1.3 The peripheral cardiovascular system
The peripheral cardiovascular system has an essential role in the distribution of blood to supply the requirements of tissues and to remove waste products. It is not just a simple conduit, but controls the relative distribution of blood to different tissues in the body according to demands.
The aorta has a highly elastic wall, which stores the force of ventricular contraction so that blood continues to be pumped around the circulation even in diastole. The stiffness of the aorta is controlled by autonomic stimulation and affects impedance to flow. If the aorta becomes abnormally stiff, the load on the LV is increased, and circulation is less efficient.
The arterioles are also elastic, but in addition they contain smooth muscle. Their diameter, and therefore the resistance which they offer to blood flow, is controlled by direct innervation, by systemic humoral factors, and by local vasoactive compounds. The relative size of arterioles in different tissues controls the relative distribution of blood within the body. A co-ordinated response is required so that blood pressure is maintained.
The capillaries are the site of exchange of nutrients and waste products. The permeability of the capillary wall can alter under some circumstances. The balance of osmotic and hydrostatic pressures across the capillary wall controlsthe net flow of fluid and electrolytes into the extracellular space. Selective perof the capillary walls is required to prevent too much protein crossing the wall and altering the osmotic pressure of the intestinal fluid.
The venules collect blood from the capillaries and return it to the great veins. The majority of circulating blood volume is contained within the venules; control of the tone of their walls plays an important role in maintaining blood pressure. When there is a demand for increased cardiac output, constriction of the venules maintains an adequate preload to allow cardiac output to respond to demands.
The great veins play a similar role to venules in control of blood pressure and circulating blood volume. The spleen also acts as a reservoir for blood, and can release a large number of red cells into the circulation during exercise to increase the oxygen carrying capacity of blood.
The lymphatic system is responsible for returning excess interstitial fluid into the venous circulation. This ensures that the interstitial fluid maintains a suitable osmolarity and concentration of electrolytes and proteins.