Capillary Circulatory

CAPILLARY CIRCULATION The capillary circulation links the arterial (oxygenated) system to the venous (deoxygenated) system. The capillaries form an intimate relationship both structurally and functionally with the tissues. It is at the capillary level that exchange of dissolved glucose, amino acids, oxygen etc takes place.

Blood enters the arterial end under high blood pressure and due to this high pressure blood undergoes filtration at the capillary wall. The simple squamous epithelium acts as the filter, so that the filtrate consists mainly of water and all its dissolved contents of plasma. This filtrate is referred to as the tissue fluid and this process of pressure filtration of blood is called ultra-filtration.

The tissue fluid plays a very integral part in the maintenance of Homeostasis. (Homeostasis is defined as “the maintenance of a constant internal environment”) The tissue fluid, being a direct filtrate of blood, bathes the cells and tissue thereby supplying it with nourishment and oxygen. Since the tissue fluid comes into direct contact with the cytoplasm it is crucial that its temperature, pH, and contents remain constant at all times. Therefore, as stated earlier, since tissue fluid is a direct filtrate of blood the liver and other organs strive at all times to keep the level of glucose, oxygen, salt/water (osmotic pressure), temperature and pH of blood constant. This will ensure that these criteria are thus maintained in the tissue fluid and cytoplasm.

The tissue fluid exits the capillary at the arterial end due to the blood pressure being high. The tissue fluid contains everything in blood except the red blood cells, white blood cells and plasma proteins. Approximately midway along the capillaries, the pressure exerted by the plasma proteins, the osmotic oncotic pressure, exceeds the blood pressure so the tissue fluid re-enters the venous end of the capillary. However, due to pressure indifferences not all the fluid is returned to the capillaries. Instead the remaining fluid is drained through the lymphatic capillaries, which originate at the tissues.

The lymphatic vessels carry the lymph, which is similar to plasma, except that it lacks fibrinogen, upwards until it returns this lymph to the venous circulation when the thoracic ymph duct empties into the left subclavian vein just near the superior venecava. Thus the lymphatics act as a drainage system returning the balance tissue fluid to the blood circulation.

CARDIAC EXCITATION Embedded within the walls of the myocardium is specialised conducting tissue that carries a wave of depolarization through the heart resulting in its smooth coordinated contraction. This tissue is believed to be specialised neural tissue which when it contracts, makes the myocardium contract with it. First is the SAN(sino-atrial node) that acts like the pace maker and it is embedded in the wall of the right atrium. It initiates the impulse which travels along the walls of the atria and converges at the AVN (atrio-ventricular node). The time taken for the impulse to travel from the SAN to the AVN corresponds to atrial ststole. The AVN is surrounded by non-conductive tissue which slows down the passage of the impulse. Thus there is a delay at the AVN which is very important in ensuring that atrial systole is totally over before ventricular systole begins. From the AVN, rapid impulses travel down the Bundle of His and Purkinje fibres to the apex of the heart. Ventricular systole thus begins at the apex and radiates upwards so that both ventricles contract from the apex upwards forcing the blood in an upward direction into the two great arteries. Remember that cardiac muscle is myogenic and thus does not need a nerve supply to contract. However, Adrenaline and the sympathetic nervous system can accelerate the heart rate by telling the SAN to increase its discharge. Similarly, the parasympathetic nervous system can slow the heart rate. Thus the heart rate can be altered and modified when necessary.