Lymphatic system and disorders

MDCAT Biology · Biology · Circulation. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Blood pressure at the arterial end of a capillary pushes water and small solutes out through the thin wall. This liquid is tissue fluid. It bathes the cells and has no red cells and very little protein. At the venous end the blood pressure is low, so the osmotic pull of the plasma proteins draws most of the fluid back.

The fluid left behind drains into blind-ended lymph capillaries and is now called lymph. Lymph vessels have valves, pass through lymph nodes and empty into the veins near the heart.

In atherosclerosis, fatty deposits narrow an artery. A clot can then block it. A blocked coronary artery causes a heart attack.

Rules to remember

Net filtration pressure = blood (hydrostatic) pressure − osmotic pressure of the plasma
Arterial end: hydrostatic pressure is greater, so fluid leaves. Venous end: osmotic pressure is greater, so fluid returns
Lymph nodes filter lymph and hold lymphocytes and macrophages; lacteals in the villi absorb digested fats
Oedema = swelling from excess tissue fluid (low plasma protein, blocked lymph vessels or high blood pressure)
Atherosclerosis = cholesterol-rich plaque (atheroma) in the artery wall; arteriosclerosis = hardening of the wall, with calcium deposits
Thrombus = a clot fixed where it formed; embolus = a clot or fragment carried in the blood
Hypertension = blood pressure that stays at about 140/90 mm Hg or above; normal is about 120/80 mm Hg

Common mistake

Students mix up angina and heart attack. In angina the heart muscle gets too little blood and hurts but does not die; in a heart attack (myocardial infarction) a coronary artery is blocked and part of the muscle dies.

Worked example

At the venous end of a capillary the blood pressure is 15 mm Hg and the osmotic pressure of the plasma is 25 mm Hg. Which way does fluid move, and under what net pressure?

Blood pressure pushes fluid out; osmotic pressure pulls fluid in.
Net pressure = 15 − 25 = −10 mm Hg.
The negative sign means the inward pull is greater.

Answer: Fluid moves from the tissue fluid back into the capillary under a net pressure of 10 mm Hg.

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