Module 2 - Exchange and Transport

Blood is a suspension of red (erythrocytes) and white (leukocytes) cells and platelets in plasma. The plasma contains many dissolved substances, including oxygen, carbon dioxide, salts, glucose, fatty acids, amino acids, hormones and plasma proteins.

When left to settle, or spun in a centrifuge, blood separates into three components, bottom layer of Red blood cells, a thin middle layer of White blood cells and platelets and a top layer of Plasma.

Tissue fluid is a colourless fluid that is formed from blood plasma by pressure filtration through capillary walls. It surrounds all cells of the body. Its role is to transport O2 & nutrients from blood to cells, and to carry CO2 and other wastes back to the blood.

Lymph is tissue fluid that has drained into lymphatic vessels. It passes through lymph nodes, where it gains white cells and antibodies. Lymphatic vessels absorb hormones from some endocrine glands and fat in small intestine.

When an artery reaches the tissue it branches into smaller arterioles, then into a network of capillaries which eventually link up with venules to carry blood back to the veins. So blood flowing into an organ or tissue is contained in the capillaries. At the arterial end of a capillary, the blood is under high pressure due to the contraction of the heart muscle. This is known as hydrostatic pressure. It will tend to push the blood fluid out of the capillaries. The fluid can leave through tiny gaps in the capillary wall.

The fluid that leaves the blood consists of plasma with dissolved nutrients and oxygen. All the red blood cells, platelets and most of the white blood cells remain in the blood, as do the plasma proteins. These are too large to be pushed out through the gaps.

The fluid that leaves pressure of the blood is not the only force acting on the fluid. The fluid surrounds the body cells, so exchange of gases and nutrients can occur across the cell surface membranes. This exchange occurs by diffusion and facilitated diffusion. Oxygen and nutrients enter the cells; carbon dioxide and other wastes leave the cells.

Hydrostatic pressure of blood is not the only force acting on the fluid. The tissue itself has some hydrostatic pressure, which will tend to push the fluid back into the capillaries. Both the blood and the tissue fluid also contains solutes, giving them a negative water potential. The water potential of the tissue fluid is less negative than that of blood. This means that water tends to move back into the blood from the tissue fluid by osmosis, down the water potential gradient.

At the venous (vein) end of the capillary, the blood has lost its hydrostatic pressure. The combined effect of the hydrostatic pressure in the tissue fluid and the osmotic force of the plasma proteins is sufficient to move fluid back into the capillary. It carries with it any dissolved waste substances, such as CO2, that have left the cells.

Some tissue fluid, that's not returned to the blood capillaries, is drained into the lymphatic system. This system consists of a number of vessels (tubes) that are similar to capillaries. They start in the tissues and drain the excess fluid into larger vessels, which eventually rejoin the blood system in the chest cavity.

Lymph fluid is similar to tissue fluid and contains the same solutes. It has less O2 and fewer nutrients, as these are absorbed by body cells. Will also be more CO2 and wastes that have been released from body cells. Has more fatty material, absorbed from intestines

Main difference between tissue fluid and lymph is that the lymph contains many lymphocytes. These are produced in the lymph nodes. The lymph nodes are swellings found at intervals along the lymphatic system. They filter any bacteria and foreign material from the lymph fluid. The lymphocytes can then engulf and destroy these bacteria and foreign particles. This is part of the immune system that protects the body from infection.