Overview
This lecture explains how blood is distributed throughout the body at rest and during exercise, focusing on vascular control mechanisms and the role of the vasomotor control center in the brain.
Blood Distribution at Rest
- The heart pumps blood from the aorta to various organs and tissues.
- At rest, 80-85% of cardiac output goes to organs such as liver, kidneys, skin, and brain.
- Only 15-20% of cardiac output at rest goes to working (mainly postural and breathing) muscles.
Blood Redistribution During Exercise
- During exercise, only 15-20% of blood is sent to other organs.
- 80% of cardiac output is redirected to working muscles (e.g., legs during running).
- This shift ensures muscles receive more oxygen and nutrients when active.
Mechanisms of Blood Flow Regulation
- Blood vessels (arterioles) have smooth muscle in their walls that can contract (vasoconstrict) or relax (vasodilate).
- Vasodilation increases vessel diameter, lowering resistance and increasing blood flow to tissues.
- Vasoconstriction decreases vessel diameter, raising resistance and reducing blood flow.
- At rest, arterioles to organs are more dilated, while those to muscles are more constricted.
- During exercise, vasoconstriction occurs in organs, and vasodilation occurs in arterioles supplying muscles.
Capillary Beds and Pre-capillary Sphincters
- Arterioles lead to capillary beds where exchange of oxygen and nutrients occurs.
- Pre-capillary sphincters are circular muscles that regulate blood flow into capillary beds.
- Sphincter constriction reduces blood flow through the capillaries, shunting blood to required areas.
Vasomotor Control Center (VCC)
- The VCC is located in the medulla oblongata of the brain.
- VCC regulates vasomotor tone via sympathetic (vasoconstriction) and parasympathetic (vasodilation) stimulation.
- The VCC receives input from:
- Chemoreceptors (detect chemical changes e.g., pH)
- Baroreceptors (detect blood pressure changes)
- Proprioceptors (detect muscle and tendon tension)
Key Terms & Definitions
- Cardiac Output — The volume of blood pumped by the heart per minute.
- Vasodilation — Widening of blood vessels to increase blood flow.
- Vasoconstriction — Narrowing of blood vessels to decrease blood flow.
- Arteriole — A small artery leading to capillaries.
- Capillary Bed — Network of capillaries where exchange with tissues occurs.
- Pre-capillary Sphincter — Circular muscle regulating blood flow into capillary beds.
- Vasomotor Tone — Degree of tension in blood vessel walls.
- Vasomotor Control Center (VCC) — Brain region controlling vessel diameter via nervous signals.
- Sympathetic Stimulation — Increases vasomotor tone, causing vasoconstriction.
- Parasympathetic Stimulation — Decreases vasomotor tone, causing vasodilation.
Action Items / Next Steps
- Review mechanisms of vasoconstriction, vasodilation, and pre-capillary sphincter function.
- Study sources and function of neural input to the vasomotor control center.