Lecture 14 - Capillary Exchange PDF

Body Systems The Cardiovascular System BIOL10811 Lecture 14: Capillary Exchange Dr Nick Stafford Division of Diabetes, Endocrinology & Gastroenterology School of Medical Sciences Faculty of Biology, Medicine and Health [email protected] November 2024  Objectiv...

Body Systems The Cardiovascular System BIOL10811 Lecture 14: Capillary Exchange Dr Nick Stafford Division of Diabetes, Endocrinology & Gastroenterology School of Medical Sciences Faculty of Biology, Medicine and Health [email protected] November 2024  Objectives A successful student should be able to Describe the processes through which substances are exchanged at the capillary Understand the dynamics of capillary exchange and the determinants of net filtration pressure Describe the circulation in organs with special considerations such as heart, lungs and brain  Content 1) Introduction to Capillary Exchange 2) Capillary Structure 3) Transport Mechanisms in Capillary Exchange 4) Net Filtration Pressure Net Hydrostatic Pressure Net Osmotic Pressure 5) The Dynamics of Capillary Exchange 6) Special Circumstances for Capillary Exchange and Blood Flow Pulmonary Circulation Coronary Circulation Cerebral Circulation What substances/molecules are exchanged at the capillary?  1) Introduction to Capillary Exchange Blood flow to tissues is regulated by the SNS and local metabolic factors… …as a minimum pressure (capillary hydrostatic pressure) is required to exchange substances/fluids across capillary networks Capillary Exchange O2 & Nutrients Cells rely on capillary exchange to: Waste Obtain nutrients and oxygen Remove metabolic wastes eg CO2 Capillaries are the only site at which exchange can occur between blood and interstitial fluid  Content 1) Introduction to Capillary Exchange 2) Capillary Structure Why are capillaries good sites for substance exchange?  2) Capillary Structure Capillary vessels and beds are perfectly designed for the 2-way process of exchange Short distance for Large surface area for Blood flows slowly diffusion exchange  Large cross-sectional  Thin walls (1µm)  10+ billion capillaries  Small diameter (8µm) area of capillary network  ~600m2  Close proximity to cells  2) Capillary Structure Continuous (most capillaries) Fenestrated (endocrine organs, intestine, kidneys) Permeability Water-filled pores → rapid exchange of water and solutes Sinusoid (endocrine organs, liver, bone marrow, spleen) Large clefts between endothelial cells and incomplete BM → free exchange of water and larger solutes eg plasma proteins  Content 1) Introduction to Capillary Exchange 2) Capillary Structure 3) Transport Mechanisms in Capillary Exchange  3) Transport Mechanisms in Capillary Exchange Substance exchange across capillary walls occurs as molecules move down concentration or pressure gradients The principal transport mechanisms involved are: Diffusion Bulk Flow Transcytosis Down concentration gradient Down pressure gradient By vesicular transport  3) Transport Mechanisms in Capillary Exchange Capillary Lumen Diffusion Transcytosis Bulk Flow Through EC Through Between Through clefts/pores In vesicles membrane channels ECs Reabsorption Water Filled Pore Endothelial Cleft Cell Filtration Lipid-soluble Smaller Larger Ions Small water- Macro gases & soluble molecules Water-soluble molecules Eg Na+ molecules molecules K+ Ca2+ Eg Eg O2 CO2 Cl- Eg Water Ions Glycoproteins Eg Water Ions Fatty acids Glucose Urea Nutrients Waste Amino Acids Interstitial Fluid  Content 1) Introduction to Capillary Exchange 2) Capillary Structure 3) Transport Mechanisms in Capillary Exchange 4) Net Filtration Pressure Net Hydrostatic Pressure Net Osmotic Pressure  4) Net Filtration Pressure Bulk flow between capillaries and interstitial fluid is determined by the net pressure difference across capillary walls Four forces influence fluid & solute movement Capillary Hydrostatic Blood Colloid Pressure Osmotic Pressure The blood pressure exerted The plasma osmotic on capillary walls “pushing” pressure “pulling” fluid in fluid out Interstitial Fluid Hydrostatic Pressure Interstitial Fluid Colloid Osmotic Pressure The pressure exerted on the outer capillary The osmotic pressure of walls by the IF, the interstitial fluid “pushing” fluid in “pulling” fluid out Whether fluid is driven out of or into the capillary depends upon The Net Filtration Pressure  4) Net Filtration Pressure Net Filtration Net Hydrostatic _ Net Osmotic Pressure = Pressure Pressure CHP _ BCOP IHP ICOP  Net Hydrostatic Pressure Generally Decreases negligible along capillary (~0 mmHg) Net Hydrostatic Capillary Hydrostatic _ Interstitial Fluid Pressure = Pressure (CHP) Hydrostatic Pressure (IHP) Favours “pushing” fluid from capillary to IF Filtration CHP BCOP IHP ICOP  Net Osmotic Pressure Generally Affected by negligible blood volume (~0 mmHg) Net Osmotic Blood Colloid _ Interstitial Fluid Colloid Pressure = Osmotic Pressure Osmotic Pressure (BCOP) (ICOP) IF has few plasma proteins in suspension Favours “pulling” fluid into capillary from IF Reasbsorption CHP BCOP IHP ICOP  Content 1) Introduction to Capillary Exchange 2) Capillary Structure 3) Transport Mechanisms in Capillary Exchange 4) Net Filtration Pressure Net Hydrostatic Pressure Net Osmotic Pressure 5) The Dynamics of Capillary Exchange  5) The Dynamics of Capillary Exchange Net Hydrostatic Net Osmotic _ Net Filtration Pressure Pressure = Pressure CHP - IHP BCOP - ICOP CHP > BCOP +ve NFP Filtration CHP < BCOP -ve NFP Reasbsorption +ve NFP -ve NFP Reabsorptio Filtration n  5) The Dynamics of Capillary Exchange CHP declines from BCOP is constant arterial to venous end along capillary 35 18 mmHg 25 mmHg CHP = BCOP From arteriole No net To venule movement CHP > BCOP CHP < BCOP 35 30 25 25 18 25 25 25 Absorption into Filtration out of capillary capillary Nutrients to tissues Waste from tissues  5) The Dynamics of Capillary Exchange To Lymphatic System 3.6 L/day Filtration Reabsorption 24 L/day 20.4 L/day Max filtration pressure… …is greater than max absorption pressure So the transition point is located towards the venous end More filtration takes place along the capillary than absorption What happens to net filtration pressure in someone with hypertension?  When Net Filtration Pressure Changes… Typical values CHP 35 - IHP 0 _ BCOP ICOP 25 - 0 = NFP 10 At arterial end Hypertension CHP 45 - IHP 0 _ BCOP ICOP 25 - 0 = NFP 20 Increased filtration  Fluid collects in extremities  Systemic Oedema What happens to net filtration pressure after severe haemorrhage?  When Net Filtration Pressure Changes… Typical values CHP 18 - IHP 0 _ BCOP ICOP 25 - 0 = NFP -7 At venous end Haemorrhage CHP 5 - IHP 0 _ BCOP ICOP 25 - 0 = NFP -20 Increased reabsorption  Fluid recalled from tissue into bloodstream  Increases BP & CO What happens to net filtration pressure when you are dehydrated?  When Net Filtration Pressure Changes… Typical values CHP 18 - IHP 0 _ BCOP ICOP 25 - 0 = NFP -7 At venous end Dehydration CHP 18 - IHP 0 _ BCOP ICOP 30 - 0 = NFP -12 Increased reabsorption  Fluid recalled from tissue into bloodstream  Delays onset of symptoms  What happens to net filtration pressure when you have tissue damage (eg ankle sprain)?  When Net Filtration Pressure Changes… Typical values CHP 18 - IHP 0 _ BCOP ICOP 25 - 0 = NFP -7 At venous end Tissue Damage CHP 18 - IHP 0 _ BCOP ICOP 25 - 10 = NFP +3 Increased filtration  Plasma proteins leak into IF increasing ICOP  Local swelling (oedema)  Content 1) Introduction to Capillary Exchange 2) Capillary Structure 3) Transport Mechanisms in Capillary Exchange 4) Net Filtration Pressure Net Hydrostatic Pressure Net Osmotic Pressure 5) The Dynamics of Capillary Exchange 6) Special Circumstances for Capillary Exchange and Blood Flow Pulmonary Circulation Coronary Circulation Cerebral Circulation 13 ure ct 6) Special Circumstances: Pulmonary Circulation Le Enhances O2 absorption Enhances O2 delivery Pulmonary circulation supplies >300 million alveoli  6) Special Circumstances: Pulmonary Circulation The pulmonary circulation must enable gas exchange to occur constantly Pulmonary vascular CHP is lower than in Arteries are more resistance is very low systemic circulation distensible Arterioles are shorter, wider and 10 mmHg rather than 35 mmHg Can accommodate increased CO have thinner walls with little increase in pressure CHP < BCOP Fluid absorbed along entire capillary length 6) Special Circumstances: Pulmonary Circulation If pulmonary CHP exceeds 25 mmHg… Fluid leaks into alveoli Pulmonary Oedema impacting respiration  Special Circumstances: Coronary Circulation 13 ure ct Le SNS/Adrenaline Promotes coronary artery vasodilation Coronary flow increases when vasoconstriction predominates elsewhere  Special Circumstances: Coronary Circulation Coronary blood flow is affected by Coronary flow is highest the cardiac cycle during diastole enabled Coronary flow is by arterial elastic recoil restricted during systole due to compression of the left coronary artery To compensate:  Cardiomyocytes have high O2 reserves  The myocardium has high capillary density → increases O2 extraction  Special Circumstances: Cerebral Circulation 13 ure ct Le The brain:  Consumes 12% CO for 2% body mass  Flow rate = 750 ml/min  Neurons have poor metabolic reserves  4 arteries used to supply the brain which anastomose inside cranium  Flow can be maintained if there is a disruption  Objectives A successful student should be able to  Describe the processes through which substances are exchanged at the capillary  Understand the dynamics of capillary exchange and the determinants of net filtration pressure  Describe the circulation in organs with special considerations such as heart, lungs and brain