Local and Humoral Control

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Questions and Answers

What substance is primarily released by the vascular endothelium in response to shear stress to cause vasodilation in upstream vessels?

Epinephrine

Angiotensin II

Nitric oxide (correct)

Endothelin

Which factor is NOT listed as a possible reason for baseline tone in blood vessels?

Presence of Ca++

Presence of Mg++ (correct)

Myogenic activity

High oxygen content of arterial blood

Which of the following is a vasoconstrictor released from the posterior pituitary and contributes to neurohumoral control?

Angiotensin II

Vasopressin (ADH) (correct)

Norepinephrine

Epinephrine

What substance is produced by damaged endothelial cells to induce local vasoconstriction and reduce bleeding?

Endothelin Signup and view all the answers

Which factor does NOT contribute to vasodilation in vascular smooth muscle?

Increased Ca++ Signup and view all the answers

What range of blood pressures does long-term regulation of blood flow effectively manage?

50-250 mm Hg Signup and view all the answers

What happens when a blood vessel relaxes?

It dilates, allowing more blood and oxygen to be provided Signup and view all the answers

What is reactive hyperemia?

Sudden increase in blood flow to several times the baseline value after occlusion Signup and view all the answers

Which theory proposes that excess oxygen or other nutrients can lead to increased smooth muscle contraction and decreased blood flow?

Metabolic Theory Signup and view all the answers

What is the Myogenic Theory of autoregulation independent of?

Both neurohumoral influences and endothelium integrity Signup and view all the answers

What does upstream regulation of vasodilation affect?

Large upstream arteries Signup and view all the answers

What effect does an increase in pressure initially have according to the Myogenic Mechanism of Autoregulation?

Increases flow Signup and view all the answers

Variation of smooth muscle in the resistance vessels does NOT allow for?

Vasoconstriction only Signup and view all the answers

How does smooth muscle contraction in resistance vessels differ between smooth muscle and skeletal muscle?

It develops slowly and maintains contraction for long periods unlike skeletal muscle Signup and view all the answers

What is/are the core mechanism(s) of acute control of tissue blood flow?

Local and neurohumoral mechanisms Signup and view all the answers

What happens in the absence of adequate oxygen according to the Oxygen Lack Theory of local blood flow regulation?

The smooth muscle cannot maintain contraction Signup and view all the answers

Bradykinin contributes to capillary leakage in inflammation.

True Signup and view all the answers

Histamine is a vasoconstrictor released from damaged endothelial cells.

False Signup and view all the answers

Acetate decreases vascular smooth muscle contractile state.

True Signup and view all the answers

Study Notes

Mechanisms of Local and Humoral Control of Tissue Blood Flow

Acute control occurs within seconds to minutes via local and neurohumoral mechanisms
Long-term control occurs over days to months due to changes in size or number of blood vessels

Baseline Tone and Variation in Tissue Blood Flow

Baseline tone: partial constriction of resistance vessels allowing for vasodilation and further vasoconstriction
Variation in tissue blood flow: due to changes in metabolism, oxygen supply, and other nutrients

Smooth vs Skeletal Muscle

Smooth muscle contraction:
- Develops slowly
- Develops high forces
- Maintains contraction for long periods
Mechanisms of contraction:
- Electromechanical coupling: increased myoplasmic Ca++ via voltage-gated calcium channels
- Pharmacomechanical coupling: via receptor-mediated calcium channels, occurs in the absence of electrical excitation

Substances Involved in Pharmacomechanical Coupling

Catecholamines
Histamine
Acetylcholine
Serotonin
Angiotensin
Adenosine
Nitric oxide
CO2
Potassium
Prostaglandins

Theories of Local Blood Flow Regulation

Vasodilator Theory: vasodilator substances produced and released due to increased metabolism or decreased oxygen supply
Oxygen Lack Theory: smooth muscle relaxation in response to oxygen lack, leading to vasodilation

Acute Control of Local Blood Flow

Reactive Hyperemia: increased blood flow following occlusion of blood supply
Active Hyperemia: increased blood flow in highly active tissue, e.g., exercising skeletal muscle

Autoregulation of Blood Flow

Maintenance of normal blood flow following changes in arterial pressure
Two proposed mechanisms:
- Metabolic Theory: similar to oxygen lack theory
- Myogenic Theory: change in transmural pressure initiates contraction or relaxation of vascular smooth muscle

Upstream Regulation of Vasodilation

Locally mediated vasodilation affects small arteries and arterioles, but not larger upstream arteries
Increased flow through larger arteries upstream releases EDRF (primarily nitric oxide), causing vasodilation

Baseline Vascular Tone

Independent of nervous system input
Possibly due to myogenic activity, high oxygen content of arterial blood, presence of Ca++, or unknown plasma factor

Neurohumoral Control of Blood Flow

Vasoconstrictors:
- Norepinephrine
- Epinephrine
- Angiotensin II
- Vasopressin (ADH)
- Endothelin
Vasodilators:
- Bradykinin
- Histamine

Other Factors Affecting Vascular Smooth Muscle Contractile State

Vasodilation:
- Increased K+
- Increased Mg++
- Increased H+
- Acetate
- Citrate
- Increased CO2 (cerebral)
Vasoconstriction:
- Increased Ca++
- Decreased H+
- Increased CO2 (systemically via sympathetic nervous system)

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