Transport in Humans

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8.1: The Need for a Transport System

Simple diffusion cannot bring oxygen and food materials to cells in complex multicellular organisms. There must be a transport system to carry substances from one part of the body to the other.

8.2: Structure and Composition of Blood

Mammalian blood is composed of 55% plasma (the fluid part of blood) and the other 45% is made up of blood cells and platelets.

Main Constituents of Blood:

Plasma

Red Blood Cells

White Blood Cells

Platelets

Plasma

Plasma is a pale, yellowish liquid. Plasma is about 90% water and the rest is a complex mixture of various dissolved substances such as:

Soluble proteins (fibrinogen, prothrombin and antibodies)

Dissolved mineral salts (hydrogencarbonates, chlorides, sulfates and phosphates of calcium, sodium and potassium)

Food substances (glucose, amino acids, fats and vitamins)

Excretory products (urea, uric acid and creatinine)

Hormones (insulin)

Plasma transports these substances, along with the red blood cells, around the body. The amounts of soluble proteins, mineral salts and glucose in the blood plasma are kept relatively constant.

Red blood cells (aka erythrocytes)

In each cubic mm of blood, there are about 5 million red blood cells.

Each mammalian red blood cell has the following features:

Contains haemoglobin (which combines reversibly with oxygen to transport it to all parts of the body)

Circular, flattened, biconcave shape (Increases surface area to volume ratio, allowing for faster rate of diffusion and absorption of oxygen)

Does not possess a nucleus, enabling it to carry more haemoglobin and oxygen

Elastic and can turn bell-shaped to squeeze through blood vessels smaller than itself in diameter (<0.01mm)

Red blood cells are produced by the bone marrow. A red blood cell's life span is 3-4 months.

White blood cells

White blood cells larger than red blood cells but are fewer in number.

Ratio of red blood cells to white blood cells is 700:1.

Each white blood cell has these features:

Colourless and does not contain haemoglobin

Irregular in shape and does not contain a nucleus

It can move, change its shape and squeeze through the walls of the thinnest blood capillaries into the spaces among tissue cells.

There are two main kinds of white blood cells: lymphocytes and phagocytes.

The white blood cells play a vital role in keeping the body healthy by fighting diseases.

Blood platelets or thrombocytes

Blood platelets or thrombocytes are not true cells. They are membrane-bound fragments of cytoplasm from certain bone marrow cells. They are responsible for the clotting of blood.

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8.3: Functions of Blood

Transport function of blood

Blood performs three important functions:

Acts as a transport medium carrying various substances from one part of the body to another.

Protects the body against disease-carrying organisms (pathogens)

Blood clotting at wounds prevents excessive loss of blood. Clotting seals the wound and prevents the entry of bacteria into the bloodstream

Blood transports the following:

Digested food (Intestines -> Other parts of the body; excess mineral salts are transported to the kidneys)

Excretory products (All parts of the body -> Kidneys and Lungs)

Hormones (Glands -> Target organs)

Heat (Respiring body tissues -> All parts of the body to maintain a uniform body temperature)

Oxygen (Lungs -> All parts of the body for cellular respiration)

Blood transports oxygen to the cells of the body through this process:

As blood passes through the lungs, oxygen diffuses from the alveoli to the blood.

Haemoglobin has a great affinity for oxygen. Haemoglobin can combine reversibly with oxygen. Haemoglobin combine with the oxygen to form oxyhaemoglobin (bright red in colour)

As blood through tissues containing very little oxygen, the oxyhaemoglobin releases its oxygen. The oxygen then diffuses in solution into the tissue cells. In this way, every cell in the body receives its supply of oxygen.

Protective function of blood

Protective functions of blood include:

Blood clotting

Phagocytosis

Antibody protection

Clotting or coagulation of blood

Blood exposed to air will soon clot or form a solid lump. The clot seals the wound, preventing an excessive loss of blood.

People with haemophilia will have their normal blood-clotting mechanism impaired, and slight injuries can cause such a person to bleed to death or die of internal bleeding.

The mechanism of clotting is a complicated process:

When blood vessels are damaged, damaged tissues and blood platelets release an enzyme known as thrombokinase.

Thrombokinase converts the protein prothrombin, normally present in the plasma, into thrombin. Calcium ions must be present before this can take place.

Thrombin catalyses the conversion of the soluble protein fibrinogen to insoluble threads of fibrin.

Fibrin threads entangle blood cells and the whole mass forms a clot or scab. The clot seals the wound, preventing entry of microorganisms.

Clotting process can be summarised in three reactions:

Damaged tissues and platelets -> Thrombokinase

Prothrombin (inactive) -> Thrombin (active)

Fibrinogen -> Insoluble fibrin threads form a mesh to trap blood cells

Normally in undamaged blood vessels, the blood does not clot. This is due to the presence of an anti-clotting substance called heparin, produced from the liver. When thrombokinase is released, it neutralises the action of heparin so that clotting can take place. When blood clots, a yellowish liquid called serum is left behind. Serum has the same composition as plasma except that it lacks the clotting factors.

Phagocytosis

Phagocytosis is the process of engulfing or ingesting foreign particles, such as bacteria, by the white blood cells.

Production of antibodies

When pathogens gain entry into the bloodstream, they stimulate lymphocytes to produce certain chemical substances called antibodies. Antibodies protect our bodies against diseases.

Antibodies act by:

Destroying the bacteria

Causing the bacteria to clump together or agglutinate so that they can be easily ingested by the phagocytes

Neutralising the toxins produced by bacteria

The immune system includes white blood cells and their products. The immune system causes an immune response to foreign particles, keeping the body free from disease.

Organ transplant and tissue rejection

When tissue or organ transplants occur, it must not be rejected by the recipient's immune system. If rejected by any organ, the recipient's lymphocytes may respond by producing antibodies to destroy the transplanted organ.

Tissue rejection will not be a problem if the tissue to be transplanted comes from the same person.

Preventing tissue rejection

Tissue match. Ensure that the tissues of both the donor and recipient be as genetically close as possible. Siblings, parents and close relatives are likely to have similar genes.

Use immunosuppressive drugs to inhibit the responses of the recipient's immune system. However, the recipient has lower resistance to many kinds of infection, and has to continue taking the drugs for the remainder of his/her life.

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8.4: The Circulatory System

In vertebrates, the blood flows through a closed system of blood vessels called the circulatory system. This blood flow is called blood circulation.

The circulatory system comprises of:

1) The Heart

The blood is kept circulating throughout the body by this organ. When the heart relaxes, it fills up with blood, and when it contracts, the blood is squeezed out with great force. The blood then circulates through the blood vessels which direct the blood flow around the body.

2) Arteries

Carry blood away from the heart. The large artery that leaves the left side of the heart is the aorta. It branches to form smaller arteries.

3) Arterioles

The arteries branch again to form tiny vessels called arterioles. The arterioles divide and ultimately their branches become very tiny blood vessels called capillaries.

4) Capillaries

Microscopic blood vessels that are found between the cells of almost all tissues. Walls are made up of a single layer of flattened cells (endothelium), which is partially permeable, enabling certain substances to diffuse quickly through the capillary walls.

Capillaries branch repeatedly, providing a large surface area for the exchange of substances between the blood and the tissue cells. This lowers the blood pressure, and the flow of blood is slowed down, giving more time for the exchange of substances.

5) Venules

Before the capillaries leave an organ or tissue, they unite to form small veins called venules.

6) Veins

Venules in turn join to form bigger veins. Veins carry blood back to the heart.

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