Microcirculation and Lymphatics PDF

MICROCIRCULATION AND LYMPHATICS Reference: Ch: 16, Guyton & Hall (14th Ed.) Ch: 4, Linda S. Costanzo (6th Ed.) Dr. Waqas Hameed MBBS MPhil PhD  Sess...

MICROCIRCULATION AND LYMPHATICS Reference: Ch: 16, Guyton & Hall (14th Ed.) Ch: 4, Linda S. Costanzo (6th Ed.) Dr. Waqas Hameed MBBS MPhil PhD  Session Learning Objectives At the end of this session, students will be able to: 1. Describe the functional anatomy of the microcirculation 2. Explain the role of arteriolar vasomotion in determining capillary flow 3. Describe main solute exchange mechanisms that move solutes across the capillary wall 4. Describe the relation between diffusion equilibrium time and transit time in capillaries 5. Describe what determines net fluid movement across capillaries 6. Summarize the Starlings forces that decide bulk flow of water across capillary walls 7. Describe factors promoting tissue interstitial fluid formation 8. Describe functions of lymph vessels & summarize causes of edema  The Microcirculation Consist of: ▪ arterioles ▪ numerous “exchange vessels” (capillaries & postcapillary venules) ▪ larger collecting venules Blood vessels which allow exchange: Terminal arterioles only little oxygen Capillaries All solutes & water Post-capillary venules only little water (increased during inflammation)  ARTERIOLES Major site for regulating systemic vascular resistance - Arteriolar resistance regulates capillary flow - have basal tone - undergoes rhythmic variation (contraction & relaxation) – Vasomotion VASOMOTION Intermittent contraction of the metarterioles & precapillary sphincters depends on conc. of O2 ↑ O2 usage by tissue - tissue oxygen conc. decreases below normal ↑ intermittent periods of capillary blood flow  CAPILLARIES Large surface area Relatively high permeability to fluid & macromolecules Primary site of exchange for fluid, electrolytes, gases & macromolecules  CAPILLARY TYPES CONTINUOUS Basement membrane - continuous Intercellular clefts - tight (i.e., have tight junctions) - have the lowest permeability Found in: Skeletal muscle Myocardium Connective tissue CNS  CAPILLARY TYPES FENESTRATED Perforations (fenestrae) in endothelium Relatively high permeability Found in: Kidneys Glandular tissue Intestinal mucosa  CAPILLARY TYPES DISCONTINUOUS / SINUSOIDS Large intercellular gaps & gaps in basement membrane Extremely high permeability Permits WBC & RBC to enter & exit Found in: Liver Spleen Bone marrow  Venules & Terminal Lymphatics Venules Fluid & macromolecular exchange at venular junctions Sympathetic innervation - alters venular tone thus regulating CAPILLARY HYDROSTATIC PRESSURE Terminal Lymphatics Endothelium with intercellular gaps surrounded by highly permeable basement membrane Spontaneous & stretch-activated vasomotion is present which serves to "pump" lymph Interstitium Space b/w cells Interstitial Fluid Fluid in this space  LYMPHATICS Lymphatics - an accessory route by which fluid & protein can flow from interstitial spaces to the blood Important in preventing Edema Major route for absorption of nutrients from GI tract An impt. role in immune system Capillary recruitment More blood into area Maintains diffusion gradient Increases surface area for bulk flow & diffusion Reduces diffusion distance Reduces diffusion time Endothelial cell permeability Increases diffusion rate  Exchange of Solutes across Capillary Wall Lipid-soluble molecules (e.g., O2 & CO2) Trans-membrane diffusion Lipid-insoluble small solutes (e.g., hormones, glucose & water) Inter-cellular pathways Fenestrations Lipid-insoluble large solutes (e.g., plasma proteins) Vesicular transport (endocytosis, exocytosis) Trans-endothelial channels Inflammation-induced endothelial gaps Four general mechanisms by which materials are exchanged across the capillary wall  Bulk Transfer (Flow) across Capillary Wall Flow across capillary wall Water moves (flows) through inter-cellular clefts & cellular pores (fenestrae) Dissolved small solutes also carried with water flow Water movement (bulk flow, filtration) across capillary wall determined by ‘Starling Forces’  HYDROSTATIC PRESSURE vs OSMOTIC PRESSURE HYDROSTATIC PRESSURE - forces fluid & dissolved substances through capillary pores into interstitial spaces OSMOTIC PRESSURE - caused by plasma proteins (colloid osmotic pressure) - causes fluid movement by osmosis from interstitial spaces into blood Determinants of Net Fluid Movement across Capillaries Capillary hydrostatic pressure (Pc) ▪ PROMOTES filtration ▪ tends to force fluid outward through the capillary membrane Interstitial fluid pressure (Pif) ▪ OPPOSES filtration when value is positive Plasma Colloid Osmotic Pressure (πp) Colloid Osmotic Pressure of ▪ OPPOSES filtration Plasma ▪ osmosis of water inward through membrane Albumin - 75% Globulin - 25% Interstitial fluid colloid pressure (πif) ▪ PROMOTES filtration ▪ causes osmosis of fluid outward through membrane  STARLING FORCES Net filtration pressure (NFP) = Kf × (PC – Pif) – ( πP – πif)  STARLING FORCES Net filtration pressure (NFP) = Kf × (PC – Pif) – ( πP – πif) If NET FILTRATION PRESSURE (sum of these forces) is POSITIVE - there will be net fluid filtration across the capillaries If SUM OF STARLING FORCES is NEGATIVE - there will be net fluid absorption from interstitial spaces into capillaries Filtration rate = net filtration pressure (NFP) multiplied by filtration coefficient NET FILTRATION - slight excess of filtration, this fluid must be returned to the circulation through lymphatics Lymph - derived from interstitial fluid that flows into lymphatics 2-4 liters/day returned LYMPHATIC SYSTEM Relationship between initial lymphatics & blood capillaries Relationship of lymphatic system to circulatory system Arrangement of endothelial cells in an initial lymphatic Lymphatic capillaries: Closed-end tubules that form vast networks in intercellular spaces One-way valves direct lymph away from tissue & back into systemic circulation via thoracic duct & subclavian veins  Factors that Determine Lymph Flow ↑ Interstitial fluid hydrostatic pressure - ↑ Lymph flow Two primary factors: 1. The Interstitial Fluid Pressure 2. Activity of the Lymphatic Pump Rate of lymph flow = (interstitial fluid pressure) X (activity of lymphatic pump)  Lymphatic Pump Increases Lymph Flow Collecting lymphatic vessels or larger lymph vessel stretched with fluid, the smooth muscle in wall of vessel automatically contracts Each segment of lymph vessel between successive valves functions as a separate automatic pump  External Intermittent Compression of Lymphatics Helps Lymphatic pump activity Any external factor that intermittently compresses the lymph vessel also can cause pumping e.g., 1. Contraction of surrounding skeletal muscles 2. Movement of the parts of the body 3. Pulsations of arteries adjacent to the lymphatics 4. Compression of the tissues by objects outside the body During Exercise - lymphatic pump very active During periods of rest - lymph flow sluggish to almost zero  Functions of lymphatic system Transports interstitial (tissue) fluid back to the blood Maintains volume of Interstitial fluid Concentration of proteins in Interstitial fluids Immunological defenses against pathogens Transports absorbed fat from small intestine to blood Interstitial fluid pressure  EDEMA An abnormal collection of fluid in tissue spaces & cavities Factors causing edema Increased formation of tissue interstitial fluid Increased capillary pressure Reduced capillary (plasma) oncotic pressure Due to decreased concentration of plasma proteins Increased tissue oncotic pressure Increased capillary permeability Reduced drainage of interstitial fluid Blocked lymphatics Surgical removal of lymph nodes  Diseases/Conditions leading to Edema Increased net hydrostatic pressure in capillaries Arteriolar dilatation Venous obstruction Heart failure Reduced net plasma oncotic pressure Protein deficiency (less intake/synthesis, excessive loss)  Diseases/Conditions leading to Edema Increased capillary permeability Inflammation Reduced lymph flow Blocked lymphatics e.g., filariasis after surgical removal of lymph nodes in a region JazakAllah