Hemodynamics and Distribution of blood in the systemic circulation
Physiology of circulation (Hemodynamics)
Circulation is a continuous circuit through which the same amount of blood flow through each subdivision of circulatory system is maintained by the heart. In a resting animal about 25% of the blood volume is in the central circulation (pulmonary and coronary circulation) and the rest 75% in the systemic circulation.
Distribution of blood in the systemic circulation
| Arteries and arterioles | 15% |
| Capillaries | 5% |
| Venules and veins | 80% |
Aorta is the biggest blood vessel, originates from the left ventricle of the heart and continues as large arteries. Both the aorta and the large arteries have more elastic tissues with less smooth muscle; hence known as elastic vessels. Arteries are the high-pressure passage way and deliver blood to capillaries. These vessels function in the maintenance of blood pressure during diastole by their elastic recoiling effect.
The large arteries form the main arterial branches thar give rise to the terminal branches or smaller arteries, which have smooth muscles in their walls. They are continued as the small arterioles or the true arterioles, which are continued as meta-arterioles. Small arteries and arterioles have thick walls with less elastic tissue and more of smooth muscle, called as muscular vessels. These arterioles are gates of the systemic circulation, which may constrict or dilate to regulate blood flow to capillaries.
Capillaries are tiny tubular network composed of only a thin layer of endothelium that favours selective permeability of water, oxygen, nutrients, CO2 and other metabolic waste between blood and tissues and they are known as exchange vessels. In capillaries the blood cells flow in single layer with RBCs in the centre and WBCs in the periphery. Capillaries form the highest cross-sectional area in the circulatory system.
The true capillaries are interconnected which forms the place of nutrient exchange. At the point the capillaries branch off from metarterioles, the capillaries are enriched by smooth muscle called precapillary sphincter. The metarterioles and the precapillary sphincters are not innervated and are controlled by local conditions in the tissues.
Blood flows from the arteriole directly into metarteriole then leads to capillaries and capillary like channels which connect directly with veins called as A – V shunts.
When the tissues are at rest and large amount of blood is not required, the sphincters may be completely closed. When large amount of nutritive blood flow is required for active tissues, the metarterioles and the sphincters dilate to shunt the blood nearly the entire length into the capillary bed. The precapillary sphincter undergoes periodical contraction at intervals of 15 second to 3 minutes, called as vasomotion.
Vasomotor mechanism causes either vasoconstriction (decreased diameter) or vasodilatation (increased diameter) of the arteries and arterioles. It involves an active contraction of the smooth muscles of arterioles and precapillary sphincters, whereas vasodilatation is a passive event involving only relaxation of smooth muscle.
Arteriovenous anastomoses or shunt occurs between arteries and veins. It is not involved either in blood flow through true capillaries or in the exchange of materials between blood and extracellular fluid. These shunts are primarily by passing the resistance pathways of the precapillary area in tissues during a period of low requirement for nutritive blood supply. In dogs arteriovenous anastomose is primarily functional in thermoregulation allows increased blood flow to the tongue for evaporatory heat loss through panting. This structure is under separate neural and hormonal control.
The terminal vascular beds consist of small arterioles, metarterioles, capillaries and venules forms microcirculation. Capillaries continue as venules, a thin walled network which unit to form terminal veins. Venules and veins are thin walled structures contain elastic tissues and smooth muscles. Veins function as a blood reservoir called as capacitance vessels. When dilated, they can store large amount of blood and by constriction shift blood volume and affect cardiac venous return.
The veins continue as vena cava, which drains its content into the right atrium. Vena cava has the highest diameter.
Veins in the extremities have valves. These valves prevent back flow of blood, thus permits unidirectional blood flow towards heart. There are no valves in the major abdominal and thoracic veins.
Hemal nodes are the nodal structure found in cattle and sheep on the course of small blood vessels. They are similar in structure and function like spleen. Erythropoiesis occurs during fetal period and granulopoiesis occur in postnatal life.
Compliance (capacitance)
Compliance (capacitance) is the total storage capacity in a given portion of the circulatory system for every mm Hg of pressure rise. Veins have about 24 times more capacitance than arteries since about 80% of the total blood volume is located within the venous system.