An illness causes Doug to experience severe diarrhea and an accompanying loss of bicarbonate-rich secretions. How can this metabolic acidosis be compensated?

A. protein-regulated diffusion. Large plasma proteins like albumin are too big to pass through the filtration membrane and remain in the capillaries. They actually create a colloid osmotic pressure that pulls water back into the blood, opposing filtration. Diffusion is a passive movement of solutes, not the primary mechanical force driving the high-volume ultrafiltration of plasma.

B. glomerular hydrostatic pressure (glomerular blood pressure). This is the blood pressure within the glomerular capillaries, which is typically much higher than in other capillary beds due to the high-resistance efferent arteriole. It serves as the dominant outward force that physically pushes water and small solutes through the filtration slits. It is the fundamental driver of the glomerular filtration rate.

C. the size of the pores in the basement membrane of the capillaries. The fenestrations and filtration slits determine the permeability and selectivity of the filter, essentially acting as a sieve. While these pores permit the passage of substances, they do not provide the energy or force to move them. They represent a physical constraint on what can pass rather than a driving force.

D. the ionic electrochemical gradient. Electrochemical gradients primarily drive the movement of specific ions across tubular epithelial cells during reabsorption and secretion. Glomerular filtration is a non-selective, bulk-flow process driven by mechanical pressure rather than individual ion concentrations. The process is governed by hydrostatic and osmotic pressures according to Starling's law.

A. plasma: Plasma is the liquid component of blood that remains in the glomerular capillaries if it contains large proteins and cells. While the fluid in the capsular space is derived from plasma, it lacks the protein density of true plasma. Plasma stays within the vascular compartment.

B. urine: Fluid is only technically classified as urine once it has exited the collecting ducts and entered the renal calyces. At this stage, no further reabsorption or secretion occurs. The fluid in the capsular space will undergo significant modification before becoming final urine.

C. blood: Whole blood contains formed elements such as erythrocytes, leukocytes, and platelets which are too large to pass through the filtration membrane. Under normal physiological conditions, blood should never enter the capsular space. Presence of blood in this space indicates basement membrane damage.

D. filtrate: The fluid that enters the capsular space is an ultrafiltrate of blood plasma. It contains water and small solutes like glucose, amino acids, and nitrogenous wastes that pass through the filtration slits. This liquid is the raw material that the tubules process into urine.