Circulatory System in Humans
Senior 3 · Biology
Circulatory System in Humans
Lesson 5 of 5
25 min
In 2016 Rwanda began delivering blood to hospitals by drone. A study of 20 health facilities found that, between March 2017 and December 2019, drones delivered 12 733 orders of blood products, on average 79 minutes faster than the road, and that blood wasted by passing its expiry date fell by about two-thirds. But a bag of blood that arrives in minutes is useless, or deadly, if it does not match the patient. This lesson explains why, and also how your blood stops itself leaking out, and how the fluid that leaks out of your capillaries gets back.
Definition
Blood clotting: the process that turns liquid blood into a solid plug at a wound.
Antigen: a molecule, usually a protein, on the surface of a cell that the immune system can recognise. Antibody: a protein made by lymphocytes that binds to one particular antigen.
Agglutination: the clumping of red blood cells when antibodies bind to the antigens on them.
Blood transfusion: giving blood from one person (the donor) to another (the recipient).
Lymph: tissue fluid that has drained into the vessels of the lymphatic system.
When a blood vessel is cut, it first narrows and platelets stick to the damaged edges. Then a chain of reactions in the plasma makes a solid clot:
Figure 1: Blood clotting.
Clotting stops the loss of blood and keeps pathogens out of the wound while it heals underneath. In haemophilia, an inherited condition, one of the clotting factors is missing, so even small cuts and bruises can bleed for a long time. Clots that form inside vessels, where they should not, are also dangerous: a clot in a coronary artery causes a heart attack, as you saw in Lesson 3, and one in the brain causes a stroke.
Everyone's red cells carry, or do not carry, two antigens called A and B. Your plasma contains antibodies against whichever of the two you lack. That gives four groups:
| Group | Antigens on red cells | Antibodies in plasma | Can receive red cells from | Can give red cells to |
|---|---|---|---|---|
| A | A | anti-B | A, O | A, AB |
| B | B | anti-A | B, O | B, AB |
| AB | A and B | none | A, B, AB, O | AB |
| O | none | anti-A and anti-B | O | A, B, AB, O |
The rule is simple: the donor's red cells must not carry an antigen that the recipient's plasma has antibodies against. If they do, the antibodies bind to the donated cells and make them agglutinate; the clumps block small vessels and the cells burst, which can kill. Group O red cells carry neither antigen, so they can be given to anyone (O is the universal donor); group AB plasma has neither antibody, so AB people can receive red cells of any group (the universal recipient).
A second system, the Rhesus (Rh) factor, depends on another antigen, D. People who have it are Rh positive; those who lack it are Rh negative and can make anti-D antibodies if Rh positive blood ever enters their body. So Rh negative patients should receive Rh negative blood. In practice, hospitals give blood of the same ABO and Rh group, and first mix a little of the donor's and the patient's blood to check that they match (a cross-match). All donated blood is also tested for infections such as HIV and hepatitis.
Worked example 1: a grouping test
Drops of four blood samples were mixed with anti-A serum and with anti-B serum on a white tile. Identify each blood group.
Figure 2: Results of a blood grouping test (illustrative).
Worked example 2: choosing a donor
A patient of group B needs red cells urgently. Donors of groups A, B, AB and O are available. Which can give, and why?
As blood flows through a capillary, the pressure at the arterial end pushes some plasma out through the thin wall. This tissue fluid bathes every cell, bringing oxygen and food and taking away wastes. Most of it soaks back into the capillary at the venous end, where the pressure is lower, but each day a few litres are left over. This excess drains into tiny, blind-ended lymph capillaries and is then called lymph.
Figure 3: Tissue fluid and lymph in a capillary bed.
Lymph capillaries join into larger lymph vessels, which, like veins, have valves and are squeezed by the muscles around them. On the way the lymph passes through lymph nodes, small swellings found in the neck, armpits, groin and abdomen. Finally it empties back into the blood, into veins near the heart. The lymphatic system has three jobs:
If lymph vessels are blocked, fluid collects and the tissues swell. In lymphatic filariasis, a tropical disease spread by mosquitoes, tiny worms block the lymph vessels and can cause great swelling of the legs.
Warning
Common mistakes. (1) Saying a group A person has anti-A antibodies: they have anti-B; nobody makes antibodies against their own antigens. (2) Saying group O can receive from anyone: O plasma has both anti-A and anti-B, so O can receive only O. (3) Calling clotting "drying of blood": it is a chain of chemical reactions that makes insoluble fibrin. (4) Saying lymph is pumped by the heart: it is moved by muscles squeezing the lymph vessels, with valves keeping it flowing one way.
Why do blood types matter?, Natalie S. Hodge, TED-Ed. Watch how antigens and antibodies decide which blood a person can safely receive, and what the Rh factor adds.
Tip
Memory hook: "You make antibodies against what you lack." Group A lacks B, so it has anti-B. Then check: does the donor's cell carry an antigen the patient's plasma attacks?
Practice: clotting, blood groups and lymph
Key points
Further reading
Sources and credits
Written by Ganzaa: the text, Figures 1 to 3 (Figure 2 shows illustrative results), the table, the worked examples and the practice questions.
