18.5 TRANSPORT Blood and Circulation

Mammalian Transport system  The transport system in humans is typical of all mammals. Materials are transported throughout the body in a liquid medium. Three different fluids are involved: tissue fluid, blood and lymph.  The tissue (or intercellular) fluid surrounds individual cells. It acts as a mediating fluid between the blood and the cells.

Mammalian Transport system  Blood moves through a series of tubular vessels. These vessels, the arteries, veins and capillaries, vary in size, in the structure of their walls and in function. The pumping action of the muscular heart moves the blood through the vessels. This is aided by other movements of the body.  Lymph is carried in lymphatic vessels. It picks up tissue fluid and particulate matter and returns these to the blood system. The lymph is very important in the body’s immune response

STRUCTURE AND FUNCTION OF BLOOD  Blood is a highly specialised connective tissue consisting of several cell types suspended in a fluid medium, the plasma. Figure shows the types of blood cells found in mammals and the functions they perform.

Composition and function of transporting fluids in mammals

Red blood cells (erythrocytes)

 The main function of the red blood cells (erythrocytes) is to transport oxygen from the respiratory surfaces to the tissues. Mature red blood cells have no nucleus and must therefore be constantly produced in the bone marrow. This occurs at a rate of about 1.5 million per second.  Their biconcave shape ensures a maximum ratio of surface area to volume for the uptake and release of respiratory gases.  The shape also ensures smooth flow through fine blood vessels, their diameter being about the same as that of the finest capillaries.

Haemoglobin - Hb

 The inside of a red blood cell is packed with the red pigment haemoglobin. This is a complex protein containing four iron haem groups. Each haem group can carry one oxygen molecule. The lack of a nucleus makes it possible for more haemoglobin to be included within the cell, and thus for a large amount of oxygen to be carried. A single red blood cell can carry 1000 million oxygen molecules.

Carriage of oxygen  Oxygen diffuses into the red blood cell across its thin elastic membrane and combines with the haemoglobin to form oxyhaemoglobin. This is a loose union which allows rapid attachment and detachment.

Haemoglobin – respiratory pigment  Haemoglobin has a strong affinity for oxygen. In areas of the body where the oxygen concentration is high—for example at the alveoli of the lungs— oxygen is rapidly taken up by the haemoglobin. When blood carrying oxyhaemoglobin encounters an area of low oxygen concentration at actively respiring tissues, the oxygen is unloaded (or dissociates) from the haemoglobin. This allows the blood to transport oxygen at a rate sufficient to supply all the cells with their needs.

Oxygen dissociation curve The curve indicates that blood can become fully saturated at a relatively low oxygen tension. If the relationship were linear (the broken line), the percentage oxygen saturation at any particular oxygen tension would be lower. For example, at 6.0 kN/m2, the actual saturation is 90 per cent, whereas a linear relationship would give about 48 per cent saturation of blood.

The Bohr Effect An increase in carbon dioxide levels, as occurs in the working tissues, has a profound effect on the oxygen dissociation curve. It reduces the amount of oxygen that can be carried by oxyhaemoglobin. The oxygen dissociation curve is shifted downwards and to the right. This shift is named the Bohr effect.

Carriage of carbon dioxide Carbon dioxide diffuses from the tissues into the blood. As a result of chemical reactions, most of the carbon dioxide is transported as bicarbonate ions in the red blood cells, and some is combined with haemoglobin.

Exchange of gases at the respiratory surfaces and in tissues