CARDIOVASCULAR 5 BLOOD FLOW

Blood Flow to the Organs Cardiac output is distributed unequally to different organs due to unequal resistance to blood flow through the organs.

Physical Laws Describing Blood Flow Blood flows from a region of higher pressure to a region of lower pressure. The rate of blood flow is proportional to the differences in pressure. ΔP = pressure difference Mean arterial pressure = 100 mmHg Mean arterial pressure = average pressure in arteries

FRICTIONAL RESISTANCE IMPEDES FLOW The rate of blood flow is directly proportional to the change in pressure and inversely proportional to the frictional resistance within the vessels. blood flow ♾ ΔP resistance ΔP = pressure difference between the two ends of the tube What is resistance? Resistance is measured as: resistance = Lη r4 L = length of the vessel η = viscosity of the blood r = radius of the blood vessel

Poiseuille’s Law Poiseuille's law: the velocity of the steady flow of a fluid through a narrow tube (as a blood vessel or a catheter) varies directly as the pressure and the fourth power of the radius of the tube and inversely as the length of the tube and the coefficient of viscosity. (From Google) Jean-Louis Marie Poiseuille (1799-1869), French physicist Blood flow = ΔPr4(π) ηL(8) Vessel length (L) and blood viscosity (η) do not vary normally. Mean arterial pressure (P) and vessel radius (r) are therefore the most important factors in blood flow. Vasoconstriction of arterioles provides the greatest resistance to blood flow and can redirect flow to/from particular organs You will not be asked to know the formula per se

Pressure Differences in Different Parts of Systemic Circulation Blood flow to an organ is determined by Vasoconstriction/vasodilation of arterioles Arterioles provide the greatest resistance to blood flow Main pressure drop occurs in arterioles

Blood Flow to Organs Runs in Parallel Arterial blood passes through only one set of resistance vessels before returning to the heart One organ is not downstream from another organ

Total Peripheral Resistance Total Peripheral Resistance = The sum of all vascular resistance in systemic circulation Because flow to organs runs parallel to each other, a change in resistance within one organ may not affect another. However, vasodilation in a large organ may decrease total peripheral resistance and mean arterial pressure. Normally, increased cardiac output and vasoconstriction elsewhere make up for this. Example: During exercise, vasodilation occurs in skeletal muscle, and this would lead to a lowered mean arterial pressure. BUT, vasoconstriction occurs in viscera and cardiac output rises at the same time. Therefore, blood pressure actually increases.

Sympathetic Regulation of Peripheral Resistance Sympathoadrenal system  increases cardiac output  increases total peripheral resistance Alpha adrenergic receptors are stimulated by norepi Cause vasoconstriction in smooth muscle throughout the digestive tract, kidneys, skin Beta adrenergic receptors are stimulated by epinephrine (sent from adrenal medulla) Cause vasodilation in skeletal muscle Cholinergic receptors are stimulated by Ach Cause contraction of skeletal muscle Cause vasodilation in skeletal muscles

Parasympathetic Regulation of Peripheral Resistance Parasympathetic  always cholinergic  acts only on blood vessels surrounding the digestive tract, external genitalia and salivary glands Promotes vasodilation Less important than sympathetic system in control of peripheral resistance.

Paracrine Regulation of Blood flow Paracrine = molecules produced by one tissue regulate a nearby tissue Examples of paracrine smooth muscle vasodilators: Bradykinin – produced by sweat; dilates skin blood vessels Nitric oxide – leads to vasodilation; nitroglycerin (which can be converted to NO) is used to treat angina pectoris Prostaglandin I2 – administered to people with pulmonary hypertension Example of paracrine smooth muscle vasoconstrictor: Endothelin-1: produced by endothelium  binds its receptor  vasoconstriction

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INTRINSIC Regulation of Blood Flow: Autoregulation – the ability of an organ to regulate its own blood flow. Myogenic means that the smooth muscle of an organ regulates the blood flow Metabolic changes signal a need for vasodilation to bring in more oxygen Changes: 1) increased metabolic rate requires more O2 2) high CO 3) Low pH due to lactic acid build up 4) Release of K+ and paracrine regulators Examples: Reactive hyperemia- restrict blood flow for a short time and then remove the constriction. Active hyperemia- increase in blood flow that accompanies muscle contraction Example – if the brain is not getting adequate blood flow, it will dilate its own vessels