a) STATE THE CHARACTERISTICS OF RESPIRATORY SURFACES.[4m] b) DESCRIBE THE MECHANISMS OF BREATHING IN AND OUT IN MAMMALS[16]

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a) State the characteristics of respiratory surfaces.

- Highly vascularised / Dense  network of blood capillaries
-          Large surface area (for gaseous exchange)
-          Thin membranes /epithelium / one cell thick wall;
-          Moist lining  
(4 Marks)

b) Describe the mechanisms of breathing in and out in mammals
 Breathing in
External intercostal muscles contract, 
Internal intercostal muscles relax;
The rib cage lifts upwards and outwards, 
Muscles of diaphragm contract; hence if flattens; 
The volume of the thoracic cavity increases, 
The air pressure in the thoracic cavity decreases, 
Higher air pressure in the atmosphere forces air into the lungs (through the nose) (8 Marks)
Breathing out
External intercostal muscles relax;
Internal intercostal muscles contract; 
The rib cage downwards and inwards, 
The muscles of diaphragm relax, hence the diaphragm assumes dome shape; 
The volume of the thoracic cavity decreases; 
The air pressure of the thoracic cavity increases; 
The higher air pressure forces air out of the lungs (through the nose) (8 marks)
                        Maximum 20 Marks)

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