Transport in Humans

Transport in humans In large organisms, diffusion is too slow for the efficient exchange of materials needed for survival. Transport system in mammals consists of the blood system and the lymphatic system.

Structure and composition of blood Blood is a fluid tissue made up of 55% plasma and 45% blood cells and platelets.

Plasma Contains dissolved substances including some protein, mineral salts, food substances, hormones (eg insulin) and excretory products (eg urea).

Red blood cells Contains the pigment haemoglobin which is an iron-containing compound. Haemoglobin combines reversibly with oxygen, allowing transport of oxygen from the lungs to other parts of the body. Does not have a nucleus and has a biconcave shape which increases surface area to volume ratio, allowing absorption and release of oxygen at a faster rate. Elastic and bell-shaped in or to squeeze through blood vessels smaller than itself. Produced in the bone marrow and broken down in the spleen, releasing haemoglobin which is broken down in the liver into iron (stored in liver) and bile pigments (excreted into the gut in bile).

White blood cells

Features

• Colourless and does not contain haemoglobin • Irregular in shape and contains nucleus • Can change its shape and move

Consists of phagocytes (digests foreign particles) and lymphocytes (produces antibodies). They play a vital role in keeping the body healthy by fighting diseases.

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Platelets Membrane-bound fragments of cytoplasm from certain bone marrow cells. Plays a part in the clotting of blood.

Blood groups Red blood cells carry proteins called antigens on their cell surfaces. Blood plasma contains antibodies. People’s blood groups are classified according to antibodies and antigens present in their blood. A blood group is named after the antigen present. Agglutination occurs when the blood group of the donor is incompatible with the recipients.

• If you belong to blood group B, your red blood cells contain antigen B and your serum contains antibody a. Therefore, you cannot receive blood containing antigen A or your blood would agglutinate. • Blood group O does not contain any antigens. Thus, blood group O is considered the ‘universal donor’.

Functions of blood Blood transports • • • •

Oxygen and digested food to all parts of the body Excretory products to excretory organs Hormones to parts of the body where they are needed Distributes heat

Oxygen combines with haemoglobin to form oxyhaemoglobin in the lungs. In higher altitudes, oxygen levels are lower than at sea level. The body increases the number of red blood cells and hence the haemoglobin content in the body to allow more oxygen intake. Blood serves a protective function

• Clotting seals wounds • Antibodies destroy bacteria and viruses, cause clumping of bacteria and neutralise toxins • Phagocytosis destroys foreign particles

Clotting An enzyme known as thrombokinase is released by damaged tissue and platelets when blood vessels are damaged. Chapter 8 - Transport in humans

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Thrombokinase converts prothrombin and calcium into thrombin. Thrombin then catalyses the conversion of fibrinogen to insoluble threads of fibrin, forming a clot.

Immune system Consists of white blood cells and their products. Helps the body fight diseases.

Organ transplant and tissue rejection A recipient's lymphocytes may respond to a transplant by producing antibodies to destroy the transplant. Tissue rejection will not be a problem if the tissue to be transplanted comes form the same person. The risks of tissue rejection can also be reduced by using immunosuppressive drugs which inhibits the patient’s immune system. However this causes problems such as a lower resistance to many kinds of infection as well as over dependency on the drug.

The circulatory system Consists of the heart, arteries, arterioles, capillaries, venules, and veins.

The heart Muscular organ which drives blood around the whole body. When the heart contracts, blood is squeezed out, and when the heart relaxes, it fills up with blood. These contractions and relaxations circulates blood around the body.

Arterioles Arteries branch to form tiny vessels called arterioles which branch into tinier blood vessels called capillaries.

Venules Before capillaries leave an organ or tissue, they unite to form small veins called venules. Chapter 8 - Transport in humans

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Capillaries Microscopic blood vessels that are found between the cells of almost all the tissues Walls are one cell thick called the endothelium. The endothelium is partially permeable, enabling selected substances to diffuse through quickly. Capillaries branch repeatedly, lowering the blood pressure in the capillaries in order to slow down the flow of blood, giving more time for the exchange of substances.

Arteries and Veins Arteries

Veins

Carry blood away from the heart

Carry blood towards the heart

High blood pressure, hence blood flows in spurts

Low blood pressure, hence blood flows more slowly and smoothly

Have think and elastic muscular walls

Have thin and slightly muscular walls

Carry oxygenated blood (except pulmonary arteries)

Carry deoxygenated blood (except for pulmonary veins)

Semi-lunar valves are absent

Semi-lunar valves are present to prevent back-flow of blood

Have a smaller lumen when compared to a vein of similar external diameter

Have a larger lumen when compared to an artery of similar external diameter

Double circulation in mammals The blood flows through the heart twice in one circuit. Consists of the pulmonary circulation and the systemic circulation.

Pulmonary circulation From the heart, the pulmonary arteries carry the blood to the lungs. Oxygenated blood is returned to the heart by the pulmonary veins.

Systemic circulation Oxygenated blood leaves the left side of the heart and is distributed by arteries to all parts of the body (except the lungs). Chapter 8 - Transport in humans

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Veins carry the blood from all parts of the body back to the right side of the heart.

Advantages of a double circulation Blood enters the lungs at a lower pressure compared to blood leaving the heart, giving more time for the exchange of gases in the lungs since blood travels slower there. Blood is pumped to the rest of the body at high pressure, hence oxygenated blood is distributed to the body tissues more quickly, helping maintain the high metabolic rate in mammals.

The heart A muscular pump that keeps blood flowing through the blood vessels of the body. Supplied with food and oxygen by a set of blood vessels called the coronary arteries.

Structure Four chambers • • • •

Left atrium Left ventricle Right atrium Right ventricle

The left ventricle has thicker walls than the right ventricle. The median septum separates the left and right chambers of the heart.

Path taken by blood through the heart

a brief summary to prevent insane boredom

Deoxygenated blood from various parts of the body is returned to the right atrium. When the right atrium contracts, blood flows into the right ventricle. Blood leaves the right ventricle through the pulmonary arch. Oxygenated blood from the lungs is brought back to the left atrium by the pulmonary arteries.

The cardiac cycle

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Both the atria and ventricles are relaxed. The left atrium receives blood from the pulmonary arteries while the right atrium receives blood from the venae cavae. When the atria contracts, blood is forced into the ventricles.

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The ventricles then contract after a short pause. This contraction is known as the ventricular systole. This increase in pressure forces the bicuspid and tricuspid valves to close, preventing back-flow of blood into the atria. This produces a louder ‘lub’ sound. When the pressure in the left ventricle becomes higher than the pressure in the aorta, the semi-lunar valves in the pulmonary and aortic arches are forced open. Blood flows from the left ventricle into the aortic arch. As the ventricles contract, the atria relax. The ventricles then relax, which is called the ventricular diastole. The drop in pressure in the ventricles causes the semi-lunar valves in the two arches to close, preventing back-flow of blood into the ventricles. The produces the softer ‘dub’ sound. The drop in pressure in the ventricles also causes the bicuspid and the tricuspid valves to open. The whole cycle then repeats.

Main arteries and veins Arteries

• The hepatic artery delivers blood to the liver • The renal arteries deliver blood to the kidneys • The pulmonary artery delivers blood to the lungs

Veins • • • •

The hepatic vein returns blood from the liver to the heart The renal veins return blood from the kidneys to the heart The pulmonary veins return blood from the lungs to the heart The hepatic portal vein delivers blood from the gut to the liver

Tissue fluid and lymph Tiny spaces between tissue cells contain a colourless liquid known as tissue fluid. Tissue fluid is blood plasma that has moved out of the capillaries to the surrounding tissue cells. Tissue fluid does not contain plasma proteins, but it does have white blood cells that have moved out of the capillaries.

Coronary heart disease Blood pressure is the force exerted by blood on the walls of blood vessels. If the coronary arteries become blocked, blood supply to the heart is reduced, resulting in either angina (chest pains usually in women) or heart attack. Atherosclerosis is the deposition of fatty substances on the inner surface of an artery. A diet consisting mainly of saturated fats, smoking and stress can increases risks of atherosclerosis and coronary heart disease. Cutting down on the aforementioned activities and exercising regularly can reduce the risk of coronary heart disease. Chapter 8 - Transport in humans

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