Ch. 13/14 Day 5 Flashcards
There are pressure differences in different parts of systemic circulation. Where is the steepest P drop?
In the “resistance vessels”
Total Peripheral Resistance
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 CO and vasoconstriction elsewhere make up for this
Extrinsic Regulation of Blood Flow: Autonomic and Endocrine Control of Blood Flow
A. Sympathetic nerves (adrenergic)
- -increase in CO and increase total peripheral resistance through release of norepinephrine onto smooth muscles of arterioles in the viscera and skin to stimulate vasoconstriction (alpha-adrenergic)
- -during “fight or flight” blood is diverted to skeletal muscles
- -adrenal epinephrine stimulates beta-adrenergic receptors for vasodilation
B. Parasympathetic nerves (cholinergic)
- -ACh stimulates vasodilation
- -limited to digestive tract, external genitalia, and salivary glands
- -less important in controlling total peripheral resistance due to limited influence
Paracrine Regulation of Blood Flow
Molecules produced by one tissue control another tissue w/in the same organ
–ex: tunica interna produces signals to influence smooth muscle activity in tunica media
Smooth muscle relaxation influenced by bradykinin, nitric oxide, and prostaglandin I2 to produce vasodilation
Endothelin-1 stimulates smooth muscle contraction to produce vasoconstriction and raise total peripheral resistance
Intrinsic Regulation of Blood Flow
Used by some organs (brain and kidneys) to promote constant blood flow when there is fluctuation of BP; also called autoregulation
Intrinsic Regulation of Blood Flow: Myogenic Control Mechanisms
Vascular smooth muscle responds to changes in arterial blood pressure
Intrinsic Regulation of Blood Flow: Metabolic Control Mechanisms
e. g. local vasodilation controlled by changes in:
- -decrease Po2/increase Pco2 due to increased metabolism
- -decrease tissue pH (due to CO2, lactic acid, etc.)
- -release of K+ and paracrine signals
Intrinsic Regulation of Blood Flow: Active Hyperemia
Matches blood flow to increased metabolism
Intrinsic Regulation of Blood Flow: Reactive Hyperemia
Follows a period of decreased blood flow
Blood Flow to Heart and Skeletal Muscles: 1. Aerobic Requirements of Heart
- Coronary arteries feed large number of capillaries (2,500-4,000 per cubic mm tissue)
- Unlike most organs, blood flow is restricted during systole (due to compression by squeezing during contraction). Thus cardiac tissue has myoglobin to store oxygen during diastole to be released in systole
- Cardiac tissue is metabolically very active (increase mitochondria and respiratory enzymes)
- Large amounts of ATP produced from aerobic respiration of fatty acids, glucose, and lactate
- During exercise, blood flow through coronary arteries increases from 80 to 400 mL/minute/100g tissue
Blood Flow to Heart and Skeletal Muscles: 2. Regulation of Coronary Blood Flow
- Norepinephrine from sympathetic nerve fibers (alpha-adrenergic) stimulates vasoconstriction raising vascular resistance at rest
- Adrenal epinephrine (beta-adrenergic) stimulates vasodilation and thus decreases vascular resistance during exercise
- Vasodilation is enhanced by intrinsic metabolic control mechanisms - increased CO2, K+, paracrine regulators
Blood Flow to Heart and Skeletal Muscles: 3. Effect of Exercise Training on Coronary Blood Flow
- Increase density of coronary arterioles and capillaries
- Increased production of NO (nitric oxide) to promote vasodilation
- Decreased compression of coronary arteries during systole due to lower cardiac rate (in highly trained athletes)
Blood Flow to Heart and Skeletal Muscles: 4. Regulation of Blood Flow Through Skeletal Muscles
- Arterioles have really high vascular resistance at rest (alpha-adrenergic effect)
- ->even at rest, skeletal muscles still receive 20-25% of body’s blood supply (due to their total mass over the entire body)
- Blood flow decreases during contraction (squeezing of arterioles, as in heart) and can stop completely beyond 70% of maximum contraction (result pain, fatigue during sustained isometric contraction)
- Vasodilation stimulated by both adrenal epinephrine (beta-adrenergic effect) and cholinergic ACh
- Intrinsic metabolic controls enhance vasodilation during exercise
Circulatory Changes during Dynamic Exercise
Vascular resistance through skeletal and cardiac muscles decreases due to:
- -increased CO
- -metabolic vasodilation
- -diversion of blood away from viscera and skin
Blood flow to brain increases a small amount w/ moderate exercise and decreases a small amount during intense exercise
CO can increase 5x due to increased cardiac rate
SV can increase due to increased venous return from skeletal muscle pumps and respiratory movements
Ejection fraction increases due to increased contractility (Frank-Starling effect)
Cardiovascular Response to Exercise
CO and muscle blood flow increase
Reflex: working muscles –> motor cortex –> CV control center in medulla –> cardiac contractility and HR
Vasodilation in muscle + vasoconstriction in other tissues –> diversion of up to 90% of CO to the working muscle
