BIOH 113 1st Edition Lecture 13Outline of Last Lecture I. Factors Affecting Stroke VolumeII. Regulation of Heart RateIII. Blood VesselsIV. CapillariesOutline of Current Lecture I. Capillary BedsII. Venous SystemIII. Vascular AnastomosesIV. Blood FlowV. ResistanceVI. Systemic Blood PressureVII. Arteriol Blood PressureVIII. Capillary Blood PressureIX. Venous Blood PressureX. Maintaining Blood PressureXI. Blood flow through TissuesXII. Capillary ExchangeCurrent LectureI. Capillary BedsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.a. Microcirculation of interwoven networks of capillaries consisting of:i. Vascular shunts: metarteriole-thoroughfare channel connecting an arteriole directly with a postcapillary venuleii. True capillaries: 10-100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bedb. Blood flow through capillary beds:i. Precapillary sphincter1. Cuff of smooth muscle that surrounds each true capillary2. Regulates blood flow into the capillaryii. Blood flow is regulated by vasomotor nerves and local chemical conditions, so it can either bypass or flood the capillary bedII. Venous Systema. Venules:i. Formed when capillary beds unite1. Allow fluids and white blood cells to pass from bloodstream to tissuesii. Postcapillary venules: smallest venules, composed of endothelium and some connective tissueiii. Large venules have one or two layers of smooth muscle (tunica media)b. Veins:i. Formed when venules convergeii. Composed of three tunics, with a thin tunica media and a thick tunica externa consisting of collagen fibers and elastic networksiii. Blood reservoirs that contain 65% of the blood supplyiv. Veins have much lower blood pressure and thinner walls than arteriesv. To return blood to the heart, veins have special adaptations:1. Large diameter lumens, which offer little resistance to flow2. Valves (resembling semilunar heart valves) which prevent backflow of bloodvi. Venous sinuses: specialized, flattened veins with extremely thin wallsIII. Vascular Anastomosesa. Merging blood vessels—more common in veins than arteriesb. Arterial anastomoses provide alternative pathways for blood to reach a given body regioni. If one branch is blocked, the collateral channel can supply the area with adequate blood supplyc. Thoroughfare channels are examples of arteriovenous anastomosesIV. Blood Flowa. Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period:i. Is measured in ml/minii. Is relatively constant when at restiii. Varies widely through individual organs, according to immediate needsb. Fluid dynamics (hemodynamics):i. Blood flow: equivalent to cardiac output and is the flow of blood flowing through the circulatory system in a given periodii. Blood pressure: the force per unit area that is exerted on the vessel wall by the contained blood; pressure gradient that provides the driving force that keeps blood moving from an area of high pressure to an area of low pressure throughout the bodyiii. Resistance: measure of the amount of friction blood encounters as it passes through the vessels and its opposition to flowc. Blood flow (F) is directly proportional to the difference in blood pressure between two points in the circulationi. If change in blood pressure increases, blood flow speeds up and vice versaii. Blood flow is inversely proportional to resistance (R)1. If R increases blood flow decreasesiii. R is more important than the difference in blood pressure in influencing local blood pressureV. Resistance—opposition to flowa. Measure of the amount of friction blood encounters as it passes through vesselsb. Referred to as peripheral resistancec. The three important sources of resistance are blood viscosity, total blood vessel length, and blood vessel diameterd. Resistance factors that remain relatively constant are:i. Blood viscosity: thickness of the blood1. Change viscosity by dehydration or increased hematocritii. Blood vessel length; the longer the vessel the greater the resistance1. Change length by weight fluctuations e. Changes in vessel diameter are frequent and significantly alter peripheral resistancef. Resistance varies inversely with the fourth power of vessel radiusg. Small diameter arterioles are the major determinants of peripheral resistanceh. Fatty plaques from atherosclerosis:i. Cause turbulent blood flowii. Dramatically increase resistance due to turbulenceVI. Systemic Blood Pressurea. Pumping action of the heart generates blood flow through the vessels along a pressure gradient, always moving from higher to lower pressure areasb. Pressure results when flow is opposed by resistancec. Systemic pressure:i. Is highest in the aortaii. Declines throughout the length of the pathwayiii. Is 0mmHg as the vena cava enter the right atriumd. The steepest change in blood pressure occurs in the arteriolesVII. Arteriol Blood Pressurea. Arteriol BP reflects two factors of the arteries close to the hearti. Their elasticity ii. The amount of blood forced into them at any given timeb. BP in elastic arteries near the heart is pulsatile (rises and falls)c. Systolic pressure: pressure exerted on arterial walls during ventricular contractiond. Diastolic pressure: lowest level of arterial pressure during a ventricular cyclee. Pulse pressure: difference between systolic and diastolic pressuref. Mean arterial pressure (MAP): pressure that propels the blood to the tissuesVIII. Capillary Blood Pressurea. Capillary BP ranges from 20-40 mmHgb. Low capillary pressure is desirable because high BP would rupture the capillariesc. Low BP is sufficient to force filtrate out into the interstitial space and distribute nutrients, gases, and hormones between blood and tissuesIX. Venous Blood Pressurea. Venous BP is steady and changes little during the cardiac cycleb. Pressure gradient in the venous system is only about 20 mmHgc. A cut vein has even blood flow; a lacerated artery flows in spurtsd. Factors aiding venous return—venous BP alone is too low to promote adequate blood return and is aided by:i. Respiratory pump: pressure changes created during breathing suck blood toward the heart by squeezing local veinsii. Muscular pump: contraction of skeletal muscles push blood toward the heartiii. Valves prevent backflow during venous returnX. Maintaining Blood