Why a Small Cut Stops Bleeding on Its Own

Blood looks like a single red liquid, but spin it in a tube and it separates into a straw-coloured fluid and a layer of cells. Together they carry oxygen, fight infection and seal wounds.

This part covers plasma and the formed elements, blood groups, clotting, and lymph.

Blood is a fluid connective tissue made of plasma, about 55 per cent of its volume, and formed elements, about 45 per cent: RBCs carry gases, five kinds of WBCs defend the body, and platelets start clotting.

Plasma:

- 90 to 92 per cent water, 6 to 8 per cent proteins
- Fibrinogen for clotting, globulins for defence, albumins for osmotic balance

Erythrocytes (RBCs):

- **5 to 5.5 million per mm${}^3$, formed in red bone marrow in adults
- Biconcave, with no nucleus in most mammals
- Carry haemoglobin, 12 to 16 g per 100 mL of blood
- Live about 120 days

Leucocytes (WBCs): 6000 to 8000 per mm${}^3$.

- Granulocytes — neutrophils (60 to 65 per cent, most abundant, phagocytic), eosinophils (2 to 3 per cent, infections and allergies), basophils (0.5 to 1 per cent, least abundant, release histamine, serotonin and heparin in inflammation)
- Agranulocytes — lymphocytes (20 to 25 per cent, B and T cells for immunity), monocytes (6 to 8 per cent, phagocytic)

Platelets (thrombocytes): fragments of megakaryocytes; 1.5 to 3.5 lakh per mm${}^3$; release clotting factors.

An everyday example. During dengue, doctors track the platelet count closely, because a sharp fall raises the risk of bleeding.

The substance. Serum is not the same as plasma** — serum is plasma with its clotting factors removed.

ABO groups depend on the A and B antigens on RBCs, and plasma carries antibodies against the antigens a person lacks, so donor RBCs must not carry antigens the recipient's antibodies will attack; an Rh-negative mother can make anti-Rh antibodies that destroy the RBCs of a later Rh-positive foetus.

ABO grouping:

- Group A — antigen A; anti-B antibodies; receives from A and O
- Group B — antigen B; anti-A antibodies; receives from B and O
- Group AB — antigens A and B; no antibodies; receives from all groups — the universal recipient
- Group O — no antigens; anti-A and anti-B; receives only from O — the universal donor

Worked example — check a transfusion. A group B patient is offered group AB blood. The donor RBCs carry antigen A, and the patient's plasma has anti-A, so the cells would clump: not safe.

Rh grouping. RBCs carrying the Rh antigen are **Rh${}^{+}$; those without it are Rh${}^{-}$**. An Rh${}^{-}$ person exposed to Rh${}^{+}$ blood forms anti-Rh antibodies, so Rh must be matched too.

Erythroblastosis foetalis:

- An **Rh${}^{-}$ mother carries an Rh${}^{+}$ foetus
- Some foetal blood may reach the mother at the first delivery
- The mother then makes anti-Rh antibodies
- In a later Rh${}^{+}$ pregnancy, these antibodies cross into the foetus and destroy its RBCs, causing severe anaemia and jaundice
- It is prevented by giving the mother anti-Rh antibodies right after the first delivery

An everyday example. Hospital blood banks keep O negative blood ready for emergencies when there is no time to test a patient's group.

The substance. The first Rh${}^{+}$ baby is usually unharmed** — the danger lies in later pregnancies.

An injury sets off a cascade that forms thrombokinase, which converts inactive prothrombin into active thrombin; thrombin then turns soluble fibrinogen into insoluble fibrin threads that trap cells into a clot, with calcium ions needed along the way.

The cascade:

- Injury stimulates platelets to release factors, and damaged tissues release factors too
- These start a chain of enzyme reactions
- An enzyme complex, thrombokinase, forms
- Thrombokinase converts prothrombin into thrombin
- Thrombin converts fibrinogen into fibrin
- Fibrin threads form a mesh that traps dead and damaged formed elements, making the clot

$$\text{Prothrombin}\underset{{\text{Ca}}^{2+}}{\overset{\text{thrombokinase}}{\to }}\text{Thrombin}\text{Fibrinogen}\stackrel{\text{thrombin}}{\to }\text{Fibrin}$$

Worked example — predict the effect. Blood is collected in a tube containing a chemical that binds free Ca${}^{2+}$. It does not clot, because thrombin cannot form.

An everyday example. The dark reddish-brown crust on a scraped elbow is a clot of fibrin threads and trapped blood cells.

The substance. Prothrombin and fibrinogen circulate all the time — the cascade only switches them on where there is injury.

As blood passes through capillaries, water and small solutes leak into the spaces between cells as tissue fluid; this fluid, collected into lymphatic vessels, is lymph, which returns fluid to the blood, carries absorbed fats from the intestine and carries lymphocytes for immunity.

Formation:

- Water and dissolved substances move out of capillaries into tissue spaces, leaving larger proteins and most formed elements behind
- This interstitial (tissue) fluid has the same mineral distribution as plasma

Lymph:

- Collected by a network of lymphatic vessels and drained into major veins
- A colourless fluid containing specialised lymphocytes

Functions:

- Immunity — lymphocytes in lymph drive immune responses
- Fat absorption — fats are absorbed into lymph through lacteals in the intestinal villi
- Fluid balance — returns leaked fluid, maintaining blood volume

An everyday example. Swollen, tender glands in the neck during a sore throat are lymph nodes where lymphocytes are multiplying to fight infection.

The substance. If lymph drainage is blocked, tissue fluid collects and the part swells, as in the leg swelling of filariasis.