells diffuses into lymph and is conveyed to the respiring cells. Carbon dioxide and waste substances from the cells are transported by the lymph to the blood stream. Most of the hormones reach the blood system via the lymph. The lymph nodes filter out the dust particles and bacteria thus preventing their entry into the bloodstream. What are lymph nodes? Lymph nodes are swellings found along the lymph vessel. There are large lymph nodes in the groin, under the arms and in the neck. Lymph nodes often swell up if you have an infection. Functions: 1. Remove particles of debris. 2. Manufacture of lymphocyte. 3. Ingestion of bacteria. (Large phagocytes are attached to the walls of these nodes) What are lymphatic ducts? There are two lymphatic ducts: The thoracic duct collects lymph from the left side of the body and empties it into the left subclavian vein. The right lymphatic duct collects lymph from the right side of the body and empties it into the right subclavian vein. 11 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

12 The Heart 12 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

13 It is a muscular pump, made up of special muscle known as cardiac muscle, which has the ability to contract rhythmically even without stimulation (myogenic). It never tires or suffers from cramp and beats about 70 times per minute in the average adult at rest. It has four chambers, all with similar volumes when full: The top two chambers are the atria (s - atrium) and the bottom two are the ventricles (s - ventricle) Feature Wall Blood flow Prevention of blood flow On contraction blood is pumped Atria (also known as auricles) Thin walls Less force is required for contraction to transport blood in a short distance Right atrium receives blood from the superior and inferior vena cavae. Left atrium receives blood from the pulmonary veins. By closing valves in the vena cava and the pulmonary veins. Blood flows from atria to ventricles. Ventricles Thick walls particularly left ventricle as blood is travelling at long distances. This requires a very strong contraction force The right ventricles receive blood from the right atrium. The left ventricle receives blood from the left atrium. By closing of tricuspid calve on the right side of the heart and bicuspid valve on the left side of the heart. Blood flows into the pulmonary artery and aorta The heart is protected by the vertebral column at the back, sternum and the rib cage in front. The heart lies in a fluid filled cavity called the pericardial cavity. This acts as a lubricant. The right and left side of the heart is separated by septum (separates the oxygenated and deoxygenated blood) Heart muscles receive their oxygen and nutrient supply from two coronary arteries, which branch from the aorta. A system of valves ensures one-way flow of blood through the heart. Between the right atrium and right ventricle is the tricuspid valve (three pockets / flaps). Between the left atrium and left ventricle is the bicuspid or mitral valve (two pockets / flaps). Between the arteries leading away from the heart and the ventricles are the semi-lunar valves (three pockets / flaps) Note: The valves between the atria and the ventricles are also known in general as atrioventricular valves. The main blood vessels of the heart and its function: MAIN BLOOD VESSELS Superior vena cava Inferior vena cava Pulmonary artery Pulmonary vein Aorta Renal artery Renal vein Hepatic artery Hepatic vein Hepatic portal vein FUNCTION Transports deoxygenated blood from the upper parts of the body to the right atrium. Transports deoxygenated blood from the lower parts of the body to the right atrium. Transports deoxygenated blood from the lungs to the left atrium. Transports oxygenated blood from lungs to the left atrium Transports oxygenated blood from the left ventricle to all parts of the body. Transports oxygenated blood from the heart to the kidneys. Transports deoxygenated blood from the kidney to the heart. Transports oxygenated blood from the heart to the liver. Transports deoxygenated blood from the liver to the heart. Transports blood rich in digested food from villi of small intestine to the liver. 13 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

14 The Cardiac Cycle The rhythmic contraction and relaxation of the heart to produce one heartbeat ( lub-dub sound). Systole: cardiac muscles contraction. Diastole: cardiac muscles relaxation. There are three main stages in a single cardiac cycle: 1. Atrial systole Both atria contract (ventricles relax). Blood in atria pushed into the ventricles. Blood at high pressure pushing the bicuspid and tricuspid valves open. 3. Diastole (pause between lub-dub sound) Both atria and ventricles relax. Blood in aorta and pulmonary artery under high pressure causing the semi-lunar valves to close producing the dub sound (prevents blood from flowing back into the ventricles). Blood from the veins (vena cava and pulmonary veins) flow into atria. The whole cycle is then repeated 2. Ventricular systole Both ventricles contract (atria relax). Blood in ventricles pushed into the arteries (pulmonary artery and aorta). Blood at high pressure pushing the semilunar valves open. Simultaneously, bicuspid and tricuspid valves close (to prevent backflow into atria) to produce lub sound. A graph showing the changes in blood pressure in one cardiac cycle. QUESTION 1: MATCH THE LETTERS ON THE GRAPH WITH THE FOLLOWING EVENTS: Semi lunar valves open: Semi lunar valves closes: Atrioventricular valves open: Atrioventricular valves closes: QUESTION 2: HOW MANY HEARTBEATS WOULD THERE BE IN 1 MINUTE? 14 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

15 Blood Pressure and Pulse Blood pressure is the force exerted by blood on the walls of the arteries as a result of the contraction and relaxation of the heart. This succession surge or wave of pressure can be felt as a pulse. The pulse beat in an artery can be located by gently pressing on the wrist at the base of the thumb using your middle and index fingers (radial pulse) or at the neck (carotid pulse). The pulse rate is the same as the number of heartbeats per minute. There is no such surge in the veins where blood flows smoothly under much lower pressure. Veins, however, have semi-lunar valves which ensure that blood continues to flow towards the heart. Otherwise, veins rely on the movement of the nearby skeletal muscles to massage the blood from one set of semi-lunar valves to the next. The heart muscles will not get enough nutrients and oxygen. This will lead to their degeneration, resulting in a severe heart pain a heart attack. Other than high animal fats diet, drinking alcohol also increases atheroma formation. Coronary Heart disease Diet is important for a healthy heart but like all muscles, the heart benefits from exercise. Fat deposits (atherosclerosis) and blood clots (thrombosis) on the inner walls of the coronary arteries can partially block their lumen, obstructing blood flow to the heart muscles. 15 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

16 The heart is also affected by cigarette smoking. Carbon monoxide in cigarette smoke encourages the build-up of atheroma. Nicotine triggers the release of adrenaline, which increases rate of heartbeat and constricts the blood vessels. Nicotine also increases the tendency for blood to clot. If these (atheroma, thrombosis) happen in the coronary arteries causing an occlusion of these arteries, they may therefore not supply enough blood (oxygen and nutrients) to the heart muscle causing heart attack (myocardial infarction severe damage or death of heart muscles) People who lead stressful lives are also at risk of heart disease. Stress increases the release of adrenaline, which constricts artery walls. In this way, stress adds to existing problems of partially blocked arteries. One of the early symptoms of a heart disease is angina (pain in the centre of the chest during exercise and disappears at rest). To decrease the risk of heart disease: Restrict the intake of animal fats and cholesterol. Avoid obesity (control weight). Do not smoke and do not comsume alcoholic drinks. Settle for a less stressful lifestyle. Take regular exercise. 16 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S