Capillaries Phys Notes

Capillaries are the smallest blood vessels in your body. Think of them as tiny bridges connecting the arterial side of circulation to the venous side.

Their walls are extremely thin—basically one cell layer thick—which allows substances to move between your blood and tissues.

A simple path to remember is: Heart → arteries → arterioles → CAPILLARIES → venules → veins → heart

At the capillaries, exchange happens. Blood delivers things the cells need, especially oxygen and nutrients, while substances such as carbon dioxide and other wastes move from the tissues into the blood.

Perfusion basically means blood flow to a tissue or organ.

For example, your heart pumps blood toward your leg. If enough blood travels through the capillaries in your leg and supplies those cells with oxygen, the tissue is being well perfused.

Good perfusion = enough blood reaching tissue → enough oxygen/nutrients delivered

Poor perfusion = not enough blood reaching tissue → cells may not receive enough oxygen

Perfusion depends on things like blood pressure, cardiac output, and how constricted or dilated the blood vessels are.

One thing you may hear about: capillary exchange

There are a few ways substances move across capillaries.

Diffusion: substances move from an area of higher concentration → lower concentration. For example, oxygen can move from the blood into tissues where oxygen levels are lower.

Filtration: pressure pushes water and small dissolved substances OUT of the capillary.

Reabsorption: fluid moves back INTO the capillary.

A helpful way to picture it:CAPILLARY → oxygen + nutrients → CELLS CAPILLARY ← carbon dioxide + wastes ← CELLS

If you understand just “capillaries are where exchange happens” and “perfusion means blood reaching and supplying tissue,”