1.The nicotine (of tobacco) stimulates adrenal glands to secrete adrenaline and nor-adrenaline; both these hormones raise the blood pressure.
2. Smoking increases the carbon monoxide level of blood, which competes with oxygen for transport.
3. As the concentration of haembound oxygen decreases, there is oxygen deficiency in the body.

(i) HCI of stomach belongs to the physiological barrier.
(ii) Macrophages come under cellular barrier.

(i) The mucus coating of the gut epithelium is a barrier that traps microbes.
(ii) The hydrochloric acid secreted by gastric glands retards the growth of microbes.
This type of immunity is called innate immunity, which occurs right from birth.

(a) Contact inhibition is the property shown by normal cells,by the virtue of which contact with other cells inhibits their uncontrolled proliferation and growth.
(b) All normal cells have cellular oncogenes (c-onc) or proto-oncogenes;when activated under certain conditions,such genes could lead to oncogenic transformation of cells,i.e.they become cancerous.

(i) Cancer cells grow rapidly,invade and damage the surrounding normal cells.
(ii) They starve the normal cells by competing for vital nutrients.
(iii) Some cells get sloughed off and reach distant sites through body fluids and start a new (secondary) tumour there,i.e. metastasis.