Concepts: Lymph and the blood vessels

2.1 The fluid between the cells

Not all the fluid that leaves the blood capillaries is taken back by them. Some of it remains in the spaces between the cells as tissue fluid, and this fluid is collected by a second set of vessels. The fluid inside those vessels is called lymph. Lymph is colourless, contains fewer proteins than plasma, and carries lymphocytes, so it is an important part of the body's defence.

The lymphatic system has three jobs. It drains the excess tissue fluid back into the blood circulation, so that the tissue does not become waterlogged. It carries absorbed fats from the intestine in the lacteals of the villi. And it provides the sites where lymphocytes multiply and where the immune response is organised, in the lymph nodes and the spleen. Lymph vessels do not form a closed loop with the blood vessels; they empty their fluid into the large veins near the heart, and they have valves that keep the lymph moving in one direction.

2.1.1 Open and closed circulation

In an open circulatory system the blood leaves the vessels and flows through a system of sinuses, bathing the tissues directly before it returns to the heart. Insects and many molluscs have this arrangement. In a closed circulatory system the blood stays inside vessels for the whole circuit and exchanges materials with the tissues through the thin capillary walls. Vertebrates and earthworms have a closed system. The closed system allows a higher pressure and a faster, more directed flow, which is one reason larger and more active animals have it.

2.1.2 Arteries, veins and capillaries

Arteries carry blood away from the heart. Three coats make up the wall. The innermost coat is a lining of squamous endothelium, called the tunica intima. Outside it lies the tunica media, built of smooth muscle and elastic fibres. The outermost coat, the tunica externa, is fibrous connective tissue. The tunica media is thick and elastic in arteries, so they can stretch when the heart pushes blood into them and then recoil to keep the flow going between beats.

Veins carry blood back to the heart. Their tunica media is comparatively thin and they have less elastic tissue, so they cannot drive blood by recoil; instead they have valves that prevent backflow, and the movement of the surrounding muscles squeezes the blood along. The lumen of a vein is wider than that of an artery of the same size.

Capillaries are the smallest vessels, only one cell thick in their wall, and they link the smallest arteries to the smallest veins. Their thin wall and enormous total surface area make them the actual exchange region where oxygen, nutrients and waste products pass between blood and tissue fluid.

2.1.3 Lymph in motion

Lymph moves slowly, because the lymphatic vessels do not have a pump like the heart. Movement of the surrounding skeletal muscles squeezes the vessels, and the valves inside them stop the fluid from sliding back. Breathing also helps, because the pressure changes in the chest draw lymph towards the large veins at the base of the neck. This is one reason a person who sits still for many hours notices swelling in the ankles: the muscle pump is idle and tissue fluid collects. Walking or raising the legs restores the flow, and the swelling goes down.

The lymph nodes are the filters on this route. They contain dense populations of lymphocytes and macrophages, which examine the lymph as it passes and trap microbes and damaged cells. When an infection is active the nodes in the region become enlarged and tender, because the cells inside them are multiplying. This is why a doctor feels the neck, armpit and groin for swollen nodes during an examination. The spleen performs a related filtering job for the blood, and it is also a store of lymphocytes; together the nodes, spleen and the other lymphoid tissues form the defensive network of the body.

2.1.4 Why the vessels are built differently

Each kind of blood vessel matches its job. Arteries face the pressure wave from each heartbeat, so their middle layer is thick with smooth muscle and elastic fibres. The elastic recoil keeps the blood moving between beats and smooths the flow. Veins face almost no pressure, so they are thin walled and wide, and they rely on valves and the muscle pump. Capillaries are built for exchange, so their wall is a single layer of endothelial cells with a basement membrane. The diameter of an arteriole is controlled by its smooth muscle, and that control decides how much blood reaches each capillary bed. During exercise the arterioles of the muscles widen while those of the gut narrow, so the same cardiac output is redirected to where it is needed.

2.1.5 Practice from the NCERT exercise, question 5

Exercise 5. Blood and lymph differ in composition and in flow. Blood has red cells, white cells and platelets in plasma and flows in a closed circuit, while lymph is tissue fluid with lymphocytes and fewer proteins and flows in lymph vessels towards the veins. Blood carries oxygen and nutrients, while lymph drains tissue fluid, carries absorbed fats and supports immunity.