Net hydrostatic pressure forces water ____ a capillary; net osmotic pressure reabsorbs water _____ a capillary.
(a) into, out of
(b) out of, into
(c) out of, out of
(d) into, into

Verified step by step guidance

1. Step 1: Understand the concept of hydrostatic pressure in capillaries. Hydrostatic pressure is the force exerted by a fluid against the walls of its container. In capillaries, this pressure tends to push water and solutes out of the capillary into the surrounding interstitial fluid.
2. Step 2: Understand the concept of osmotic pressure in capillaries. Osmotic pressure is the force exerted by solutes (mainly plasma proteins like albumin) that draws water into the capillary from the interstitial fluid, promoting reabsorption.
3. Step 3: Recognize that net hydrostatic pressure generally forces water out of the capillary because blood pressure inside the capillary is higher than in the interstitial space.
4. Step 4: Recognize that net osmotic pressure generally pulls water back into the capillary because the concentration of plasma proteins inside the capillary is higher than in the interstitial fluid, creating an osmotic gradient.
5. Step 5: Combine these concepts to answer the question: net hydrostatic pressure forces water out of a capillary, and net osmotic pressure reabsorbs water into a capillary.

Key Concepts

Hydrostatic Pressure in Capillaries

Hydrostatic pressure is the force exerted by fluid against the walls of blood vessels. In capillaries, it pushes water and solutes out of the capillary into the surrounding tissue, promoting filtration. This pressure is higher at the arterial end and decreases toward the venous end.

Osmotic Pressure in Capillaries

Osmotic pressure, mainly due to plasma proteins like albumin, draws water back into the capillaries from the interstitial fluid. This reabsorption helps maintain fluid balance by pulling water into the capillary, especially at the venous end where hydrostatic pressure is lower.

Fluid Exchange Across Capillary Walls

Fluid movement across capillary walls depends on the balance between hydrostatic and osmotic pressures. Hydrostatic pressure drives fluid out, while osmotic pressure pulls fluid in. Understanding this balance explains how nutrients and wastes are exchanged between blood and tissues.