To show that when cardiac output uses, the 'rap' falls. Fig. 5.5.6. See text FACTORS INFLUENCING VENOUS RETURNS

mm Hg This equilibrium point is relatively stable. For example. let at a given moment, the CVP is suddenly raised. The first heart beat after the 1, because of the Frank-Sterling mechanism, will have a greater stroke volume But because of this greater stroke volume, translocation of blood from the venous side in the arterial side will be greater and so the CVP will be somewhat lesser in the following beat Therefore, immediately after 1, the equilibnum pomp) 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 uses, the 'rap' falls. Fig. 5.5.6. See text FACTORS INFLUENCING VENOUS RETURNS 1. When the cardiac output rise, the velocity of circulation rise which results in greater cardiac inflow (= venous return to the heart) per unit time Explanation; (i) as the cardiac output rise. as explamel) above, the central venous pressure falls hence pressure in the peripheral veins also falls (ll) as the cardie output rises, a little extra blood is pushed in to the arterial system per beat. As the arterial system resists Extension even a slight increase in blood volume in it (the artenal system), causes a sharp rise of arterial BP When the above two factors, viz, (i) and (ii) are considered together, in becomes obvious that the pressure gradience between The arterial and venous system. (= the difference between arterial BP and venous BP) rises. This increase in the pressure gradiance results ingreater rate of flow of blood from the artenal 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 the intrathoracic pressure as a whole becomes more negative. The intra abdominal pressure however rise due to the descent of the diaphragm. Because of the increasing negativity, the diameters of the were cave increase and the BP within Them till; So the pressure gradience (The difference of BPs between the intra abdominal part of the inferior vena cava and the right atrium) increases — flow of blood towards the right atrium increases. Conclusion, therefore, is. more vigorous it the respiration, speedier is the venous return (cardiac inflow) This suction action of respiration it called the respiratory pump In violent muscular exercise, its affect may be spectacular. 3. The muscle pump When the skeletal muscles contract 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 exerise) the venous return increases. 4. Gravity On assumption of erect posture. the venous damage from head and neck is facilitated by gravity where as the same from the inferior extremity is opposed by gravity. 5. Venomotor Tone Veins are supplied by the sympathetic fibers. When the sympathetic fibers are stimulated, the veins undergo constriction and the blood within it is driven towards the heart more speedily Therefore sympathetic, stimulation leads to venoconstriction 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 Divisions 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 as branches of the aorta The branches repeatedly give rise to further branches, which are narrower and narrower and ultimately they become arterioles. The arterioles eventually open into the capillaries, the capillaries into the venules (= very delicate veins). The venules unite with other venules to form veins, the veins unite with other veins and ultimately two great veins, viz, the superior and inferrior vene cave (singular, vena cava) are formed which open into the right atrium. This portion ofthe vascular Iree is called the systemic circurl. and its circulation called greater systemic circulation From the right ventricle, arises the mam pulmonary artery, which divides into two branches, the right and the left pulmonary artries, destined to supply (he right and the left lung respectively Blood from the two Jungs are eventually drained by The four pulmonary veins which open mto the lefl 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 (tig.5.10.4) 1. Windkessel vessels', also called elastic arteries. 2. Precapillary resistance vessels' or the arterioles, also called, muscular arteries. 3. Precapillary sphincters'. 4. 'E-change vessels'or capilaries. 5. Post capillary resistance vessels'chiefly the venules. 6. Large veins or the 'capacitance vessels' A term 'microcirculstion' is very popular amongst the physiologists and clinicians. Micro circulation includes the pracapillary sphincter region, capillaries and the smallest venules The name (i.e. microcirculation') owes its ongin to the fact that they can be seen only under the microscope. GENERAL STRUCTURE OF THE VASCULAR TREE To start with.a big artery, e.g. the aorta, may be considered Such an artery has three coats, viz. (i) tun