Capillaries are the primary site of exchange between the blood and the body's cells. They form vast networks called capillary beds, which provide a massive surface area for diffusion.
Their walls consist of a single layer of endothelial cells, making them one cell thick. This adaptation minimizes the diffusion distance for oxygen, glucose, and carbon dioxide.
Capillary walls contain small gaps or pores between cells, allowing small molecules and liquid (plasma) to leak out while retaining large proteins and blood cells within the vessel.
Veins carry blood back to the heart at significantly lower pressures than arteries. Consequently, their walls are much thinner with less muscular and elastic tissue.
To accommodate large volumes of blood, veins have a wide lumen, which reduces resistance to flow. This is critical because the driving force (blood pressure) is low by the time blood reaches the venous system.
A defining feature of veins is the presence of valves. These structures ensure unidirectional flow by closing to prevent the backflow of blood, which is especially important in limbs where blood must move against gravity.
Tissue fluid is the liquid that surrounds cells, formed from blood plasma that escapes capillaries. It facilitates the final delivery of nutrients and the removal of waste.
At the arterial end of a capillary, the hydrostatic pressure (blood pressure) is higher than the osmotic pressure (the pull of solutes). This net outward force pushes water and small solutes out into the surrounding tissue.
At the venous end, hydrostatic pressure drops due to friction and loss of fluid volume. The osmotic pressure, maintained by large plasma proteins that remained in the capillary, is now greater than the hydrostatic pressure, drawing water back into the vessel by osmosis.
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Pressure | High | Low | Low/Decreasing |
| Wall Thickness | Thick | Thin | Extremely Thin (1 cell) |
| Lumen Size | Narrow | Wide | Very Narrow (1 RBC wide) |
| Valves | Absent (except at heart) | Present | Absent |
| Function | Transport from heart | Transport to heart | Exchange of substances |
Link Structure to Function: When describing a vessel, always state the structural feature first, then explain exactly how it aids the vessel's specific role. For example, 'Thick elastic walls (structure) allow the artery to stretch and recoil to maintain high blood pressure (function).'
Recoil vs. Contraction: A common mistake is confusing elastic recoil with muscle contraction. Elastic fibers recoil passively to maintain pressure; smooth muscle contracts actively to narrow the lumen and redirect flow.
Tissue Fluid Balance: Always check the balance of pressures. If hydrostatic pressure is greater than osmotic pressure, fluid moves OUT. If osmotic pressure is greater, fluid moves IN.
The Pulmonary Exception: Remember that while most arteries carry oxygenated blood, the pulmonary artery carries deoxygenated blood. Similarly, the pulmonary vein carries oxygenated blood.
