The lymph is mainly formed from the plasma of blood and it surrounds the cells. The main difference between blood and lymph is:

Correct — A, Lymph does not contain red blood cells. The stem has already told you how lymph forms — plasma seeps out of the blood capillaries into the spaces between cells. The capillary wall behaves as a filter: water, salts, glucose and small molecules cross it easily, most large plasma proteins cross poorly, and red blood cells do not cross at all. What collects in the tissue spaces and drains into the lymphatic vessels is therefore plasma without its red cells, which is why lymph is colourless while blood is red. White blood cells are a different matter — lymphocytes move freely out of the capillaries and are abundant in lymph, which is exactly what makes the lymphatic system part of the body's defences. So lymph is neither cell-free nor red-celled; it lacks one kind of cell and carries the other.

• (b)Lymph does not contain white blood cells. — The opposite is true. Lymph is rich in lymphocytes, and the lymph nodes through which it passes are where those cells multiply and where infection is fought.
• (c)Lymph contains both red blood cells and white blood cells. — Half right at best. The white cells are there, the red cells are not, and if red cells were present lymph would not be the colourless fluid it is.
• (d)Lymph does not contain any cells. — Lymph is not a cell-free filtrate. It carries lymphocytes, which is the reason it has an immune function at all.

Blood does not reach cells directly. It runs through capillaries whose thin walls let plasma leak into the surrounding spaces, and that tissue fluid is what actually bathes the cells and exchanges materials with them. Most of it returns to the capillaries, and the remainder drains into blind-ended lymphatic capillaries as lymph, travels through lymph nodes, and is finally returned to the bloodstream at the base of the neck where the great lymphatic ducts empty into the subclavian veins.

The four options are a clean test of one idea: which components of blood can get through a capillary wall. Red blood cells are the largest formed elements and are retained; plasma proteins are large molecules and mostly retained; water, ions, glucose and white blood cells get out. Reading the options that way turns the question into a filtration problem rather than a memory test. A useful physical check is colour — lymph is described as colourless or pale yellow, and only haemoglobin inside red cells makes blood red, so any option putting red cells in lymph fails on appearance alone.

• Lymph is formed from plasma that filters out of the blood capillaries into the spaces between cells.
• It contains no red blood cells and is colourless, in contrast with blood.
• It does carry white blood cells, mainly lymphocytes, which is the basis of its role in immunity.
• Its protein content is lower than that of plasma, because large plasma proteins cross the capillary wall poorly.
• Lacteals in the villi of the small intestine absorb digested fat into the lymph, making intestinal lymph milky — the fluid called chyle.
• Lymph is returned to the blood where the thoracic duct and the right lymphatic duct open into the subclavian veins.

• Reading lymph as a cell-free fluid because it is colourless.
• Assuming that anything derived from plasma must contain everything plasma contains.
• Confusing lymph with serum, which is plasma from which the clotting proteins have been removed.

Framed as four statements about the same difference, three of them variations on the wrong idea about which cells are present, so careful reading of each option matters as much as the biology.

• practice — not a real PYQ

  The milky fluid found in the lacteals of the intestinal villi after a fatty meal is called

  • (a)chyme
  • (b)chyle
  • (c)serum
  • (d)plasma

  Answer(b) chyle — lymph carrying absorbed fat, which gives it a milky appearance. Chyme is the partly digested food leaving the stomach.

• practice — not a real PYQ

  Lymph is finally returned to the blood circulation at the

  • (a)hepatic portal vein
  • (b)subclavian veins
  • (c)renal artery
  • (d)pulmonary vein

  Answer(b) subclavian veins — the thoracic duct and the right lymphatic duct empty there, at the base of the neck.