FACTORS INFLUENCING VENOUS RETURNS

mm Hg. This equilibrium point is relatively viable For example, let at a given moment. I, the CVP is suddenly raised The first heart beat after the , because of the Frank-Starting mechanism, will have a greater stroke volume But because of this greater stroke volume, translocation of blood from the venous side to the arterial side will be greater and so the CVP will be somewhat lesser in the following beat Therefore, immediately after .the equilibrium point will shift no doubt, but it will try. to return to the original point gradually. Fig. 5.5.5. To show that when cardiac output rises, the rap' falls Fig. 5.5.6. See text FACTORS INFLUENCING VENOUS RETURNS 1. When the cardiac output rises, the velocity of circulation raised which results in greater cardiac inflow (= venous return to the. heart) per unit time Explanation (I) as the cardiac output rises, as explained above, the central venous pressure falls hence pressure in the peripheral veins also falls. (II) as the cardiac output rises, a little extra blood is pushed into the arterial system per beat As the arterial system resists distension even a slight increases in blood volume in it (the arterial system). causes a sharp rise of arterial BP When the above two factors, vast (I) and (ii) are considered together, it becomes obvious that the pressure gradients between the arterial and venous system (= the difference between arterial BP and venous BP) rises. This increase in the pressure radiance results in greater rate of flow of blood from the arterial to the venous side, stated simply this means greater venous return per unit time. 2. The respiratory pump During inspiration, the intrapleural pressure as well as (he intrathoracic pressure as a whole becomes more negative The intra abdominal pressure however rises due to the descent of the diaphragm Because of the increasing negativity, the diameters of the vane cave increase, and the BP within them falls So the pressure radiance (the difference of BP s between the intra abdominal part of the inferior vane cava and The right atrium) increases — flow of blood towards the right atrium increases. Conclusion, therefore, is, more vigorous is the respiration, speedier are the venous returning (cardiac inflow) This suction action of respiration is called the respiratory pump In violent muscular exercise, its effect may be spectacular. 3. The muscle pump When the skeletal muscles contact they exert a squeezing action on the veins Because of the presence of valves in them, when the veins are squeezed, blood moves only towards The heart. Conclusion is. when the skeletal muscles are working hard (e.g. muscular exercise) the venous rectum increases. 4. Gravity On assumption of erect posture. the venous drainage from head and neck is facilitated by gravity where as the same from the inferior extremity is opposed by gravity. 5. Vasomotor tone Veins are supplied by the sympathetic fibers When the sympathetic fibers are stimulated, the vein undergo constriction and The blood within it is driven towards the heart more speedily. Therefore sympathetic, stimulation leads to venison striation and greater cardiac inflow In some organs, veins normally hold great quantities of blood On sympathetic stimulation, the venous blood from these organs are evacuated and Thus the cardiac inflow increases Such organs are called reservoirs of blood ANATOMICAL CONSIDERATIONS AND THE STRUCTURE FUNCTION INTER RELATIONSHIP 1. The Major Division of the Vascular Tree. 2. General Structure of the Vascular Tree. 3. The Individual Segments of the Vascular Tree their functions and structural peculiarities The Vascular tree' begins at the beginning of the aorta. continues at branches of the aorta The branch repeatedly give rise to further branches, which are narrower and narrower and ultimately they become arterioles The areoles eventually open into the capelins. the capillaries into The venues (= very delicate veins) The venues unite with other venues To form veins. the veins unite with other veins and ultimately two great veins, via, the superior and inferior vane cave (singular, vena cava} are formed which open into the right atrium This portion of the vascular tree is called the systemic circuit, and its circulation called greater or systemic Circulation From the right ventricle, arises the main pulmonary artery, which divides into two branches. The right and the left pulmonary arteries, destined to supply the right and the left lung respectively Blood from the two lungs are eventually drained by the four pulmonary veins which open into the left atrium. This portion of The vascular tree is called the pulmonary circuit. and its circulation called, lesser or pulmonary circulation For descriptive purposes end viewed from functional point of view, the vascular tree is usually subdivided into following divisions (segments) or regions (fig. 5.10.4.). 1. Windless vessels', also called elastic arteries. 2. Precapillary resistance vessels' or the arterioles, also called, muscular arteries. 3. precapiltary sphincters .'4. 'Exchange vessels' or capillaries. 5. "Post capillary resistance vessels'chtefly the venues .6. Large veins or the 'capacitance vessels' A term 'microcirculation' is very popular amongst the physiologists and clinicians Micro circulation includes the precapillary sphincter region, capillaries and the smallest venues The name (i.e. 'microcirculation') owes its origin to the fact That they can be seen only under the microscope GENERAL STRUCTURE OF THE VASCULER TREE To start with. a big artery. e.g. the aorta, may be considered Such an artery has three coats, via (I)ton