Blood
Blood is the body's transport medium, carrying oxygen, nutrients, hormones, and immune cells, and helping regulate temperature and pH.
Composition of blood
An adult has about 5 litres of blood. About 55% is plasma, a straw-coloured fluid that is about 92% water carrying proteins, nutrients, hormones, gases, and wastes. About 45% is red blood cells; white blood cells and platelets make up less than 1%. The percentage of red cells is the haematocrit.
Plasma proteins
Albumin, made by the liver, is the most abundant plasma protein and holds water in the blood by osmosis. Globulins include antibodies and transport proteins. Fibrinogen is needed for clotting. When the liver fails, low albumin lets fluid leak into tissues, causing swelling (oedema).
Red blood cells
Red blood cells (erythrocytes) are biconcave discs with no nucleus, packed with haemoglobin, which binds oxygen using iron. They live about 120 days and are made in red bone marrow, stimulated by the hormone erythropoietin (EPO) from the kidneys when oxygen is low. Anaemia is a shortage of red cells or haemoglobin, causing tiredness and breathlessness.
White blood cells
White blood cells (leukocytes) defend against infection. Neutrophils, the most numerous, engulf bacteria. Lymphocytes (B and T cells) carry out specific immunity. Monocytes become macrophages that engulf debris. Eosinophils fight parasites and are involved in allergies; basophils release histamine. A high white count often signals infection.
Platelets and haemostasis
Platelets are cell fragments that help stop bleeding (haemostasis) in three steps: the vessel constricts (vascular spasm), platelets stick together to form a plug, and a clotting cascade converts fibrinogen into fibrin threads that mesh into a clot. Vitamin K is needed to make several clotting factors. Haemophilia is a genetic lack of a clotting factor.
Blood types
Red cells carry surface antigens. Type A has A antigens, type B has B, type AB has both, and type O has neither. Plasma contains antibodies against the antigens a person lacks. Rh-positive blood carries the Rh(D) antigen. Mismatched transfusions cause the recipient's antibodies to clump the donor cells, a dangerous reaction.
Transfusion compatibility
Type O negative red cells lack A, B, and Rh antigens, so they can be given in emergencies to almost anyone: universal donors. Type AB positive people have no anti-A or anti-B antibodies and can receive red cells of any ABO type: universal recipients. Hospitals still cross-match blood whenever possible.
Worked example
A patient with type A positive blood needs red cells. Their plasma has anti-B antibodies, so they can receive A or O cells, either Rh-positive or Rh-negative. Giving type B or AB cells would trigger a reaction against the B antigens.
Key takeaways
Blood is about 55% plasma and 45% cells. Albumin holds water in vessels; fibrinogen enables clotting. Red cells carry oxygen on haemoglobin, last 120 days, and are boosted by EPO. White cells defend; neutrophils are most common. Platelets and the clotting cascade stop bleeding. ABO and Rh antigens determine transfusion compatibility; O negative is the universal donor.
Flashcard
Liquid part of blood, about 55%
Check your understanding
Why can type O negative red cells be given to almost any patient in an emergency?