Blood Vessels, Tissue Fluid and Lymph

Arteries have strong walls containing elastic tissue, collagen and smooth muscle, suited to carrying blood at relatively high pressure. Elastic stretch and recoil help maintain flow between heartbeats. Arterioles use changes in smooth-muscle tone to vary lumen diameter and resistance, redistributing blood flow to tissues.

Veins return blood at lower pressure and generally have a wider lumen relative to wall thickness. Valves and surrounding skeletal-muscle contraction can assist return and prevent backflow. Capillaries have thin walls and extensive branching, placing blood close to tissue cells and providing a large total exchange area.

Blood hydrostatic pressure tends to force water and small dissolved substances out through capillary walls. Cells and most large plasma proteins are retained, so tissue fluid resembles plasma in some small-solute components but has much less protein and normally no red blood cells.

Oxygen and nutrients can diffuse from tissue fluid to cells, while metabolic products move in the opposite direction. The balance depends on local concentration gradients and tissue activity. Hydrostatic filtration is not the same process as diffusion of each dissolved substance, so do not use the words interchangeably.

Plasma proteins lower blood water potential and contribute to an inward osmotic effect opposing outward hydrostatic filtration. As capillary hydrostatic pressure falls along a bed, this opposing effect becomes relatively more important. Some fluid can return to capillaries under appropriate conditions, while lymph vessels collect excess tissue fluid and return it to the venous circulation.

Actual fluid balance varies among tissues; the simple school model is not a universal claim that every capillary reabsorbs a fixed percentage at its venous end. Raised capillary hydrostatic pressure, reduced plasma protein concentration or impaired lymph drainage can favour tissue-fluid accumulation and oedema. Identify which part of the balance changes in the case provided.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.