BSC 2086 1st Edition Lecture 12 Outline of Last Lecture I. Introduction to Blood Vessels and CirculationII. Structure of Blood VesselsIII. ArteriesIV. Capillaries Outline of Current Lecture I. Capillaries ContinuedII. VeinsIII. Blood Distribution in Vessels IV. Pressure and ResistanceV. Arterial Blood PressureVI. Venous Pressure and ReturnVII. Capillary Pressures Current LectureI. Capillaries Continued a. Thoroughfare Capillariesi. Direct capillary connections between the arterioles and the venulesii. Controlled by metarterioles smooth muscle segments b. Collaterals i. Multiple arteries contributing to one capillary bedii. Allows circulation if one artery is blocked prevents too much damageiii. Arterial anastomosis 1. Union of 2 collateral arteriesiv. Arteriovenous anastomoses 1. Direct connectors of arterioles and venules which bypass the capillary bedc. Angiogenesisi. The making of new blood vesselsii. VEGF vascular endothelial growth factoriii. Location:1. Embryos as tissues and organs develop2. In response to factors released by hypoxic (oxygen starved) cellsa. Very important in cardiac muscle d. Vasomotion i. Cycle of relaxation and contraction of capillary sphincters ii. Allows the blood flow in the capillary beds to constantly change routes II. Veins a. General facts:i. Collect blood from the capillaries found in tissues and organsii. Take blood back to the heartiii. Larger in diameter than arteries but have thinner walls and lower blood pressure b. Types of veins:i. Venules 1. Very small veins that get blood from capillariesii. Medium-sized veins1. Have a thin tunica media and few smooth muscle cells2. Tunica externa has longitudinal bundles of elastic fibers iii. Large veins1. Have all 3 tunica layers2. Thick tunica externa and thin tunica mediac. Venous valves i. Folds of the tunica intima that prevent blood from flowing backwardsii. Compression pushes the blood towards the heart III. Blood Distribution in Blood Vesselsa. 30 – 35% of blood volume found in heart, arteries and capillariesb. 60 – 65% of blood volume found in venous systemi. 1/3 of this blood is found in large venous networks of liver, bone marrow and skinc. Capacitance of a blood vesseli. Relationship between blood volume in a vessel and blood pressureii. Capacitance vessels (veins) stretch more than arteries iii. High capacitance = expands easily at low pressure like a rubber ballooniv. Low capacitance = expands only with high pressure like a tire d. Blood loss response of veinsi. Sympathetic nerves stimulated by vasomotor centers 1. Venoconstriction: systemic veins contrict2. Veins found in liver, skin and lungs redistribute venous reserve a. Venous reserve = 20% of total blood volume IV. Pressure and Resistancea. Total capillary blood blowi. = Cardiac outputii. Determined by pressure and resistance b. Pressure (P)i. Generated to overcome resistance (R)ii. Pressure Gradient (ΔP) difference between pressure at one end of vessel to another end1. Circulatory pressure2. More important than absolute pressurec. Flow (F)i. Proportional to the pressure gradientii. Divided by Rd. Measuring Pressurei. Blood pressure (BP)1. Pressure in arteries (mm Hg)ii. Capillary hydrostatic pressure (CHP)1. Pressure within capillary bedsiii. Venous pressure1. Pressure in veins e. Circulatory pressurei. ΔP across the systemic circuit 1. About 100 mmHgii. Must overcome total peripheral resistance (R of entire cardiovascular system)1. Total peripheral resistance due to:a. Vascular resistance i. Friction between blood and vessel wallsii. Depends on length and diameter of vessel1. Vessel length Is constant in adults (angiogenesis does not normally occur in adults except in those with cancer)2. Diameter varies by vasodilation and vasoconstriction 3. R increases exponentially as diameter decreases because the greater the diameter, the less blood coming into contactwith the vessel walls, therefore the less friction b. Viscosity i. R caused by molecules and materials suspended in liquidii. Blood viscosity is 4x that of wateriii. Can be affected by anemia, polycythemiac. Turbulencei. Swirling action that will disrupt the flow of liquidsii. Occurs in the great vessels and the heart chambersiii. Will hardly occur in small vessels unless their walls are damaged 1. Atherosclerotic plaques f. Relationships of Blood Circulationi. Flow is directly proportional to blood pressure (BP), and inversely proportional to peripheral resistance (PR)ii. Resistance is inversely proportional to the 4th power of the vessel radius V. Arterial Blood Pressure a. Systolic pressure vs. Diastolic Pressurei. Systolic: peak arterial pressure during systole ii. Diastolic: minimum arterial pressure 1. Although there is no force pushing from the ventricles, there is still pressure due to the vena cava and aorta being elastic arteries. In order to return to original shape they squeeze remaining blood and use their pressure reservoir. a. This is the reason why the diastolic pressure doesn’t drop to zeroiii. Pulse pressure: difference between systolic and diastolic pressures iv. Mean arterial pressure (MAP)1. MAP = diastolic pressure + 1/3 pulse pressure a. Example: if BP is 120/90 theni. MAP = 90 + 1/3(120-90) = 100 mmHg b. Abnormal BP i. Normal = 120/80 (systolic pressure /diastolic pressure)ii. Hypertension = greater than 140/901. Abnormally high BPiii. Hypotension is abnormally low BP 1. Usually caused by overly aggressive drug treatment of hypertensionc. Elastic reboundi. Arterial walls stretch during systole and rebound during diastole ii. Allows blood to keep flowing during diastoled. Pressure in small arteries and arteriolesi. Pressure and distance relationship1. MAP and pulse pressure decrease as the vessel gets farther from the heart2. BP decreases with friction3. Pulse pressure decreases due to elastic rebound VI. Venous Pressure and Returna. Indicator of how much blood will arrive at the atrium each minuteb. Low effective pressure in the venous systemc. Low resistance due to:i. Muscular compression of peripheral veins1. Blood pushed towards heart by skeletal muscle compressiona. One way valvesii. Respiratory pump 1. Action of the thoracic cavity2. Inhaling decreases the thoracic pressure a. Pulls air into lungs and the blood into the inferior vena cava3. Exhaling increases thoracic pressure a. Venous blood pushed into right atriumVII. Capillary Pressuresa. Vital to homeostasisb. Materials move across the capillary walls by:i.
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