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. Blood Flow to 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

Physical Laws Describing Blood Flow The rate of blood flow is also inversely proportional to the frictional resistance to blood flow within the vessels. blood flow = ΔP resistance ΔP = pressure difference between the two ends of the tube 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 adds in physical constants 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 Allows you to calculate the rate of blood flow Jean-Louis Marie Poiseuille (1799-1869), French physicist

Pressure Differences in Different Parts of Systemic Circulation

Total Peripheral Resistance = The sum of all vascular resistance in systemic circulation Blood flow to organs runs parallel to each other, so a change in resistance within one organ does not affect another. Vasodilation in a large organ may decrease total peripheral resistance and mean arterial pressure. Increased cardiac output and vasoconstriction elsewhere make up for this.

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