5.1 The Microcirculation and Lymphatic System.pptx

Microcirculation and Lymphatic System: Capillary Fluid Exchange, Interstitial Fluid, and Lymph Flow Lecture Outline I. Structure of the microcirculation and capillary system II. Flow of Blood in the Capillaries- Vasomotion III. Exchange of Water, Nutrients, and Other Substances Between the Blood and...

Microcirculation and Lymphatic System: Capillary Fluid Exchange, Interstitial Fluid, and Lymph Flow Lecture Outline I. Structure of the microcirculation and capillary system II. Flow of Blood in the Capillaries- Vasomotion III. Exchange of Water, Nutrients, and Other Substances Between the Blood and Interstitial Fluid IV. Interstitium and Interstitial Fluid V. Fluid Filtration Across Capillaries is Determined by Hydrostatic and Colloid Osmotic Pressures and the Capillary Filtration Coefficient 1 Microcirculation and Lymphatic System: Capillary Fluid Exchange, Interstitial Fluid, and Lymph Flow Objectives 1. Know the structure and function of the blood vessels and microcirculation 2. Explain vasomotion in the microcirculation 3. Know what determines net fluid movement (bulk flow) across capillaries (Starling Forces) 4. Describe how excess fluid is returned to circulation via the lymphatic system 5. Describe how edema is formed in the interstitium 2 References Assigned reading from your text: Hall Chapter 16 Selected images from Moore Chapter 1 3 Tunics of Blood Vessels  Three tunics and two main cell types present in arteries and veins: • Tunica adventitia – outer connective tissue sheath • Tunica media – smooth muscle • Tunica intima – layer of endothelium Microcirculation 4 Vessel Types  • • • Arteries- high pressure reservoirs contain ~20% of blood Large elastic arteries- (eg aorta) receive cardiac output Medium muscular arteries- regulate blood flow Small arteries and arterioles- capillary filling and arterial pressure regulated by degree tonus here  Veins- low pressure reservoirs contain ~80% of blood • Venules- smallest veins • Medium veins of limbs- passive valves prevent retrograde flow – No valves in the small veins, SVC, IVC, or pulmonary veins, or veins from brain and viscera • Large veins- have wide bundles of longitudinal smooth muscle and a welldeveloped adventitia (eg the SVC)  Capillaries are thin-walled vessels that provide a large surface area for exchange • Composed of a single layer of endothelial cells • At any time- only about 5% circulating blood in capillaries • The most important 5% blood volume- exchange of O2, CO2, nutrients here 5 The Function of the Microcirculation  Over 10 billion capillaries with surface area of 500–700 square meters perform the function of solute and fluid exchange • Transports nutrients to tissues • Removes waste from tissues Vasomotion  Blood flow is intermittent in capillaries • Vasomotion- intermittent contraction of metarterioles and precapillary sphincters • Capillaries collapsed in resting tissue • Dilate when active- due to metabolites • Not innervated • Lack of O2 most important factor in increasing periods of capillary flow in most tissue • Excess CO2 in brain • Empty when stimulated mechanically (white reaction) due to sphincters Ganong- FIGURE 31–16 The microcirculation. Arterioles give rise to metarterioles, which give rise to capillaries. The capillaries drain via short collecting venules to the venules. The walls of the arteries, arterioles, and small venules contain relatively large amounts of smooth muscle. There are scattered smooth muscle cells in the walls of the metarterioles, and the openings of the capillaries are guarded by muscular precapillary sphincters. The diameters of the various vessels are also shown. Structure of Capillary Wall • Unicellular layer of endothelial cells surrounded by a basement membrane • Diameter of capillaries • ~5 micrometer diameter at arterial end • ~9 micrometer diameter at venous end • Solute and water move across capillary wall via intercellular cleft (space between cells) or by plasmalemma vesicles • These intercellular clefts may contain tight junctions Modes of Exchange Across Capillaries- and Types of Pores 1- Lipid soluble molecules pass across cells (O2, CO2) 2- Membrane carriers facilitate movement of ions across both surfaces of cells 3- Transcytosis moves large molecules by shuttling endocytic and exocytic vesicles 4- Paracellular tight junctions are a major source of small solute movement (eg ions) – Intercellular clefts between endothelial cells contain tight junctions 5- Fenestrations of glomerular capillaries permit large amounts of small molecules/ions through 6- Sinusoids- found in the liver and bone marrow- allow large plasma proteins to pass 9 Three Main Types of Capillaries • Continuousmost blood capillaries • Fenestrated- glomerular capillaries of kidney • Sinusoid- of liver 10 Passage of Molecules Across the Capillary Wall • Substances pass through endothelium by diffusion and filtration (bulk flow) • Diffusion is the most important means of transfer of substances between plasma and interstitial fluid – O2 and glucose higher in plasma and diffuse to interstitial fluid – CO2 diffuses in opposite direction – Concentration differences across capillary enhance diffusion • Bulk flow movement of relatively large volume of fluid and dissolved contents through pores – Result of different inward (osmotic) and outward (hydraulic) forces – Pressure gradient determines the direction • Filtration from capillaries into interstitium • Reabsorption from interstitium into capillaries Passage of Molecules Across the Capillary Wall • Lipid-insoluble substances such as H2O, Na, Cl, glucose cross capillary walls via intercellular clefts (~6–7 nanometers wide) – Water molecule is ~ .27 nanometers – RBC is ~ 7 micrometers (7000 nanometers) • The permeability of the capillary pores for different substances varies according to their molecular diameters – Permeability of pores in muscle capillaries: water =1.0, glucose = 0.6, albumin = 0.001 • The capillaries in different tissues have extreme differences in their permeabilities – Eg tight junctions of brain allow only water and small molecules to pass Interstitium and Interstitial Fluid • Space between cells is called interstitium; fluid in this space is called interstitial fluid. • Two major types of solid structures in interstitium are collagen fibers and proteoglycan filaments (coiled molecules composed of hyaluronic acid). • Almost all fluid in interstitium is in the form of gel (fluid proteoglycan mixtures); there is very little free fluid (edema) under normal conditions. Hydrostatic and Colloid Osmotic Forces Determine Fluid Movement Through the Capillary Membrane • • • • Rate of filtration at any point along the capillary depends on a balance of Starling forces Hydrostatic pressure gradient– the pressure exerted by fluid in the capillary minus the hydrostatic pressure of the interstitial fluid – Directed outward Osmotic pressure gradient– tendency of a solution to take in pure solvent – across the capillary wall is the colloid osmotic pressure of plasma minus the colloid osmotic pressure of the interstitial fluid – Directed inward Imbalance of hydrostatic and osmotic pressure gradients across walls cause edema 14 Net Filtration Pressure Net Filtration Pressure = Pc − πp − Pif + πif Hydrostatic Pressures • • • • Capillary hydrostatic pressure (Pc)—tends to force fluid outward through the capillary membrane Interstitial fluid pressure (Pif)—opposes filtration when value is positive Affected by arteriole resistance and venous pressure Pressures vary between arteriolar and venular end of capillary: – Fluid moves out of arteriolar end and into venular end of capillary Osmotic Pressures • Plasma colloid osmotic pressure—opposes filtration causing osmosis of water inward through the membrane • Interstitial fluid colloid pressure—promotes filtration by causing osmosis of fluid outward through the membrane Hydrostatic and Osmotic Pressure Determinants  Normal Capillary hydrostatic pressure is approximately 17 mm H  Interstitial fluid pressure in most tissues is negative 3. • Encapsulated organs have positive interstitial pressures (+5 to +10 mm Hg) • Negative interstitial fluid pressure is caused by pumping of  lymphatic Colloid osmotic pressure is caused by presence of large system proteins • 75% of the total colloid osmotic pressure of plasma results from the presenceg/dL of albumin and 25% is due to πp(mm Hg) globulins Albumin 4.5 21.8 Globulins Fibrinogen Total 2.5 0.3 7.3 6.0 0.2 28.0  Interstitial colloid osmotic pressure The interstitial colloid osmotic pressure is normally 8 mm Hg (3 gm/dL Analysis of the Forces Causing Filtration at the Arterial End of the Capillary Filtration 19 Analysis of Reabsorption at the Venous End of the Capillary 90% filtered fluid is reabsorbed on venous end 20 Net Starling Forces in Capillaries • Net filtration pressure of .3 mm Hg causes a net filtration rate • ~2 mL/min for entire body • Remaining 10% of filtered fluid is returned to circulation Lymphatic System (Hall) • An accessory route by which fluid and protein can flow from interstitial spaces to the blood • Important in preventing edema • Lymph is derived from interstitial fluid that flows into the lymphatics • Major route for absorption of nutrients from the GI tract • Plays important role in the immune system natomy of Lymphoid System (Moore) • Right lymphatic duct drains lymph from the body’s right upper quadrant • Right IJ and subclavian • Thoracic duct drains lymph from the remainder of the body • Left subclavian and IJ • The lymphatic trunks draining the lower half of the body merge in the abdomen, sometimes forming a dilated collecting sac, 23 Lymph communicates with Venous System  Right-sided central venous access avoids thoracic duct 24 How Does the Lymphatic Pump Affect Lymph Flow? • Initial lymphatic capillaries – – – – Are highly permeable without valves or smooth muscle Found in intestines or skeletal muscle Fluid enters through loose junctions between endothelial cells Fluid massaged by muscle contractions of organs and vessels • Collecting lymphatics – Have smooth muscle and valves – Aided by skeletal musculovenous pump, negative intrathoracic pressure during inspiration, and suction of high velocity flow of veins Musculovenous Pump • Contractions are a principal factor propelling lymph • The degree of activity of the lymphatic pump • Smooth muscle filaments in lymph vessel cause them to contract • External compression also contributes to lymphatic pumping 26 How Do Changes in Interstitial Hydrostatic Pressure Affect Lymph Flow? Interstitial fluid hydrostatic pressure Lymph flow Figure 16-8 5.1 1. A. B. C. D. Valves are found in which of the following: All veins All arteries All lymph vessels None of the above 2. A. B. C. D. The Starlingforce with the highest outwardly directed filtration pressure is: Capillary hydrostatic pressure Interstitial fluid pressure Plasma colloid osmotic pressure Interstitial fluid colloid osmotic pressure 3. A. B. C. D. Most lymph of the body returnsto circulation via the: Right lymphatic duct Thoracic duct Cisterna chyli Right internal jugular and subclavian veins 28 

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