Describe the function of platelets. [3]

a. prevent excessive blood loss and entry of pathogens into the bloodstream.

b. contain an enzyme that catalyses soluble fibrinogen into insoluble fibrin threads.

c. Threads form a network that entangles red blood cells to form a clot.

Explain how red blood cell is adapted to its function. [2]

a. Contains haemoglobin that binds reversibly with oxygen.

b. Biconcave shape increases surface area to volume ratio. This increases rate of absorption and release of oxygen.

c. Flexible and can change into bell-shaped structure, allows easy flow through narrow capillaries.

d. No nucleus to store more haemoglobin.

Describe the function of phagocytes. [2]

a. carries out phagocytosis;

b. engulfs and destroys pathogens e.g. harmful bacteria and viruses

Describe the role of lymphocytes. [2]

a. produces antibodies

b. recognises pathogens (e.g. harmful bacteria and viruses) and destroys them

c. causes pathogens to clump together for easy ingestion by phagocytes

Describe how capillaries are adapted to their function. [3]

a. capillary walls are partially permeable, allows substances to diffuse quickly.

b. Numerous branches provide large surface area to volume ratio, allows higher rate of diffusion of substances between blood and tissue cells

c. Total cross-sectional area of the blood vessels increases, lowers blood pressure in the capillaries. This allows blood flow to slow down, more time for diffusion.