2.8 Role of Arterioles in Vasoconstriction and Vasodilation

Arterioles are small blood vessels that regulate blood flow to the capillary beds. They play a crucial role in controlling blood pressure and distributing blood to different parts of the body through two processes: vasoconstriction and vasodilation.

Definition

Arterioles: Arterioles are small branches of arteries that lead to capillaries. They have a diameter of about 0.2 mm and are surrounded by smooth muscle in their walls. They are the primary resistance vessels in the circulatory system.

Vasoconstriction: Vasoconstriction is the narrowing of blood vessel diameter caused by the contraction of smooth muscle in the tunica media. It decreases blood flow.

Vasodilation: Vasodilation is the widening of blood vessel diameter caused by the relaxation of smooth muscle in the tunica media. It increases blood flow.

Main Idea

• Vasoconstriction → decreases blood flow
• Vasodilation → increases blood flow

• Nervous system — sympathetic nerves
• Hormones — norepinephrine, epinephrine, histamine
• Local factors — metabolic needs of tissues
• Emotions — stress, fear, rage

The amount of blood flowing through a blood vessel can be regulated by contraction or relaxation of smooth muscle in the tunica media.

Core Concept

• Arterioles = small arteries; regulate blood flow
• Vasoconstriction = narrowing; decreased blood flow
• Vasodilation = widening; increased blood flow
• Smooth muscle = in tunica media; controls diameter
• Norepinephrine = powerful vasoconstrictor
• Epinephrine = less potent vasoconstrictor
• **Histamine** = vasodilator
• Kinins = vasodilator agents
• Prostaglandins = mostly vasodilators
• Emotions = affect vasodilation and vasoconstriction

Detailed Explanation

Role of Arterioles

The amount of blood flowing through a blood vessel can be regulated by contraction or relaxation of smooth muscle in the tunica media.

Hormonal Control of Arterioles

Blood circulation is also controlled by hormones acting on arterioles.

Vasoconstrictor Agents:

Vasodilator Agents:

Vasodilation and Vasoconstriction Reflective of Emotions

During emotional rage such as apprehension and rage, vasodilation occurs due to secretion of epinephrine.

• A hormone responsible for fear, flight, and fright conditions.
• The sympathetic vasodilator fibers are part of a regulatory system that:
• Originates in the cerebral cortex
• Ends at postganglionic neurons in blood vessels on skeletal muscles
• Activates them to release acetylcholine
• Vasodilation occurs

• Blood discharge through thoroughfare channels rather than capillaries
• Heat loss occurs
• Skin becomes hot and red

• Blood supply becomes less to skin
• Heat is preserved
• Skin becomes cold

• Shock, hypotension, and tachycardia occur by stimulation of arterial stretch receptors
• Production of hypertension and bradycardia (slowness of the heart) occur by increased intracranial pressure

Intermediate Level

Vasoconstriction vs Vasodilation

Hormones Affecting Arterioles

Emotions and Vasodilation/Vasoconstriction

Clinical Significance

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Arterioles are small arteries that regulate blood flow.
• 2. Vasoconstriction = narrowing; decreased blood flow.
• 3. Vasodilation = widening; increased blood flow.
• 4. Smooth muscle in tunica media controls diameter.
• 5. Norepinephrine is a powerful vasoconstrictor.
• 6. Epinephrine is a less potent vasoconstrictor.
• 7. Histamine is a powerful vasodilator.
• 8. Kinins are powerful vasodilators.
• 9. Prostaglandins are mostly vasodilators.
• 10. Emotions affect vasodilation and vasoconstriction.
• 11. Vasodilation occurs during fear, rage, and apprehension.
• 12. Vasoconstriction occurs during stress and cold.
• 13. Shock causes widespread vasodilation.
• 14. Vasodilation increases heat loss; skin becomes hot and red.
• 15. Vasoconstriction preserves heat; skin becomes cold.

Remember

• - Arterioles = regulate blood flow
• - Vasoconstriction = narrowing; decreased blood flow
• - Vasodilation = widening; increased blood flow
• - Smooth muscle = in tunica media
• - Norepinephrine = powerful vasoconstrictor
• - Epinephrine = less potent vasoconstrictor
• - Histamine = powerful vasodilator
• - Kinins = powerful vasodilators
• - Prostaglandins = mostly vasodilators
• - Emotions = affect vasodilation and vasoconstriction
• - **Vasodilation** = hot, red skin
• - Vasoconstriction = cold, pale skin

Common Mistakes

• 1. Confusing vasoconstriction and vasodilation — Vasoconstriction = narrowing; vasodilation = widening.
• 2. Thinking norepinephrine is a vasodilator — Norepinephrine is a vasoconstrictor.
• 3. Thinking histamine is a vasoconstrictor — Histamine is a vasodilator.
• 4. Forgetting the role of smooth muscle — Smooth muscle controls the diameter of arterioles.
• 5. Mixing up the effects on blood pressure — Vasoconstriction increases blood pressure; vasodilation decreases it.
• 6. Confusing the effects on skin — Vasodilation = hot, red skin; vasoconstriction = cold, pale skin.
• 7. Thinking emotions only affect the heart — Emotions also affect blood vessels.
• 8. Believing all prostaglandins are vasodilators — Most are vasodilators; some are vasoconstrictors.
• 9. Forgetting the role of kinins — Kinins are powerful vasodilators.
• 10. Mixing up the hormones — Norepinephrine and epinephrine = vasoconstrictors; histamine and kinins = vasodilators.

Quick Revision

Arterioles are small blood vessels that regulate blood flow to capillary beds. They control blood flow through two processes: vasoconstriction (narrowing) and vasodilation (widening). Vasoconstriction decreases blood flow and increases blood pressure; it is caused by smooth muscle contraction. Vasodilation increases blood flow and decreases blood pressure; it is caused by smooth muscle relaxation. Norepinephrine is a powerful vasoconstrictor; epinephrine is less potent. Histamine and kinins are powerful vasodilators. Prostaglandins are mostly vasodilators. Emotions such as fear, rage, and apprehension cause vasodilation; stress and cold cause vasoconstriction. Vasodilation makes the skin hot and red; vasoconstriction makes the skin cold and pale.

60-Second Revision

• Arterioles = regulate blood flow
• Vasoconstriction = narrowing; decreased blood flow
• Vasodilation = widening; increased blood flow
• Smooth muscle = in tunica media
• Norepinephrine = powerful vasoconstrictor
• Epinephrine = less potent vasoconstrictor
• Histamine = powerful vasodilator
• Kinins = powerful vasodilators
• Prostaglandins = mostly vasodilators
• Emotions = affect vasodilation and vasoconstriction
• Warning: Norepinephrine is a vasoconstrictor; histamine is a vasodilator

NEXT TOPIC

• Chapter:
• 2. Blood Circulatory System of Man

• Topic:
• 2.9 ROLE OF PRE-CAPILLARY SPHINCTERS

Source: