Capillary exchange

Hydrostatic pressure is the pressure a fluid exerts on the walls around it, such as blood pushing on a vessel wall. The pull of water toward more solute is osmotic pressure.

• Correct: The pressure a fluid exerts on the walls of its container:
• The pull of water toward a region with more solute:
• The pressure of the air in the lungs:

Water moves by osmosis toward the side with more solute that cannot cross, which is side 1. The blocked solute is exactly what creates the osmotic pull.

• From side 1 to side 2:
• Correct: From side 2 to side 1:
• It does not move, because the solute cannot cross:

Albumin, made by the liver, is the most abundant plasma protein. Fibrinogen is a clotting protein present in smaller amounts, and hemoglobin is inside red blood cells, not dissolved in plasma.

• Fibrinogen:
• Hemoglobin:
• Correct: Albumin:

Outward forces: CHP + IFCOP = 30 + 4 = 34 mm Hg. Inward forces: BCOP + IFHP = 26 + 2 = 28 mm Hg. NFP = 34 − 28 = +6 mm Hg. A positive value means filtration.

• −6 mm Hg, reabsorption into the capillary: This has the right size but the wrong sign. The outward forces (34 mm Hg) are larger than the inward forces (28 mm Hg), so the result is positive, which means filtration.
• +10 mm Hg, filtration out of the capillary: This comes from adding the interstitial hydrostatic pressure to the outward side instead of the inward side. IFHP pushes fluid into the capillary, so it belongs with BCOP.
• Correct: +6 mm Hg, filtration out of the capillary: Correct. (30 + 4) − (26 + 2) = +6 mm Hg, so fluid filters out.
• +2 mm Hg, filtration out of the capillary: This comes from subtracting IFHP from CHP but leaving out IFCOP: (30 − 2) − 26. IFCOP pulls fluid out of the capillary, so it must be added to the outward side.

The liver makes albumin, the protein that supplies most of blood colloid osmotic pressure. With less albumin, less pull holds fluid in the capillaries, net filtration pressure rises, and fluid collects in the interstitial spaces. Her ankles swell first because they sit lowest when she is upright.

• Her capillary hydrostatic pressure has risen: A rise in capillary pressure causes edema, but nothing in this case raises it. The finding that points to the cause is her low albumin.
• Her capillary walls have become leaky to protein: Leaky capillaries cause the swelling of inflammation. Her problem is too little protein in the blood, not protein escaping through the wall.
• Correct: Her blood colloid osmotic pressure has fallen: Correct. Low albumin lowers the inward pull, so more fluid filters out and stays in the tissue.
• Her interstitial fluid hydrostatic pressure has fallen: Interstitial hydrostatic pressure rises, not falls, as fluid collects in the tissue. That rise is a result of the edema, and it pushes back against more filtration.

Filtration rises when capillary hydrostatic pressure rises (dilated arterioles, higher venous pressure), when blood colloid osmotic pressure falls (less albumin), or when protein leaks out and shrinks the oncotic difference (leaky walls).

• Correct: The arterioles feeding the tissue dilate: Increases filtration. Dilated arterioles lose less pressure, so more pressure reaches the capillaries and CHP rises.
• Correct: Pressure in the veins draining the tissue rises: Increases filtration. With no large resistance between capillaries and veins, a rise in venous pressure passes back into the capillaries and raises CHP.
• Correct: Plasma albumin concentration falls: Increases filtration. Less albumin means a lower blood colloid osmotic pressure, so less inward pull opposes filtration.
• The arterioles feeding the tissue constrict: Decreases filtration. Constricted arterioles lose more pressure, so CHP downstream falls.
• Plasma albumin concentration rises: Decreases filtration. More albumin raises the inward oncotic pull.
• Correct: The capillary walls become leaky to protein: Increases filtration. Protein leaking into the interstitial fluid raises IFCOP and shrinks the oncotic difference that holds fluid in the blood.

Less plasma protein lowers blood colloid osmotic pressure. Filtration then exceeds drainage, interstitial fluid collects, and its pressure rises until a new balance is reached with visible edema.

• Blood colloid osmotic pressure: down. Plasma proteins, mainly albumin, create blood colloid osmotic pressure. Fewer proteins means less osmotic pull.
• Capillary hydrostatic pressure: no change. Capillary hydrostatic pressure depends on arterial pressure, arteriole resistance and venous pressure. None of these is changed directly by the protein level.
• Net filtration pressure: up. With less inward oncotic pull and unchanged outward push, the balance of the four forces shifts toward filtration.
• Interstitial fluid volume: up. More fluid filters out than drains away, so fluid collects between the cells: edema.
• Interstitial fluid hydrostatic pressure: up. As fluid collects in the tissue spaces, the pressure of that fluid rises. This rise partly opposes further filtration.

Outward filtration normally washes the thin space beneath the glycocalyx, keeping its protein level near zero. Once flow reverses, the washing stops, interstitial proteins diffuse into that space, and its oncotic pressure rises. The effective oncotic pull into the blood shrinks, and the inward flow dies away.

• Correct: Interstitial proteins build up in the space beneath the glycocalyx: Correct. Protein accumulation beneath the glycocalyx cancels the inward oncotic pull.
• Plasma proteins leak out through the clefts into the interstitial fluid: The glycocalyx holds plasma proteins back whichever way fluid flows. Loss of plasma protein is not what stops reabsorption.
• The arterioles dilate and raise the capillary pressure back up: Arteriole diameter is set by local and nervous signals, not by the direction of flow across the capillary wall. Reabsorption fades even when capillary pressure stays low.
• The glycocalyx dissolves, making the capillary wall fully permeable: The glycocalyx stays in place. It is damaged in some diseases, but that is not part of the normal shutoff.

In the classic picture, capillary hydrostatic pressure falls along the capillary while blood colloid osmotic pressure stays about 25 mm Hg. At the venule end, the oncotic pull is larger, so the model predicts net reabsorption of most of the fluid filtered upstream.

• Net filtration; capillary hydrostatic pressure is larger: That describes the arteriole end, where hydrostatic pressure is highest.
• Correct: Net reabsorption; blood colloid osmotic pressure is larger: Correct for the classic model. The revised Starling principle shows this reabsorption is not sustained in most tissues, but this is the answer exams expect.
• No net movement; the two forces are equal: The classic model places the point of no net movement near the middle of the capillary, where CHP equals BCOP, not at the venule end.
• Net reabsorption; capillary hydrostatic pressure is larger: Capillary hydrostatic pressure is lowest at the venule end, and a larger CHP would favor filtration, not reabsorption.

Diffusion rate rises with surface area and falls with distance. When more capillaries in a working muscle carry blood, the total exchange surface grows and each cell is closer to an open capillary, so oxygen diffuses to the cells faster.

• Higher capillary hydrostatic pressure pushes oxygen out faster by bulk flow: Oxygen leaves capillaries by diffusion down its concentration gradient. Bulk flow carries only a tiny fraction of the oxygen that diffusion moves.
• Correct: More capillaries open, adding surface area and shortening the distance: Correct. More open capillaries means more area and a shorter path.
• The capillary walls thicken and store extra oxygen: A thicker wall would lengthen the diffusion distance and slow oxygen delivery, and capillary walls do not thicken during a sprint.
• Plasma proteins carry the oxygen out through the clefts: Plasma proteins mostly stay in the blood, and oxygen does not need a carrier to cross the wall. It dissolves through the endothelial cells.