Factors Affecting Blood Flow PDF

# Factors Affecting Blood Flow ## Vascular Resistance to Blood Flow * Determines how much blood flows through a tissue or organ * Vasodilation decreases resistance, increase blood flow. * Vasoconstriction does the opposite. ## Table 14.3 Estimated Distribution of the Cardiac Output at Rest | Organ...

# Factors Affecting Blood Flow ## Vascular Resistance to Blood Flow * Determines how much blood flows through a tissue or organ * Vasodilation decreases resistance, increase blood flow. * Vasoconstriction does the opposite. ## Table 14.3 Estimated Distribution of the Cardiac Output at Rest | Organs | Milliliters per Minute | Percent Total | |---|---|---| | Gastrointestinal tract and liver | 1,400 | 24 | | Kidneys | 1,100 | 19 | | Brain | 750 | 13 | | Heart | 250 | 4 | | Skeletal Muscles | 1,200 | 21 | | Skin | 500 | 9 | | Other Organs | 600 | 10 | | Total Organs | 5,800 | 100 | ## Physical Laws Describing Blood Flow * Blood flows through vascular system when there is pressure difference (ΔP) at its two ends * Flow rate is directly proportional to difference (ΔP = P1-P2). * Flow rate is inversely proportional to resistance * Flow = ΔP/R * Resistance is directly proportional to length of vessel (L) and viscosity of blood (η). * Inversely proportional to the 4th power of the radius * So diameter of vessel is important for resistance. * Poiseuille's Law describes factors affecting blood flow. * Blood flow = ΔPr⁴(η/(8)) ## Fig 14.14 Relationship between blood flow, radius & resistance * **(A)** * Radius: 1 mm * Resistance: R * Blood flow: F * **(B)** * Radius: 2 * 1mm * Resistance: 1/16R * Blood Flow: 16F ## Extrinsic Regulation of Blood Flow * Sympathoadrenal activation causes increased CO and resistance in periphery and viscera. * Blood flow to skeletal muscles is increased * Because their arterioles dilate in response to Epi and their Symp fibers release ACh which also dilates their arterioles. * Thus blood is shunted away from visceral and skin to muscles. * Parasympathetic effects are vasodilative. * However, Parasymp only innervates digestive tract, genitalia and salivary glands. * Thus Parasymp is not as important as Symp. * Angiotenin II and ADH (at high levels) cause general vasoconstriction of vascular smooth muscle. * Which increases resistance and BP. ## Paracrine Regulation of Blood Flow * Endothelium produces several paracrine regulators that promote relaxation: * Nitric Oxide (NO), bradykinin, prostacyclin. * NO is involved in setting resting “tone” of vessels. * Levels are increased by Parasymp activity. * Vasodilator drugs such as nitroglycerin or Viagra act thru NO. * Endothelin 1 is vasoconstrictor produced by endothelium. ## Intrinsic Regulation of Blood Flow (Autoregulation) * Maintains fairly constant blood flow despite BP variation. * Myogenic control mechanism occur in some tissues because vascular smooth muscle contracts when stretched and relaxes when not stretched. * E.g. decreased arterial pressure causes cerebral vessels to dilate and vice versa. * Metabolic control mechanism matches blood flow to local tissue needs. * Low O2 or pH or high CO2, adenosine, or K+ from high metabolism cause vasodilation which increases blood flow (= active hyperemia). ## Aerobic Requirements of the Heart * Heart (and brain) must receive adequate blood supply at all times. * Heart is most aerobic tissue. Each myocardial cell is within 10m of capillary. * Contains lots of mitochondria and aerobic enzymes. * During systole coronary, vessels are occluded. * Heart gets around this by having lots of myoglobin. * Myoglobin is an O2 storage molecule that releases O2 to heart during systole. ## Regulation of Coronary Blood Flow * Blood flow to heart is affected by Symp activity. * NE causes vasoconstriction; Epi causes vasodilation. * Dilation accompanying exercise is due mostly to intrinsic regulation. ## Circulatory Changes During Exercise * At beginning of exercise, Symp activity causes vasodilation via Epi and local ACh release. * Blood flow is shunted from periphery and visceral to active skeletal muscles. * Blood flow to brain stays same. * As exercise continues, intrinsic regulation is major vasodilator. * Symp effects cause SV and CO to increase. * HR and ejection fraction increases vascular resistance. ## Fig 14.19 * **Heavy Exercise** * Cardiac Output: 25 L/min * Blood flow to: * Heart: 100% * Brain: 0.5%-1% * Skeletal Muscles: 80%-85% * GI Tract: 3%-5% * Kidneys: 2%-4% * Skin: 3%-4% * **Rest** * Cardiac Output: 5 L/min * Blood flow to: * Heart: 100% * Brain: 15% * Skeletal Muscles: 15%-20% * GI Tract: 20%-25% * Kidneys: 20% * Skin: 4%-5% ## Fig 14.20 * **↑ Cardiac output** * ↑ Cardiac Rate * ↑ Stroke Volume * ↑ Sympathoadrenal system * Improved venous return * Deeper Breathing * **↑ Blood flow to skeletal muscles** * Metabolic vasodilation in muscles * Sympathetic vasoconstriction in viscera * Skeletal muscle activity ## Cerebral Circulation * Gets about 15% of total resting CO. * Held constant (750 ml/min) over varying conditions. * Because loss of consciousness occurs after few secs of interrupted flow. * Is not normally influenced by sympathetic activity. * Is regulated almost exclusively by intrinsic mechanisms. * When BP increases, cerebral arterioles constrict; when BP decreases, arterioles dilate (= myogenic regulation). * Arterioles dilate and constrict in response to changes in CO2 levels. * Arterioles are very sensitive to increases in local neural activity (= metabolic regulation). * Areas of brain with high metabolic activity receive most blood. ## Cutaneous Blood Flow * Skin serves as a heat exchanger for thermoregulation. * Skin blood flow is adjusted to keep deep-body at 37°C. * By arterial dilation or constriction and activity of arteriovenous anastomoses which control blood flow through surface capillaries. * Symp activity closes surface beds during cold and fight-or-flight, and opens them in heat and exercise.