THE MICROCIRCULATION CAPILLARIES Lecture – 11 Dr. Zahoor Ali Shaikh

CAPILLARIES Capillaries are exchange vessels, they exchange the materials between blood and tissue cells. Materials are exchanged across capillaries walls mainly by Diffusion. Capillary walls are very thin, 1 µm in thickness and have diameter of 7µm [human hair has diameter of 100µm].

CAPILLARIES Capillary has single layer of endothelial cells and thin basement membrane. Capillary also has pores through which water and soluble material can pass. RBC is 8µm, therefore, RBC has to squeeze through a single file. No cell is away from the capillary more than 0.01cm.

CAPILLARIES Capillaries are 10-40 billion. Despite this large number, capillary have 5% of total blood volume i.e. 250ml of blood out of 5000ml. Total surface area is very big 600m2. Velocity of blood flow [cm/unit time] is slow because velocity of flow is inversely proportional to the total cross-sectional area and capillaries have very large cross-sectional area.

CAPILLARIES The most purposeful function of the circulation occurs in the microcirculation, this is transport of O2 and nutrition to the tissues and removal of CO2 and waste products.

‘PORES’IN THE CAPILLARY MEMBRANE In many capillaries, endothelial cells are closely joined but have pores at the junctions between the cells. Pores are about 4nm wide. Water soluble substance pass through the pores and lipid soluble substance pass through the endothelial cells. Plasma protein generally can not pass through the capillary wall.

‘PORES’IN THE CAPILLARY MEMBRANE Special types of pores occur in the capillaries of certain organs. Some of these characteristics are 1. In the brain – cells are joined by tight junctions, therefore, no pores are there and capillaries work as blood brain barrier. 2. In the kidney and intestine – pores are 20- 100nm called Fenestration [means windows]. They allow rapid movement of fluid across the capillaries during formation of urine and absorption of digested meal. 3. In the liver – Endothelial cells of capillaries are discontinuous, therefore, gap between cells is from 10nm – 1000nm creating very large pores. Capillaries form sinusoids [large channels].

‘PORES’IN THE CAPILLARY MEMBRANE 3. In the liver [cont] – From these large pores, protein can pass. The advantage is that as liver synthesizes plasma protein, therefore, they must pass through liver capillaries. FLOW OF BLOOD IN THE CAPILLARIES Blood flow through the capillaries is not continuous, but intermittent, turning on and off every few seconds or minutes. It is called Vasomotion.

FLOW OF BLOOD IN THE CAPILLARIES Capillaries have pre-capillary sphincter, which is ring of smooth muscle at the area where capillary arises. This pre-capillary sphincter opening is affected by the concentration of O2 in the tissues. Only about 10% of pre-capillary sphincter are open in a resting muscle at any time, therefore, blood is flowing through only 10% of muscle capillaries.

INTERSTITIAL FLUID Exchange between blood capillaries and tissue cells are not made directly, but cells are surrounded by interstitial fluid, the internal environment of cells. Therefore, cells exchange material directly with interstitial fluid by Diffusion or Active Transport. But exchange across capillary wall and interstitial fluid are largely passive and composition of plasma and interstitial fluid is same except plasma protein which can not pass through capillary wall.

BULK FLOW ACROSS CAPILLARY WALL Volume of protein free plasma is actually filtered out of capillary, mixes with surrounding interstitial fluid and then is reabsorbed. This process is called Bulk Flow. We call bulk flow because various constituents of fluid are moving together in bulk. When pressure inside the capillary is high than pressure outside, fluid is pushed outside through capillary pores and this process is called Ultra Filtration.

BULK FLOW ACROSS CAPILLARY WALL When inward driving pressure in the capillary is high than outward pressure, than inward movement of fluid from the interstitial fluid takes place through the pores. This process is called Re-absorption. Forces Influencing the Bulk Flow Bulk flow occurs because of differences in the hydrostatic and colloidal osmotic pressures between plasma and interstitial fluid.

Forces Influencing the Bulk Flow Four Forces Influence Fluid Movement across the Capillary wall: i). Capillary Blood Pressure [Pc] It is hydrostatic pressure in the capillary, exerted by the blood. This pressure forces the fluid out of capillary into the interstitial fluid. Hydrostatic pressure at the arteriolar end of capillary is 37mmHg. Hydrostatic pressure at the venous end of capillary is 17mmHg.

Forces Influencing the Bulk Flow ii). Plasma-colloid Osmotic Pressure [πP] It is also called Oncotic Pressure, it is the force due to plasma protein. It moves the fluid into the capillaries. Plasma-colloidal Osmotic Pressure is about 25mmHg. iii). Interstitial Fluid hydrostatic Pressure [PIF] This pressure is exerted by interstitial fluid on the capillary wall. This pressure forces the fluid into the capillaries. It is 1 mmHg.

Forces Influencing the Bulk Flow iv). Interstitial Fluid-colloid Osmotic Pressure [πIF] This pressure is due to plasma protein which leak into the interstitial fluid. But these protein are returned to the blood by the lymphatics. Therefore, interstitial fluid-colloid osmotic pressure is ZERO. These pressures are referred as Starling Equilibrium as E .H .Starling pointed out over a century ago.

Forces Influencing the Bulk Flow SUMMARY Two pressures that tend to force fluid out of capillary are i). Capillary Blood Pressure [Pc] ii). Interstitial Fluid-colloid Osmotic Pressure [πIF] Two pressures which are forcing the fluid to get into the capillary are i). Plasma-colloidal Osmotic Pressure [πP] ii). Hydrostatic Pressure of Interstitial Fluid [PIF]

Forces Influencing the Bulk Flow As we see in the diagram - Ultra Filtration pressure at the arteriolar end of capillary is 11 mmHg. - At the venous end, Reabsorption occurs and net inward pressure is 9 mmHg. Therefore, under normal circumstances, slightly more fluid is filtered in the interstitial fluid then reabsorbed.

LYMPHATIC SYSTEM As we saw slightly more fluid is filtered, this extra fluid is due to pressure imbalance, this fluid is absorbed by Lymphatic system. What are Lymphatic Vessels? Lymphatics are one way vessels which return the extra fluid from the interstitial fluid to the blood. Lymphatics are small blind ended terminal vessels which are present in every tissue of the body. Once interstitial fluid enters the lymphatic vessels, it is called Lymph.

FUNCTIONS OF LYMPHATIC SYSTEM Large particles e.g. plasma protein, bacteria which have escaped from the capillary wall into the interstitial fluid, go to lymphatics. Small lymphatics form large lymph vessels which empty into venous system. FUNCTIONS OF LYMPHATIC SYSTEM 1. Return of excess filtered fluid [20 liters are filtered and 17 liters are reabsorbed in the capillary]. Therefore, 3 liters are returned to circulation through Lymphatic reabsorption.

FUNCTIONS OF LYMPHATIC SYSTEM [cont] 2. Defense against Disease – as lymph passes through the lymph nodes, bacteria are destroyed by special phagocytes present within the lymph node. 3. Return of filtered protein – in the capillaries, some plasma protein are leaked during filtration. These protein pass to lymphatics and are returned to circulation.

EDEMA Edema is the abnormal collection of fluid in the interstitial space. CAUSES OF EDEMA 1. Reduced Plasma Protein - Kidney Disease [loss of protein in urine] - Liver Disease [less synthesis of plasma protein] - Diet deficient in protein - Burns [Loss of protein]

CAUSES OF EDEMA 2. Increased Venous Pressure at the Venous end of the capillary e.g. congestive heart failure. - Edema in legs and feet during pregnancy due to decrease venous return because of compression of leg veins by enlarged uterus. 3. Increased Permeability of capillary wall – it will allow more leakage of fluid and protein from plasma to interstitial fluid e.g. histamine effect in allergic reactions like Bee Sting. 4. Blockage of Lymph vessels – Excess filtered fluid will not be drained.

PERCENTAGE OF TOTAL BLOOD VOLUME IN DIFFERENT PARTS OF THE CIRCULATORY SYSTEM

WHAT YOU SHOULD KNOW FROM THIS LECTURE Microcirculation Structure of the Capillary Function of the Capillary Blood Flow in the Capillary Interstitial Fluid Pressures – At the Arterial & Venous End of the Capillary and colloid Osmotic Pressure Lymphatic System and its Functions Edema - Causes

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