(a) The main constituents of the mammalian blood are:
(i)Plasma/fluid matrix
(ii) Erythrocytes/red blood corpucles
(iii) Platelets/thrombocytes
(iv) Leucocytes/white blood cells or white corpuscles.
(b) Blood Constituents and their functions;
(i) Plasma:
(a) plasma transports end products of digestion or digested food especially glucose and amino acids.
(b) It transports waste products, hormones, heat from very actively respiring tissues, CO2, urea or waste materials, antibodies, amino acids and mineral salts.
(c) It participates in blood clotting.
(d) It transports oxygen.

Arteries

Veins

1.They carry blood away from the heart.2. Blood flows rapidly under high pressure.3. They possess narrow lumen.4. Walls have thick layer of muscles i:e. they are thick-walled. 5. They have elastic walls.6. Valves are absent.7. They start as large vessels and subsequently sub-divide. 8. They are deep-seated in the body.9. Blood flows in arteries under the pressure from the heart beats or pulse.10. Arterial blood is bright red.11. All arteries carry oxygenated blood except the pulmonary artery.

They carry blood into the heart.Blood flows gently or slowly under low pressure. They possess wide lumen.They are thin-walled. The walls are inelastic. Valves are present.They start as tiny vessels and subsequently unite or merge into large vessels They are superficially-seated in the body i.e., not deep seatedBlood flows in veins with assistance from contracting muscles and gravity and are not detectable. Venous blood is dark or dull red. . All veins carry deoxygenated blood except the pulmonary the pulmonary vei

(ii) Erythrocytes (Red blood cells):
(a) The red blood cells transport oxygen as oxyhaemoglobin.
(b) They transport small quantities of CO2 and
(c) They take part in blood clotting.
(iii)Leucocytes (White Blood Cells):
(a) Leucocytes defend the body against disease - causing organisms, for example, phagocytes engulf or ingest and breakdown disease-causing bacteria.
(b) They produce antibodies or immunity for the body.
(iv) Platelets: They aid clotting of blood.