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Structure and Function of Veins

Structure and Function of Veins. Veins Collect blood from capillaries in tissues and organs Return blood to heart Are larger in diameter than arteries Have thinner walls than arteries Have lower blood pressure. Structure and Function of Veins. Vein Categories Venules Very small veins

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Structure and Function of Veins

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  1. Structure and Function of Veins • Veins • Collect blood from capillaries in tissues and organs • Return blood to heart • Are larger in diameter than arteries • Have thinner walls than arteries • Have lower blood pressure

  2. Structure and Function of Veins • Vein Categories • Venules • Very small veins • Collect blood from capillaries • Medium-sized veins • Thin tunica media and few smooth muscle cells • Tunica externa with longitudinal bundles of elastic fibers • Large veins • Have all three tunica layers • Thick tunica externa • Thin tunica media

  3. Structure and Function of Veins • Venous Valves • Folds of tunica intima • Prevent blood from flowing backward • Compression pushes blood toward heart

  4. Structure and Function of Veins Figure 19–6 The Function of Valves in the Venous System

  5. Blood Vessels • The Distribution of Blood • Heart, arteries, and capillaries • 30–35% of blood volume • Venous system • 60–65%: • 1/3 of venous blood is in the large venous networks of the liver, bone marrow, and skin

  6. Blood Vessels Figure 19–7 The Distribution of Blood in the Cardiovascular System

  7. Blood Vessels • Capacitance of a Blood Vessel • The ability to stretch • Relationship between blood volume and blood pressure • Veins (capacitance vessels) stretch more than arteries

  8. Blood Vessels • Venous Response to Blood Loss • Vasomotor centers stimulate sympathetic nerves • Systemic veins constrict (venoconstriction) • Veins in liver, skin, and lungs redistribute venous reserve

  9. Pressure and Resistance Figure 19–8 An Overview of Cardiovascular Physiology

  10. Pressure and Resistance • Pressure (P) • The heart generates P to overcome resistance • Absolute pressure is less important than pressure gradient • The Pressure Gradient (P) • Circulatory pressure = pressure gradient • The difference between • Pressure at the heart • And pressure at peripheral capillary beds

  11. Pressure and Resistance • Force (F) • Is proportional to the pressure difference (P) • Divided by R

  12. Pressure and Resistance • Measuring Pressure • Blood pressure (BP) • Arterial pressure (mm Hg) • Capillary hydrostatic pressure (CHP) • Pressure within the capillary beds • Venous pressure • Pressure in the venous system

  13. Pressure and Resistance • Circulatory Pressure • ∆P across the systemic circuit (about 100 mm Hg) • Circulatory pressure must overcome total peripheral resistance • R of entire cardiovascular system

  14. Pressure and Resistance • Total Peripheral Resistance (R) • Vascular R • Due to friction between blood and vessel walls • Depends on vessel length and vessel diameter: • adult vessel length is constant • vessel diameter varies by vasodilation and vasoconstriction: • R increases exponentially as vessel diameter decreases

  15. Pressure and Resistance • Viscosity • R caused by molecules and suspended materials in a liquid • Whole blood viscosity is about four times that of water

  16. Pressure and Resistance • Turbulence • Swirling action that disturbs smooth flow of liquid • Occurs in heart chambers and great vessels • Atherosclerotic plaques cause abnormal turbulence

  17. Pressure and Resistance

  18. Pressure and Resistance

  19. Pressure and Resistance

  20. Pressure and Resistance • An Overview of Cardiovascular Pressures • Systolic pressure • Peak arterial pressure during ventricular systole • Diastolic pressure • Minimum arterial pressure during diastole • Pulse pressure • Difference between systolic pressure and diastolic pressure • Mean arterial pressure (MAP) • MAP = diastolic pressure + 1/3 pulse pressure

  21. Pressure and Resistance • Abnormal Blood Pressure • Normal = 120/80 • Hypertension • Abnormally high blood pressure: • greater than 140/90 • Hypotension • Abnormally low blood pressure

  22. Pressure and Resistance • Elastic Rebound • Arterial walls • Stretch during systole • Rebound (recoil to original shape) during diastole • Keep blood moving during diastole

  23. Pressure and Resistance Figure 19–9 Relationships among Vessel Diameter, Cross-Sectional Area, Blood Pressure, and Blood Velocity.

  24. Pressure and Resistance • Pressures in Small Arteries and Arterioles • Pressure and distance • MAP and pulse pressure decrease with distance from heart • Blood pressure decreases with friction • Pulse pressure decreases due to elastic rebound

  25. Pressure and Resistance Figure 19–10 Pressures within the Systemic Circuit

  26. Pressure and Resistance • Venous Pressure and Venous Return • Determines the amount of blood arriving at right atrium each minute • Low effective pressure in venous system • Low venous resistance is assisted by • Muscular compression of peripheral veins: • compression of skeletal musclespushes blood toward heart (one-way valves) • The respiratory pump: • thoracic cavity action • inhaling decreases thoracic pressure • exhaling raises thoracic pressure

  27. Pressure and Resistance • Capillary Pressures and Capillary Exchange • Vital to homeostasis • Moves materials across capillary walls by • Diffusion • Filtration • Reabsorption

  28. Pressure and Resistance • Diffusion • Movement of ions or molecules • From high concentration • To lower concentration • Along the concentration gradient

  29. Pressure and Resistance • Diffusion Routes • Water, ions, and small molecules such as glucose • Diffuse between adjacent endothelial cells • Or through fenestrated capillaries • Some ions (Na+, K+, Ca2+, Cl-) • Diffuse through channels in plasma membranes

  30. Pressure and Resistance • Diffusion Routes • Large, water-soluble compounds • Pass through fenestrated capillaries • Lipids and lipid-soluble materials such as O2 and CO2 • Diffuse through endothelial plasma membranes • Plasma proteins • Cross endothelial lining in sinusoids

  31. Pressure and Resistance • Filtration • Driven by hydrostatic pressure • Water and small solutes forced through capillary wall • Leaves larger solutes in bloodstream

  32. Pressure and Resistance • Reabsorption • The result of osmosis • Blood colloid osmotic pressure • Equals pressure required to prevent osmosis • Caused by suspended blood proteins that are too large to cross capillary walls

  33. Pressure and Resistance Figure 19–11 Capillary Filtration

  34. Pressure and Resistance • Interplay between Filtration and Reabsorption • Hydrostatic pressure • Forces water out of solution • Osmotic pressure • Forces water into solution • Both control filtration and reabsorption through capillaries

  35. Pressure and Resistance • Net Hydrostatic Pressure • Is the difference between • Capillary hydrostatic pressure (CHP) • And interstitial fluid hydrostatic pressure (IHP) • Pushes water and solutes • Out of capillaries • Into interstitial fluid

  36. Pressure and Resistance • Net Colloid Osmotic Pressure • Is the difference between • Blood colloid osmotic pressure (BCOP) • And interstitial fluid colloid osmotic pressure (ICOP) • Pulls water and solutes • Into a capillary • From interstitial fluid

  37. Pressure and Resistance • Net Filtration Pressure (NFP) • The difference between • Net hydrostatic pressure • And net osmotic pressure NFP = (CHP – IHP) – (BCOP – ICOP)

  38. Pressure and Resistance • Capillary Exchange • At arterial end of capillary • Fluid moves out of capillary • Into interstitial fluid • At venous end of capillary • Fluid moves into capillary • Out of interstitial fluid • Transition point between filtration and reabsorption • Is closer to venous end than arterial end • Capillaries filter more than they reabsorb • Excess fluid enters lymphatic vessels

  39. Pressure and Resistance Figure 19–12 Forces Acting across Capillary Walls

  40. Pressure and Resistance • Fluid Recycling • Water continuously moves out of capillaries, and back into bloodstream via the lymphoid system and serves to • Ensure constant plasma and interstitial fluid communication • Accelerate distribution of nutrients, hormones, and dissolved gases through tissues • Transport insoluble lipids and tissue proteins that cannot cross capillary walls • Flush bacterial toxins and chemicals to immune system tissues

  41. Pressure and Resistance • Capillary Dynamics • Hemorrhaging • Reduces CHP and NFP • Increases reabsorption of interstitial fluid (recall of fluids) • Dehydration • Increases BCOP • Accelerates reabsorption • Increase in CHP or BCOP • Fluid moves out of blood • Builds up in peripheral tissues (edema)

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