19 Questions
What substance is primarily released by the vascular endothelium in response to shear stress to cause vasodilation in upstream vessels?
Nitric oxide
Which factor is NOT listed as a possible reason for baseline tone in blood vessels?
Presence of Mg++
Which of the following is a vasoconstrictor released from the posterior pituitary and contributes to neurohumoral control?
Vasopressin (ADH)
What substance is produced by damaged endothelial cells to induce local vasoconstriction and reduce bleeding?
Endothelin
Which factor does NOT contribute to vasodilation in vascular smooth muscle?
Increased Ca++
What range of blood pressures does long-term regulation of blood flow effectively manage?
50-250 mm Hg
What happens when a blood vessel relaxes?
It dilates, allowing more blood and oxygen to be provided
What is reactive hyperemia?
Sudden increase in blood flow to several times the baseline value after occlusion
Which theory proposes that excess oxygen or other nutrients can lead to increased smooth muscle contraction and decreased blood flow?
Metabolic Theory
What is the Myogenic Theory of autoregulation independent of?
Both neurohumoral influences and endothelium integrity
What does upstream regulation of vasodilation affect?
Large upstream arteries
What effect does an increase in pressure initially have according to the Myogenic Mechanism of Autoregulation?
Increases flow
Variation of smooth muscle in the resistance vessels does NOT allow for?
Vasoconstriction only
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
What is/are the core mechanism(s) of acute control of tissue blood flow?
Local and neurohumoral mechanisms
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
Bradykinin contributes to capillary leakage in inflammation.
True
Histamine is a vasoconstrictor released from damaged endothelial cells.
False
Acetate decreases vascular smooth muscle contractile state.
True
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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