Ch 13/14 Cardiovascular Physiology (Day 5) Flashcards
What are resistance vessels?
steepest pressure drop
–> there are pressure differences throughout systemic circulation
What is 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 (see next slide [96] )
- Vasodilation in a large organ may decrease total peripheral resistance and mean arterial pressure.
- Increased cardiac output and vasoconstriction elsewhere make up for this.
What are the extrinsic regulators of blood flow?
- sympathetic nerves (adrenergic)
2. parasympathetic nerves (cholinergic)
Sypathetic Nerves
ADRENERGIC
- Increase in cardiac output 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 “flight or fight”, blood is diverted to skeletal muscles
- Adrenal epinephrine stimulates beta-adrenergic receptors for vasodilation
Parasympathetic Nerves
CHOLINERGIC
- Acetylcholine stimulates vasodilation.
- Limited to digestive tract, external genitalia, and salivary glands
- Less important in controlling total peripheral resistance due to limited influence
What is arteriole diameter determined by?
tonic release of norepinephrine
- increased release = constriction
- decreased release = dilation
Paracrine Regulation of Blood Flow
- Molecules produced by one tissue control another tissue within the same organ.
- ->Example: The tunica interna produces signals to influence smooth muscle activity in the 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 blood pressure; also called autoregulation.
Two types:
- Myogenic control mechanisms
- Metabolic control mechanisms
Myogenic control mechanisms
Vascular smooth muscle responds to changes in arterial blood pressure.
Metabolic control mechanisms
e.g. local vasodilation controlled by changes in:
↓PO2 /↑ PCO2 due to increased metabolism
↓ tissue pH (due to CO2, lactic acid, etc.)
Release of K+ and paracrine signals
Active hyperemia matches blood flow to increased metabolism.
- increased tissue metabolism
- increased release of metabolic vasodilators into ECF
- arterioles dilate
- decreased resistance = increased flow
- O2 and nutrient supply to tissues increases (as long as metabolism is increased)
Reactive hyperemia follows a period of decreased blood flow.
1, decreased tissue blood flow due to occlusion
- metabolic vasodilators accumulate in ECF
- arterioles dilate, but occlusion prevents blood flow
- REMOVE OCCLUSION
- decreased resistance = increased blood flow
- as vasodilators wash away, arterioles constrict and blood flow returns to normal
Blood Flow to the Heart and Skeletal Muscles: Aerobic Requirements of the Heart
Coronary arteries feed large # 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 (↑ mitochondria and respiratory enzymes)
- Large amounts of ATP produced from the aerobic respiration of fatty acids, glucose, and lactate.
- During exercise, blood flow through coronary arteries increases from 80 to 400 ml/minute/100 g tissue.
Blood Flow to the Heart and Skeletal Muscles: Regulation of Coronary Blood Flow
- Norepinephrine from sympathetic nerve fibers (alpha-adrenergic) stimulates vasoconstriction (recall sympathetic tone, slide #97), 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 the Heart and Skeletal Muscles: Effect of exercise training on coronary blood flow
- Increased density of coronary arterioles and capillaries
- Increased production of NO to promote vasodilation
- Decreased compression of coronary arteries during systole due to lower cardiac rate (in highly trained athletes)