Microcirculation and Veins-Notes.pdf

Donal S. O'Leary, Ph.D. Cardiovascular Physiology MICROCIRCULATION & VEINS Learning Objectives: 1. Describe the different types of capillaries. 2. Describe the role of the Endothelium in the control of vascular smooth muscle activity. 3. Describe the...

Donal S. O'Leary, Ph.D. Cardiovascular Physiology MICROCIRCULATION & VEINS Learning Objectives: 1. Describe the different types of capillaries. 2. Describe the role of the Endothelium in the control of vascular smooth muscle activity. 3. Describe the forces controlling fluid movement across the capillary. 4. Describe the role of the precapillary resistance in controlling hydrostatic pressure within the capillary 5. Describe the role of compliance in regulating venous volume. Lecture Outline: 1. Different capillary types 2. Starling Forces for fluid movement 3. Effects of precapillary resistance 4. Active vs. passive changes in venous volume and compliance. ANATOMY The capillaries are the final branches from the arterioles and are the smallest blood vessels. At the level of the capillaries diffusion can effectively occur over the relatively small distances between the cells of the tissue and the blood. There are three basic kinds of capillaries. Continuous capillaries have a continuous layer of endothelial cells lining the capillary broken only by small pores (no pores exist in capillaries in most of the central nervous system). Fenestrated capillaries (from the Latin for "window") have areas where the cytoplasm of the cells thins in areas. These are located in tissues which handle a lot of fluid such as the kidney. In fenestrated capillaries the "window" areas provide a low resistance pathway for diffusion. Discontinuous or Sinusoid capillaries have areas between endothelial cells through which larger materials can pass. These exist in areas of the liver. 1 Donal S. O'Leary, Ph.D. Cardiovascular Physiology CAPILLARY FLUID EXCHANGE - THE STARLING HYPOTHESIS Four forces govern the movement of fluid across the capillary wall: t - oncotic pressure in the tissue interstitium c - oncotic pressure in the capillary Pt - hydrostatic pressure within the tissue Pc - hydrostatic pressure within the capillary t - pulls fluid out of the capillary c - pulls fluid into the capillary Pt - pushes fluid into the capillary Pc -pushes fluid out of the capillary Net force for filtration = (Pc + t) - (Pt + c) This equation will yield the net force for fluid movement out of the capillary (QUESTION: what would be the formula for net force for fluid movement into the capillary?). The amount of filtration will be dependent on the net force for fluid movement times the filtration coefficient (Kf). Thus, to calculate the net amount of filtration Net Filtration = Kf [(Pc + t) - (Pt + c)] Pc does not remain constant. At the arterial end of the capillary Pc is greater than at the venous end of the capillary. This is due to the resistance to blood flow provided by the capillary causing a reduction in pressure along the length of the capillary. Thus, at the arterial end of the capillary the net forces favor filtration whereas, at the venous end of the capillary the net forces favor reabsorption of fluid into the capillary. Any excess fluid is absorbed into the lymphatic capillaries and returned to the circulation. 2 Donal S. O'Leary, Ph.D. Cardiovascular Physiology Pc= 32 mmHg c= 25 mmHg c= 25 mmHg Pc= 15 mmHg Arterial Venous end of a end of a capillary capillary Absorption Filtration H2O Solutes Lymphatic capillary EFFECT OF PRECAPILLARY RESISTANCE ON CAPILLARY PRESSURE Changes in precapillary resistance will inversely affect Pc. If resistance increases a larger pressure drop will occur across the arteriole and therefore Pc will decrease (QUESTION: how will this affect filtration/reabsorption?). If resistance decreases, a smaller pressure drop will occur across the arteriole and therefore Pc will increase. EDEMA, the buildup of fluid in the interstitium, can be caused by a number of factors. Blockage of lymphatic flow, increased Pc, decreased c, increases in Kf (if forces favor filtration). ARTERIOVENOUS ANASTOMOSES Non nutritional pathways (no diffusion) directly connecting an arteriole to a venule. Abundant in the skin and opened during hyperthermia. 3 Donal S. O'Leary, Ph.D. Cardiovascular Physiology VEINS Veins are low resistance high compliance vessels. EFFECT OF POSTURE - In the upright position, venous transmural pressure increases towards the legs thus the volume of blood below heart level increases. In man the upright posture over 70% of the total blood volume is below heart level. This is due to the high compliance of the veins. CONTROL OF VENOUS COMPLIANCE Changes in venous compliance can occur via active or passive factors: Active Changes: are due to changes in the activity of the vascular smooth muscle and result in changes in the entire venous compliance curve. Active changes in compliance can occur with changes in sympathetic stimulation, circulating vasoconstrictors, or local temperature (especially in the skin). Passive Changes: Occur with changes in the position on the same compliance curve due to changes in transmural pressure. 5 Torr 70% of blood volume BELOW heart level 22 Torr High VOLUME 40 Torr Normal 70% of blood volume at or ABOVE heart level Low 100 Torr Schematic illustration of gravitationally dependent PRESSURE distribution of blood volume and venous pressures in upright human compared with that in the dog. 4