Blood Circulation and Regulation

Overview

This lecture covers the mechanisms of blood flow through the body and explains how blood pressure is regulated and adapted in different organisms.

The heart pumps blood under high pressure through the aorta, arteries, arterioles, and capillaries.
Blood slows significantly in capillaries to enable gas and nutrient exchange with tissues.
As blood leaves capillaries, flow increases in venules and veins but remains slower than in arteries.
Blood is pushed through veins by smooth and skeletal muscle contractions and one-way valves prevent backflow.
Prolonged sitting can cause blood to pool in the legs, so regular movement is important.

Blood flow through capillary beds is regulated by nerve and hormone signals to match body needs.
Precapillary sphincters are small muscles that control blood entry into capillary beds.
Only 5–10% of capillary beds are supplied with blood at any given moment.

Plasma diffuses out of capillaries, and about 85% returns via osmosis; the remaining 15% enters lymphatic vessels.
Lymph nodes filter the lymph, removing pathogens before returning it to the bloodstream.
Lymph is moved by smooth and skeletal muscle action and valves, rejoining blood near the heart.

Long-necked animals like giraffes require high blood pressure to move blood to the brain but have mechanisms to prevent damage.
Aquatic and cold-environment animals have adaptations (e.g., countercurrent heat exchangers, blubber) to conserve heat and regulate blood flow.
Artery elasticity helps manage pressure changes during heartbeats.

Blood pressure is the force blood exerts on vessel walls; optimal adult pressure is 120/80 mm Hg.
Systolic pressure occurs during heartbeats; diastolic between beats.
Cardiac output = heart rate × stroke volume; increasing either boosts blood pressure.
Blood vessel diameter changes (vasodilation/vasoconstriction) affect blood pressure and flow.
Stress, exercise, posture, hormones, and nerve signals can all change blood pressure.

Systolic Blood Pressure — Pressure on vessel walls during heart contraction.
Diastolic Blood Pressure — Pressure between heartbeats.
Precapillary Sphincter — Smooth muscle ring controlling blood flow into capillary beds.
Varicose Veins — Enlarged veins from malfunctioning valves.
Cardiac Output — Volume of blood pumped by the heart per minute.
Peripheral Resistance — Resistance to blood flow in small blood vessels.
Lymph — Fluid from blood plasma filtered by lymphatic system before rejoining bloodstream.
Countercurrent Heat Exchanger — Vessel arrangement that conserves heat in blood flow.