TRANSPORT IN ANIMALS Summarized Notes

LEARNING OBJECTIVES

• describe the heart in terms of its gross structure and the functions of the parts
• outline how the structure ot arteries, veins and capillaries are adapted for their functions
• describe the double circulatory system of a mammal
• describe the likely causes of a heart attack Iimited to diet, smoking and stress, age and gender
• state ways of treating coronary heart disease limited to drug treatment with aspirin and surgery (stents, angioplasty and bypass)
• state that the lymphatic system consists of lymphatic vessels and lymphatic nodes
• describe the functions of the lymphatic system limited to the protection of the body from infection circulation of body fluids
• state the functions of the components of blood limited to red blood cells, white blood cells, platelets and plasma
• state the functions of lymphocytes and phagocytes as seen under the light microscope, in diagrams and images
• describe the process of clotting limited to fibrinogen only state the role of blood clotting
• describe the transfer of nutrients between capillaries and tissue fluid (details of roles of water potential and hydrostatic pressure are not required)
• define pathogen asa disease-causing organism
• define active immunity as defence against a pathogen by antibody production in the body
• state the body's defence mechanisms limited to mechanical barriers, chemical barriers, antibody production and phagocytosis (white blood cells)
• explain the role of vaccination in controlling the spread of disease
• describe the events leading to passIve and active immunity
• state that memory cells are not produced in passive immunity
• explain the importance of passive immunity for breast-fed intants
• explain how passive immunity is a short-term defence against a pathogen.

In our previous post we learnt about the double transport system in plants,  In the human body we have a double circulatory system, too. There the similarity ends! In humans, blood is pumped around the body by the heart. The heart pumps blood first to the lungs, then back to the heart before pumping blood all around the body. This is why it is described as a double circulation. Why is there a need for  transport in large organisms? Large organisms have complex systems far removed from where food is absorbed, and where oxygen is available. Toxic waste products that are produced in cells must be removed from the cells, and must be excreted ata concentrated volume, from an excreting organ. A transport system is needed to take (transport) glucose and oxygen to individual cells, and also to remove toxic substances, for example, carbon dioxide and nitrogen, from cells. In this section, you will learn about how substances or materials can be transported From one part of the body to another part. You will learn about your heart and blood vessels, and how important they are in transporting nutrients and waste products, for example, oxygen and glucose, in your body.

The Heart

The heart is part of the body's circulatory system. The human heart is about the size of a clenched fist. The heart lies between the two lungs. It is encapsulated by a double layer of tough inelastic membrane, which forms the pericardium. A fluid is secreted between the membranes allowing them to move easily over each other. The pericardium protects the heart from overexpansion caused by elastic recoil when it is beating very fast. The walls of the heart consist mainly of a special type of muscle called cardiac musde. The cardiac muscle is found only in the heart and contracts and relaxes rhythmically, without tiring, throughout your life. However, the cardiac muscles cannot be without glucose or oxygen; it dies as soon as its supply of blood is cut off. The heart is made up of two halves separated by a septum. Each side is divided into two Chambers, an upper chamber called the atrium and a lower chamber called the ventricle.

This means that there are four chambers in the heart:
• the right atrium and left atrium these are thin-walled and receive blood from the body and lungs through veins
• the right ventricle and left ventricle - these have thick, muscular walls and pump blood out of the heart to the lungs and the rest of the body through arteries.
• The septum separates the deoxygenated blood on the right side of the heart from the oxygenated blood on the left side of the heart.
• Deoxygenated blood comes from the body through the vena cava interior and from the head and arms through the vena cava superior
• It enters the right atrium and is passed through the tricuspid valve into the right ventride
• The right ventricle contracts and pumps blood through the pulmonary artery into the lungs.
• Oxygenated blood returns through the pulmonary veins into the left atrium, and then through the bicuspid valve into the left ventricle.
• The left ventricle has a very thick, muscular wall, which enables it to Contract strongly and exert sufficient pressure to pump blood into the aorta and throughout the body.
• The left ventricle pumps oxygenated blood into the aorta.
• The aorta transports oxygenated blood from the left ventricle to the head and body.
• The right ventricle pumps deoxygenated blood into the pulmonary artery and pushes the blood to the lungs.
• The pulmonary artery transports deoxygenated blood from the right ventricle to the lungs.

*Study Figures Below 

Four Chambers of The Heart

Arteries and Veins

A section through a heart

The Heart Valves

During your dissection in class if you did, you may have seen the tiny flaps of skin between the atria and ventricles. These are atrioventricular valves. Their job is to stop blood flowing from the ventricles back to the atria. This is important so that when the ventricles contract, the blood is pushed into the arteries and not back into the atria. As the ventricles contract, the pressure of the blood pushes the valves upwards. The tendons attached to them stop them from going up too far.
• The two-part bicuspid valve is between the left atrium and left ventricle.
• The three-part tricuspid valve is between the right atrium and right ventricle.

Heartbeat

The heart is made up of cardiac muscles that contract and relax about 70 times every minute. One complete sequence of contraction and relaxation is called the cardiac cycle. The cardiac cycle can be divided into three stages:
• general diastole, when the muscles of the atria and ventricles relax.
• atrial systole, when the muscles of the atria contract.
• ventricular systole, when the muscles of the ventricles contract.

As you can see in Figure a)

, the muscles of the atria and ventricles relax. The blood flows into the atria from the veins. In Figure  b)

, the muscles in the atrial wall contract and the blood is pushed through the valves into the ventricles. The bicuspid valves and tricuspid valves are closed to prevent the blood from flowing back into the atria. In Fiqure  c)

the muscles in the ventricular walls contract and the blood is pushed into the arteries. The semilunar valves at the base of the arteries are closed.to prevent blood in the arteries from flowing back into the heart.

 The Effect Of Exercise On Heartbeat

The rate of the heartbeat changes acCording to the needs of the body. For example, during exercise, when the muscles need extra oxygen, the brain sends messages along nerves to a patch of specialised cells in the right atrium, called the pacemaker, to make the heart beat faster. Your pulse rate is a measure of your rate of heartbeat. Every time your heart beats, the blood pumped into your arteries causes them to stretch and then Snap back. This sets up a pressure wave that travels along the arteries. l You can feel this pressure wave as your pulse, at points where an artery is near the skin.

Exercise requires a good supply of oxygenated blood to sustain active muscles. This is achieved by increasing the cardiac output and by pumping proportionally more of the body's blood to the muscles. Even the anticipation of exercise increases the stroke volume and heart rate. At full speed, a sprinter's heart may beat more than 200 times per minute. The heart rate remains high for a while after exercise. During exercise, the skeletal muscle receives proportionally more of the body's blood. By dilating blood vessels in one part of the body and constricting those in other parts, blood is diverted to active muscles from the intestine.

Regular exercise is important in the sense that the yentilation becomes more efficient, respiratory muscles become stronger, the blood supply to the lungs is increased and the ability to take in oxygen trom the alveoli is improved, and the blood volume and total number of red blood cells are increased. The heart becomes enlarged and the resting pulse is lowered, indicating a more efficient oxygen transport system. Regular exercise also improves the ability of cells to generate and use energy because the concentration of respiratory enzymes and number of mitochondria are increased.

 Figure shows three kinds of blood vessels:

• arteries, which transport blood away from the heart
• veins, which transport blood to the heart
• capillaries, which connect the arterial an venous systems and which are the site of exchange between the blood and the tissues.

After they leave the heart, arteries branch into narrower and narrower vessels and eventually into very fine tubes called arterioles. These lead into capillaries, which jOin up with venules. Venules join up with other venules, eventually forming veins, which return blood to the heart.

Arteries

Blond travelling in arteries floWs in spurts called pulses. The blood is forced out of the ventricles of the heart and is under high pressure. Each spurt represents the ventricle contracting and relaxing. The arteries have strong elastic walls to withstand this high oressure, so that the blood flows more smoothly.

 Veins

Blood travelling in veins flows more slowly and smoothly. The blood is returned to the atria of the heart and is ata much lower pressure than in the arteries. The veins

 have thinner elastic walls than the arteries and the space inside the vein, called the lumen, is wider than in the artery to ensure the blood can flow easily. The blood is kept moving by the contraction of the muscles that surround the veins. For example, your leg muscles squeeze the large veins in your legs when you walk. This helps to push the blood back up to your heart. Veins also have valves that allow the blood to flow in one direction and prevent the blood from flowing backwards. Figure 33

shows the action of the valves in the veins. With the exception of the aorta and pulmonary arteries, valves are not needed in arteries because the pumping action of the heart provides sufficient pressure to keep the blood flowing in one direction.

 Capillaries

Capillaries are very small blood vessels that reach every part of the body. The walle the capillaries consist of a single layer of cells so that oxygen, food substances and bther materials can diffuse out of the capillary to the cells. Waste substances, such as carh dioxide, diffuse into the blood. Figures below show the structure of a capillary.

Capillaries are adapted  to allow the effective exchange of substances between the blood and the tissues of the body.

By the time blood reaches the capillary beds from an artery, it is at high pressure and this forces blood plasma out. Ihe plasma leaves the capillary and becomes tissue fluid. As the blood plasma moves through the capillary bed towards the vein, pressure drops and stops plasma being squeezed out. Tissue fluid acts as a bridge in the diffusion of chemicals between the capillaries and the cells of the tissue. Oxygen and glucose diffuse from the blood capillary into the tissue fuid and then into the cells. Carbon dioxide and urea diffuse from the cells into the tissue fluid and then into the blood through venules.

 The Double Circulatory System

Mammals, including humans, have a double circulatory system. The double circulatory system means that the blood flows through the heart twice in order to make one complete circuit around the body:
• The pulmonary circulation transports blood between the heart and the lungs.
• The systemic circulation carries blood between the heart and all other parts of the body.

Double circulation ensures simultaneously high-pressure delivery of oxygenated blood to all regions of the body and that oxygenated blood reaches the respiring tissues. Figures below show how blood passes through the heart twice in order to pump blood around the body once. The arrows show the direction of blood flow.

If you follow the direction of the arrows in Figure 38, you will notice that blood passes through the heart twice to reach the starting point:
• through the right Side to the lungs and back to the left side of the heart, to the rest of the body and back to the right side of the heart again.

The blood flows from the lungS into the lert-hand side of the heart, and then out to the rest of the body. Follow the flow of blood round to the right-hand side of the heart then goes back to the lung again. In the double circulatory system, the blood travels through the heart twice in one complete journey around the body. The system consists of:
• pulmonary circulation, from the heart to the lungs and back to the heart.
• Systemic circulation, from the heart to the rest of the body and back to the heart.

BIood moves trom high-pressure areas to low-pressure areas down pressure gradients. The pressure gradients are produced in three main ways:
• by the pumping action of the heart
• contractions of skeletal muscles squeeze blood along veins
• Inspiratory movement of the thorax reduces the pressure inside the thoracic cavity, which helps to draw blood back to the heart.

Oxygenated And Deoxygenated Blood

The blood in the left side of the heart is oxygenated blood that has come from the lungs. The oxygen in the blood was taken up by the capillaries surrounding the alveoli of the lungs. The alveoli are tiny air sacs in the lungs where oxygen and carbon dioxide pass between the air and the bloodstream. The heart then pumps the oxygenated blood to the rest of the body. Some of the oxygen is used by the body cells for respiration. As the oxygen is used up, the blood becomes deoxygenated and is transported to the right side of the heart. The deoxygenated blood is then pumped to the lungs, where it becomes oxygenated once again.

The Coronary Arteries

The cardiac muscle of the heart has its own blood supply, which is provided by the coronary arteries. You can see these coronary arteries on the surface of the cardiac muscle of the heart in Figures below.

The coronary arteries maintain a constant supply of oxygen and glucose to the heart muscles. If the coronary muscle becomes blocked by fatty deposits or a blood clot, nutrients and oxygen cannot reach the heart muscles and the heart may stop beating. The person then suffers a heart attack.

LiKely Causes Of Heart Attack And Preventative Measures
Diseases of the heart and arteries account for many deaths in southern Africa and elsewhere, The most common arterial disease is hardening of the arteries, known as atherosclerosis. It happens especially in the coronary arteries, when a fatty layer containing cholesterol builds
up in the lining of the arteries. This causes the arteries to narrow and slow down the blood flow. It can eventually lead to one or more of the arteries becoming blocked. When the blocked arteries are the coronary arteries, the heart muscles do not get enough oxygen and glucose and they cannot function. This can result in coronary heart disease and lead to a heart attack. There is probably no single cause of coronary heart disease, but several risk factors have been identified:
• a genetic predisposition
• a high-cholesterol diet
• stress
• smoking cigarettes
• not exercising regularly
• being overweight
While people cannot change their genetic make-up, they can reduce the risk by changing their lifestyle:
• by eating a low-cholesterol diet that avoids foods rich in animal fats, for example, red meat, butter and eggs
• by not being overweight
• by not smoking
• by exercising regularly.

 Ways Of Treating Coronary Heart Disease

Apart from following a healthy lifestyle, various drugs can be used to treat coronary artery disease, as well as procedures to restore and improve blood flow.

• Aspirin: Taking a daily aspirin or other blood thinner can reduce the tendency of the blood to clot, which may help prevent obstruction of the coronary arteries. If a person has had a heart attack, aspirin can help prevent future attacks.

• Angioplasty and stent placement: The doctor inserts a long, thin tube (catheter) into the narrowed part of the artery. A wire with a deflated balloon is passed through the catheter to the narrowed area. The balloon is then inflated, compressing the deposits against the artery walls. A stent is often left in the artery to help keep the artery open. Some stents slowly release medication to help keep the artery open.

• Coronary artery bypass surgery: A surgeon creates a graft to bypass blocked coronary arteries using a vessel from another part of the body. This allows blood to flowaround the blocked or narrowed coronary artery. Because this requires open-heart surgery, it's most often reserved for cases of multiple narrowed coronary arteries.

The Lymphatic System

The main functions of the lymphatic system are:
- to collect and return tissue fluid to the circulation
- to produce lymphocytes and antibodies to defend the body against disease. Tissue fluid is formed when blood plasma leaks out of the blood capillaries through the very thin walls of the capillaries, as shown in Figure below.

Some the tissue fluid drains into lymphatic capillaries, which returns it to the blood
circulation indirectly as lymph. The lymphatic capillaries form part of the lymphatic system, as shown in Figure 42. The lymph flows through several lymph nodes that contain large numbers of lymphocytes. These destroy bacteria and toxins and protect the body against disease. The lymphatic vessels eventually join to form two large lymph ducts. You can see in Figure below that these empty the lymph into the two main veins at the side of the neck near the heart.

The lymphatic systerm has no pump to make the lymph flow. Lymph vessels do have valves in them, to make sure that movement is only in one direction. The lymphatic system is responsible for the constant circulation of fluid around all living cells and for the production of lymphocytes and their activities in defence against disease.

# Blood

Blood is made up of different types of cells floating in a liquid called plasma.

You can see many red circles in Figure above. These are the red blood cells. They are described as biconcave discs and do not have a nucleus. You can also see larger circles with dark shapes inside. These are white blood cells. The dark shape is the nucleus. You can also see in Figure 44 that there are many more red blood cells than white blood cells. The main functions of blood are:
• transport
• defence against disease

Red Blood Cells

Figure above shows a single red blood cell. You can see its biconcave shape and that it does not have a nucleus.

The main function of red blood cells is to transport oxygen from the lungs to the tissues. Red blood cells contain special molecules known as haemoglobin, which carries oxygen. Haemoglobin is a protein and contains iron. Haemoglobin combines with oxygen to form oxyhaemoglobin when the blood flows through the lungs. Oxyhaemoglobin releases its oxygen when the blood flows through the tissues. This provides oxygen to the tissues, which it needs for the process of respiration.
• Blood that is rich in oxyhaemoglobin is called oxygenated blood.
• Blood with little oxyhaemoglobin is called deoxygenated blood.

Adaptations of a red blood cell that enables it to transport oxygen efficiently include:
• not having a nucleus; this means there is more space inside for haemoglobin so the cell can carry more oxygen
• the biconcave shape that qives a large surface area for diffusion
• the thin cell membrane, which means that oxygen and carbon dioxide can diffuse in
and out rapidly .

 White Blood Cells

White blood cells defend the body against disease, unlike red blood cells, white blood cells are able to squeeze out of blood capillaries. They move through the tissues to the site of an infection.
Two important types of white blood cells are:
• neutrophils, which have a lobed nucleus
• lymphocytes, which have a large nucleus that takes up most of the cell.
The neutrophils and lymphocytes protect the body against diseases
*They form part of the immune system of the body, which we will discuss in the next sectio n*

# Platelets
Platelets are small pieces of cells that float in the blood plasma.
They play an essential role in blood clotting. The blood clots when a blood vessel is damaged. Clotting has two important functions or roles:
• It helps to stop blood moving out of the damaged vessel,
• It prevents disease-causing microorganisms described as pathogens, mainly viruses and bacteria, from getting into the bo dy.

 How Blood Clotting Happens

The platelets that are in the blood plasma collect at the site where damage to the blood vessel has occurred. They release an enzyme that sets off a chain of chemical reactions in the blood. These reactions change fibrinogen, a soluble plasma protein, into a network of insoluble fibrin threads. You can see in Figure below that this network of fibrin threads traps red blood cells. A clot is formed, which eventually turns into a hard s cab.

 Plasma

About 55% of blood is a pale yellow liquid called plasma. It contains 90% water. The other 10% is dissolved and suspended substances. One of the main functions of plasma is the transport of nutrients, such as amino acids and glucose, metabolic wastes, Such as urea and carbon dioxide, hormones, blood proteins, antibodies and body h eat.

 Defence Against Diseases

Pathogens,  which are disease-causing organisms Such as bacteria and viruses, can enter the body through a cut in the skin. The white blood cells protect the body against diseases. Two types of white blood cells from part of the immune system: neutrophils and lympho cytes.

 Neutrophils

• Squeeze out of blood capillaries into the tissues
• move to the site of an infection, such as a cut on the skin
• destroy the harmful bacteria and viruses by eating and digesting them in a process known as phagoc ytosis.

 Lymphocytes:

• produce antibodies that attack and kill the disease-cauSing organisms, in response to a ntigens.

 The Immune Response

The body is under constant attack from disease-causing microorganisms, such as harmful bacteria and viruses. If your skin is cut and damaged, these pathogens can get into your tissues. The white blood cells recognise the pathogens as foreign (*strange and unfamiliar to the body*) invaders and attack them. The body's production of disease-fighting cells and antibodies is called an immune response. Phagocytosis is a general immune response to any invading pathogen, while antibody production is a specific Immune response to a specific pathogen.

The body has effective defence mechanisms:
• The hairs and mucCus in our nose trap inhaled particles, and the walls of our respiratory tract are lined with cells that secrete mucus to trap particles and pathogens.
• The cells lining the respiratory tract have cilia (hair-like projections) that beat in a Coordinated way to sweep mucus and entrapped particles up to the pharynx, where it can be swallowed.
• The skin has a tough outer layer of cells, which is constantly renewed from below; this serves as a mechanical barrier to infection.
• The hydrochloric acid in the stomach and enzymes in the intestine destroy pathogens. The gastrointestinal tract also has cells that secrete mucus, which acts as a chemical barrier
• The white blood cells recognise and engulf pathogens or produce antibodies against them.

How do your white blood cells recognise invading pathogens, such as bacteria and viruses. Pathogens have surface molecules, usually proteins or polysaccharides, which are different from your own cell-surface molecules, These 'non-self' molecules are called antigens and are simply molecules on the surface of the invading pathogens. Your white blood cells recognise and respond to antigens. Neutrophils are specialised cells that destroy foreign bodies by recognising, engulfing and digesting them. They destroy pathogens both at the site of the intection, and in the ymph nodes.

Figure above shows the process of phagocytosis:
•Receptor proteins on the neutrophil cell membrane bind to the surface of the pathogen
• The neutrophil membrane then flows around and encloses and engulfs the pathogen in a vacuole.
• The vacuole gets pinched off inside the neutrophil.
• Enzymes inside the neutrophil move towards and fuse with the vacuole.
• Enzymes digest the ingested particle.

Antibodies are globular proteins called immunoglobulins carried in the blood. Antibodies
combine with antigens to form an antigen-antibody complex, which makes the invading pathogen harmless.

Lymphocytes produce antibodies in the lymph nodes in response to antigens. Antigens on
the surface of a pathogen stimulate lymphocytes to produce antibodies specific to that antigen
• One type of lymphocyte releases these antibodies into the bloodstream
• A second type of lymphocyte forms antibodies on its surface.

The antibody response is specific. If, for example
you have been exposed to the virus that causes
hicken-pox, your body produces antibodies that attack the chicken-pox virus and no other virus
Figure below summarises the production and action of antibodies.

You can see in Figure above the antigen attaching
itself to the lymphocyte. The lymphocyte then
produces antibodies in response. The antibodies each have a Y-shaped molecule. Each arm of the Y has a receptor site. The shape of the receptor site is specific for a particular antigen. The antibody Combines with the antigen and forms an antige- antibody complex, inactivating the  antigen.

 Immunity

Many years after you have had an infection, lymphocytes still continue to produce small amounts of antibodies that circulate in the bloodstream. This gives you protection against the disease caused by that pathogen. If you are exposed to that specific pathogen again, memory lymphocytes 'remember the earlier exposure and trigger off an immune response that is much quicker than the first time. The pathogens are killed before they can multiply and cause the disease. If, for example, you have had chicken- pox, you are unlikely to get it again. You are immune to it. You can develop immunity in two ways
• active immunity
• passive immunity.

You can acquire active and passive immunity either naturally or artificially. Active immunity gves you long-term protection against a disease. In active immunity, your body produces its own antibodies.You may remember, as a child or young adult, being immunised or vaccinated against several diseases, such as tuberculosis, measles, polio and meningitis. When you are immunised, a vaccine is either given to you orally or injected into you. The immunisation against tuberculosis is called the BCG. Vaccines can be one of the following preparations:
• a live but weakened form of a pathogen that triggers ott an immune response Without causing symptoms, for example, polio and measles vaccines.
• killed pathogens that are still able to trigger off an immune response, for example, the vaccine against whooping cough .
• poisonous toxins (made harmless by heat or chemicals) produced by a pathogen, for example, the vaccines for diphtheria and tetanus.

 Active immunity may be acquired naturally or artificially:

• Natural active immunity: You may have been exposed to a disease as a child, such as chicken-pox or measles; your body produces its own antibodies to kill the pathogens; this type of immunity is long-lasting
• Artificial active immunity: You may have been immunised against a disease, such as polio, meningitis or tuberculosis, when you are immunised, a vaccine is injected into you; your body produces its own antibodies in response to the vaccine; the mechanism of antibody production in response to vaccination is the same as the response to natural intection. The  immunity that is developed is long-lasting.

Passive immunity gives you temporary protection against a disease. In passive immunity, you are given ready-made antibodies from some other person or animal. The borrowed antibodies eventually disappear, so you have short-term protection against a disease. The body does not have the memory cells to produce more antibodies.

Passive immunity may be acquired naturally or artificially:
• Natural passive immunity: This is acquired by babies from their mothers. It comes from antibodies that pass through the placenta during pregnancy and from mother's milk after birth. This gives babies temporary immunity until their own immune systems are sutficiently developed to produce their own antibodies.
• Artificial passive immunity: This is through injection of antibodies that have been made by one mammal into another mammal. For example, horses are injected with small doses of snake venom. The antibodies produced by horses in response to the snake venom are extracted. A snake-bite victim injected with these antibodies has temporary immunity to the snake venom. This gives the victim time for his or her own immune system to respond to the venom.

Transplant Rejection

It is not only disease-causing bacteria and viruses that cause an immune response. We sometimes read in the newspaper of a transplant operation when a person receives a new kidney or heart from a donor, perhaps from someone who has been killed in a car accident. Sometimes a parent donates a kidney to his or her child. The tissues of the donor and recipient are carefully matched before the operation, but these transplanted organs are recognised by the lymphocytes as 'foreign' in the same way as pathogens are recognised as 'foreign'. The body tries to reject the transplant. To overcome this problem of organ rejection, people who receive transplants are given drugs called immunosuppressants to kill their own lymphocytes. The body cannot produce antibodies and cannot reject the new organ. This means that the patient is now unable to fight infection and needs more drugs to prevent him or her from get ting ill.

SUMMARY

• The heart forms part of the circulatory system
• The heart is made up of special muscles that do not get tired easily, and are supplied with nutrients by the coronary blood vessels.
• The heart is divided into four chambers. The ventricles pump blood out of the heart, while the atria receive the blood.
• The heartbeat is affected by any change in the amount of carbon dioxide in the blood. The heartbeat can speed up or slow down.
• Blockage of the coronary blood vessels can give rise to heart attacks.
• Coronary heart disease can be treated using aspirin, angioplasty, stent placement and coronary artery bypass surgery.
• The blood vessels (veins, arteries and capillaries) transport the blood around the body. The blood never leaves the blood vessels.
• Circulation refers to the movement of the blood around the body. If you follow the pathway of the blood, you will see that the blood has to go through the heart twice before completing the journey around the system. This is described as a double circulatory system.
• The main blood vessels in your body are: aorta pulmonary arteries and veins; vena cava; renal artery and vein; hepatic artery and vein; and hepatic portal vein.
• Blood is composed of two parts: the liquid part called blood plasma; and the blood cells.
• The red blood cells transport oxygen; the white blood cells form part of the immune system of the body;, and the platelets help the blood to clot.
• During the process of blood clotting, fibrinogen is changed into insoluble fibrin.
• The blood plasma may leave the blood vessels and give rise to tissue fluid
• The tissue fluid surrounds the cells and supplies the living cells with food substances.
• The immune system protects the body.
• Lymphocytes produce antibodies; neutrophils engulf disease-causing organisms by phagocytosis.
• There are two types of immunity: active and passive. Each can be acquired naturally. Active immunity is acquired when the body produces its own antibodies and it is usually long-lasting. Passive immunity is acquired from ready-nmade antibodies and is usually short-lived.
• The body can produce antibodies against transplanted organs that cause rejection of the organ.
• The tissue fluid is removed by lymphatic vessels from the intercellular spaces. Once the tissue fluid enters the lymph vessels, it is called lymph and not tissue fluid.
• The lymph vessels remove excess tissue fluid from the intercellular spaces and drain the fluid back in the bloodstream.

THE END SEE YOU IN THE NEXT POST, POSTED BY MRS YANG TIDAK.