Questions: Microvascular Exchange and Fluid Balance

Question 1

A patient with severe liver cirrhosis develops widespread edema and ascites. Their capillary hydrostatic pressure is measured as normal. The Starling force most responsible for their edema is:

• A. Elevated capillary hydrostatic pressure from portal venous congestion
• B. Elevated interstitial oncotic pressure from inflammatory protein leakage
• C. Reduced plasma oncotic pressure from decreased albumin synthesis by the damaged liver
• D. Lymphatic obstruction caused by liver fibrosis compressing the thoracic duct

Answer: (C)

The liver is the primary site of albumin synthesis. In cirrhosis, hepatocyte destruction reduces albumin production, lowering plasma protein concentration and therefore plasma oncotic pressure (πc). With reduced πc, the inward osmotic force that opposes filtration at the venular end is diminished — fluid that would normally be reabsorbed instead remains in the interstitium. This is why the edema of liver disease and malnutrition (kwashiorkor) can occur even with normal capillary hydrostatic pressure. The Starling equation makes the mechanism explicit: reduced πc means the (πc − πi) term shrinks, shifting net filtration pressure toward outward flow throughout the capillary bed.

Question 2

At the venular end of a systemic capillary, which force configuration produces net fluid reabsorption?

• A. Capillary hydrostatic pressure (~35 mmHg) exceeds plasma oncotic pressure (~28 mmHg)
• B. Interstitial hydrostatic pressure rises above plasma oncotic pressure, forcing fluid inward
• C. Plasma oncotic pressure (~28 mmHg) exceeds the now-reduced capillary hydrostatic pressure (~15 mmHg), creating net inward osmotic pull
• D. Lymphatic drainage reduces interstitial pressure enough to pull fluid back into the capillary

Answer: (C)

As blood travels from the arteriolar to the venular end, capillary hydrostatic pressure falls from ~35 mmHg to ~15 mmHg as energy is dissipated. Plasma oncotic pressure remains roughly constant at ~25–28 mmHg throughout (proteins don't leave the capillary in large amounts under normal conditions). At the venular end, the hydrostatic outward force (~15 mmHg) is now less than the oncotic inward force (~28 mmHg), so net flow reverses and fluid is reabsorbed. This is the classic textbook depiction of Starling equilibrium, though in reality the system is slightly net-filtration and lymphatics return the remainder.

Question 3

In normal physiology, approximately 10% of the fluid filtered from capillaries at the arteriolar end is not reabsorbed at the venular end and must be returned to circulation by the lymphatic system.

• T. True
• F. False

Answer: True

The capillary exchange system is not perfectly balanced — net filtration slightly exceeds net reabsorption over the length of a typical capillary. Approximately 2–4 liters of fluid per day escapes into the interstitium and is not recaptured at the venular end. The lymphatic system collects this excess, along with small amounts of plasma protein that leak through capillary walls, and returns them to the venous circulation via the thoracic duct. This makes lymphatics an essential component of fluid homeostasis, not a backup system. Lymphatic obstruction (filariasis, lymph node dissection) causes severe edema — lymphedema — even when all four Starling forces are completely normal.

Question 4

Most fluid filtered from capillaries at the arteriolar end is reabsorbed at the venular end, meaning the lymphatic system is primarily needed when Starling forces are abnormal.

• T. True
• F. False

Answer: False

This is the most common misconception about microvascular exchange. Under normal conditions, approximately 90% of filtered fluid is reabsorbed at the venular end, but the remaining ~10% (2–4 liters per day) requires lymphatic return. The lymphatic system is not a reserve mechanism — it is continuously active in normal physiology, draining excess fluid and the plasma proteins that leak through capillary walls. If lymphatics are blocked, fluid accumulates even when capillary hydrostatic and oncotic pressures are entirely normal. This explains why lymphedema is such a severe and chronic condition.

Question 5

Using the Starling forces framework, explain how lymphatic obstruction leads to edema even when capillary hydrostatic and oncotic pressures are normal.

Model answer: Under normal conditions, Starling forces produce slight net filtration — slightly more fluid leaves capillaries than is reabsorbed at the venular end. The lymphatic system continuously drains this excess (~10% of filtered fluid) and returns it to circulation. When lymphatics are obstructed, this drainage stops. Filtered fluid accumulates in the interstitium, raising interstitial hydrostatic pressure (Pi). As Pi rises, it increases the outward-opposing force and simultaneously elevates interstitial oncotic pressure (πi) as leaked proteins concentrate. Even though capillary hydrostatic and plasma oncotic pressures are normal, the interstitial compartment fills because the normal lymphatic outflow is blocked.

This answer demonstrates understanding of the four-force system as a dynamic equilibrium, not a static balance. The lymphatics are part of the normal steady state — the Starling equilibrium assumes their function. Removing lymphatic drainage is equivalent to adding a fourth force abnormality: interstitial pressure rises until a new (pathological) steady state is reached with a swollen interstitium. Filariasis, in which parasitic worms block lymphatic vessels, produces elephantiasis through exactly this mechanism. Cancer treatment involving lymph node dissection produces the same result in the affected limb.