Course 13.1Lesson 3 of 3

Clotting of blood and lymph

When a vessel is cut, platelets and the injured tissue set off a chain of reactions. Prothrombin becomes thrombin, with calcium ions, and thrombin changes fibrinogen into fibrin threads that trap blood cells in a clot. Tissue fluid that leaves the capillaries is collected by the lymph vessels and returned to the large veins as lymph.

about 25 min1 simulation1 practice question

Lesson 3 of 3Show all lessons

Try the code and the simulations beside the lesson.

A cut that stops bleeding

A cut bleeds, and soon the bleeding stops. A dark red lump forms over the cut. This lump is a clot, or coagulum. The process is coagulation, the clotting of blood.

A clot is a network of threads of fibrin. Dead and damaged formed elements of the blood are trapped in the network.

The steps of clotting

A flow chart of clotting. At top left, a box says injury: platelets and damaged tissue release clotting factors. A downward arrow leads to a box for thrombokinase, an enzyme complex made by a cascade. On the right, a box for prothrombin, inactive in the plasma, has a downward arrow to a box for thrombin, an enzyme, and the arrow is marked with calcium ions. A dashed purple arrow from thrombokinase points at this step, to show that thrombokinase makes the change. Below at left, a box for fibrinogen, inactive in the plasma, has a downward arrow to a box for fibrin threads. A dashed purple arrow from thrombin points at this step. A note says that the threads trap blood cells in a clot.

An injury or a trauma makes the platelets release certain factors. The damaged tissue also releases factors.
These factors, with other clotting factors that are in the plasma in an inactive state, set off a cascade. A cascade is a series of linked enzyme reactions, where each step switches on the next.
The cascade makes an enzyme complex, thrombokinase. Later books call it prothrombin activator, or prothrombinase.
Thrombokinase changes the inactive plasma protein prothrombin into the enzyme thrombin. This step needs calcium ions.
Thrombin changes the inactive plasma protein fibrinogen into fibrin threads.
The fibrin threads form a network, and blood cells are trapped in it. This is the clot.

Calcium ions play a very important part in clotting. A person who has no clotting factors, or too few platelets, loses much blood from a small wound. Basophils and some other cells release heparin, a substance that stops clotting.

Tissue fluid and lymph

The thin walls of the blood capillaries let some fluid pass into the spaces between the cells. This fluid is the tissue fluid, or interstitial fluid. It is like plasma, but the large proteins stay inside the capillary. It has the same minerals as the plasma.

The tissue fluid bathes the cells. Oxygen, food and other materials pass from the blood to the cells through it, and carbon dioxide and wastes pass back.

Most of the fluid returns to the capillaries. The rest is collected by a network of vessels called the lymphatic system. The fluid in these vessels is the lymph.

A drawing of the tissue between a blood capillary and a lymph capillary. Blood flows from an arteriole through the blood capillary to a venule. Fluid leaks out of the capillary into the tissue spaces and becomes tissue fluid, which bathes the cells. Most of it goes back into the capillary. The rest enters the blind-ended lymph capillary, becomes lymph, passes through a lymph node and a larger lymph vessel, and is drained into a large vein near the heart. In the wall of the intestine, a lacteal in a villus takes up fats.

Lymph is a colourless fluid. It contains lymphocytes, which give immunity.
It carries nutrients, hormones and other materials.
The vessels drain the lymph back into the large veins near the heart.
Fats are absorbed from the intestine through the lacteals in the villi, so the lymph that comes from the intestine is rich in fat.
Lymph passes through lymph nodes, where lymphocytes meet germs.

So the lymphatic system is a second drain of the body. It brings back the fluid that the blood did not take back. In a healthy adult this is about 2 to 4 L of lymph in a day.

This is a teaching model, not a measurement. It shows only one idea: the clot forms more slowly when the platelets or the calcium fall. This is why a person with a low platelet count may bleed for a long time. Real clotting has many more steps, and the calcium of living blood can never fall as low as the slider allows.

Try it in Python

The program prints the share of the fibrin that has formed in each period of two minutes.

Practise with new numbers

Common mistakes

Saying that thrombin is in the plasma in its active form. Prothrombin, the inactive form, is in the plasma.
Changing the order of the enzymes. Thrombokinase acts on prothrombin. Thrombin acts on fibrinogen.
Saying that the clot is made of platelets alone. It is a net of fibrin with the cells trapped in it.
Saying that lymph is red. It is colourless, because it has no red cells.
Saying that fats from the intestine enter the blood capillaries. They are mostly taken up by the lacteals into the lymph.

Remember these

Clot: a network of fibrin with trapped blood cells. Platelets and the damaged tissue start it.
Thrombokinase changes prothrombin to thrombin, with Ca $^{2+}$ . Thrombin changes fibrinogen to fibrin.
Tissue fluid comes from the plasma. The lymphatic system collects it, and the lymph returns to the large veins.
Lymph is colourless, has lymphocytes, and carries fats from the intestine through the lacteals.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

What is a clot made of?

Name the inactive substances that change to thrombin and to fibrin.

Which ion is needed for clotting?

What is the difference between tissue fluid and lymph?

How are fats absorbed from the intestine?

References

Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapters on the microcirculation and lymphatic system, and on haemostasis and blood coagulation.
Moore J. E. and Bertram C. D. (2018). Lymphatic system flows. Annual Review of Fluid Mechanics, 50, 459 to 482. doi:10.1146/annurev-fluid-122316-045259.
NCERT (2023). Biology, Textbook for Class XI, chapter 15.