Which statement about hydrostatic pressure in capillaries is correct?

Which statement about hydrostatic pressure in capillaries is correct?

• A. It rises toward the venule end.
• B. It drops as blood moves away from the arteriolar end.
• C. It remains constant along the capillary bed.
• D. It increases due to filtration.

Answer: (B)

The concept being tested is how pressure inside capillaries changes along their length. Capillary hydrostatic pressure is the pressure that pushes fluid out of the capillary into the surrounding tissue. It starts relatively high at the arteriolar end and steadily falls as blood moves toward the venous end, because friction, resistance, and the outward movement of fluid reduce the intravascular pressure along the capillary. In typical systemic capillaries, arterial-end pressures are around 35–40 mmHg and drop to roughly 15–20 mmHg by the venous end. This makes the statement that hydrostatic pressure drops as blood moves away from the arteriolar end the best description. It doesn’t rise toward the venule end, nor stay constant, and it isn’t described as increasing because of filtration—filtration occurs due to the high pressure at the arterial end, and as fluid leaves, pressure inside the capillary decreases rather than increases.

The concept being tested is how pressure inside capillaries changes along their length. Capillary hydrostatic pressure is the pressure that pushes fluid out of the capillary into the surrounding tissue. It starts relatively high at the arteriolar end and steadily falls as blood moves toward the venous end, because friction, resistance, and the outward movement of fluid reduce the intravascular pressure along the capillary. In typical systemic capillaries, arterial-end pressures are around 35–40 mmHg and drop to roughly 15–20 mmHg by the venous end. This makes the statement that hydrostatic pressure drops as blood moves away from the arteriolar end the best description. It doesn’t rise toward the venule end, nor stay constant, and it isn’t described as increasing because of filtration—filtration occurs due to the high pressure at the arterial end, and as fluid leaves, pressure inside the capillary decreases rather than increases.