Gaseous exchange in mammals e.g. man
The breathing system of a mammal consists of a pair of lungs which are thin walled elastic sacs lying in the thoracic cavity. The walls of the thorax consists of the ribs and the intercostal muscles while the floor consists of the diaphragm, a muscular flap of tissue between the thorax and the abdomen
Diag. main parts of the breathing system in man

Air enters the lungs through the trachea which is devided into two brochi, one to each lung. The trachea and bronchi have walls made up of rings of cartilage. Inside the lungs , each bronchus is divided into smaller tubes called bronchioles. The bronchioles terminate in saclike atria giving rise to numerous air sacs or alveoli. Each alveolus is a thin walled sac covered by numerous blood capillaries
Ventilation
Exchange of air between the lungs and the outside is made possible by changes in the volume of the thoracic cavity. This volume is altered by the movements of the intercostal muscles and the diaphragm.

Inspiration
The following events happen during inspiration
• The diaphragm contracts and moves downwards
• The ribs are raised upwards and outxwards by the contraction of the external intercostals muscles
• The volume of the thoracic cavity increases, thus reducing the pressure. Air then rushes into the lungs from outside through the nostrils.
Expiration
• The diaphragm relaxes and is pushed upwards by the abdominal organs. It thus assumes a dome shape
• The internal intercostals muscles contract and the ribs move downwards and inwards
• The volume of the thoracic cavity decreases, thus increasing the pressure. Air is then forced to out of the lungs

Gaseous exchange between the alveoli and the capillaries
 The walls of the alveoli and the capillaries are very thin and closely attached to each other. This makes diffusion of gases very efficient because the distance between the inside of the capillary and the inside of the alveolus is very small.
 Furthermore, the lungs have over 700 million alveoli offering a large surface area for gaseous exchange
 The walls of the alveoli are also moist, this makes oxygen dissolve easily
Blood from the tissues has a high concentration of carbondioxide and very little oxygen compared to alveolar air. The concentration gradient favours diffusion of carbondioxide into the alveolus and oxygen into the blood plasma in the capillaries. The oxygen is then picked by the haemoglobin of red blood cells and transported in combination with it as oxyhaemoglobin.
Carbondioxide which is at a higher concentration in the blood is normally carried as bicarbonate ions in the plasma. This breaks down and releases carbondioxide which then diffuses into the alveolus.