Structure of the lungs.
Structure of the respiratory system.The respiratory surface of a mammal consists of many air sacs called alveoli inside a pair of lungs. The lungs are situated next to the heart in the thoracic cavity and are connected to the atmosphere by tubes. Air passes into the lungs through these tubes. Twelve pairs of bony ribs surround and protect the lungs and heart. Intercostals muscles are attached to the ribs, and a large diaphragm separates the thorax from the abdomen. These are involved in the ventilation mechanism.Air enters the body through two nostrils, each of which possesses a boarder of large hairs, which trap particles in the air and filter them out of the system. While passing through the nasal passages the air is warmed and moistened and its odour detected. Air passes from the nasal passages, through the pharynx and into the trachea. This is a tube, which lies in front of the oesophagus and extends into the thoracic cavity. The wall of the tube is strengthened and held open by horizontally arranged C-shaped bands of cartilage. The open section of the C is next to the oesophagus. The cartilage prevents collapse of the tube during inspiration (breathing in). At its lower end the trachea splits into two
There are over 700 million alveoli present in the human lungs, representing a total surface area of 70-90m2. Without surfactant the surface tension of the fluid in the alveoli is about 10 times higher than normal and the alveoli collapse after each expiration. Babies born prematurely are at risk of being deficient in surfactant. At the same time, the diaphragm relaxes. It is first made in the lungs of a foetus when about 23 weeks old. Into each alveolar sac opens a group of alveoli. Oxygen diffuses across the thin barrier represented by the epithelium of the alveolus and the endothelium of the capillary. These events reduce the volume of the thorax and raise its pressure above that of the atmosphere. Mucus also moistens the incoming air. This means that the red blood cells have to be squeezed through the capillaries by blood pressure. During this process they bend into an umbrella shape, exposing more of their surface area to the surface of the alveolus and allowing greater uptake of oxygen. This also happens when sneezing of coughing. The respiratory muscles act as follows: The external intercostals muscles relax and the internal intercostals contract. This is a space lined by two tough, flexible, transparent pleural membranes (pleura).
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