Physiology: The Respiratory System (Revisited)
Introduction - Your cells must have oxygen to burn food, and air is about 20% oxygen. You breathe it in through your respiratory system. In the food-burning process, carbon dioxide is formed; your blood carries it to the air passages, where you breathe it out. Inspiration is breathing in and expiration is breathing out.
Nose - Air enters your body through your nose (which has two compartments, divided by a septum) and passes into the nasal cavities. Nerve endings in the septum and nasal passages provide you with the sense of smell. Two eustachian tubes go from the nasal cavity to your ears. They are needed because your middle ear has to have air in it, and the eustachian tubes equalize air pressure. As you know, they can become infected. The nasal cavities are lined with hair-like (ciliated) mucous membranes that are richly supplied with blood, which aid in warming and moistening the air before it reaches the lungs. The mucous secretion is sticky and catches impurities in the incoming air. Small hairs in the nose help stop some larger items.
Pharynx - Air passes from the nose into the tube, called the pharynx, a passageway for both air and food. It lies behind the nose and mouth. The part behind the mouth is called the throat. The tonsils are at the back of the throat. A leaf-shaped cartilage, the epiglottis, covers the air passage that leads out of the pharynx so that, when you swallow, food does not enter the lungs and choke you.
Larynx - The air passes from the pharynx into the larynx, a box-like area made of tough cartilages that are held together by ligaments and located in front of the neck. The cartilages move when you talk. Inside are two triangular folds of mucous membrane and the vocal chords which extend from back to front. They vibrate as air passes over them.
Trachea and bronchi - Air passes into the trachea, a tube made of membranes inside horseshoe-shaped bars of cartilage. At the lower end, it divides into two smaller tubes (the bronchi), one bronchus gong to each lung. Within the lungs, they divide into many smaller branches, the bronchioles. These keep thinning out (quite similar to branches of a tree) and end in sac-like air spaces, called atria. Each atria has many irregular projections, called alveoli or air cells. It resembles a bunch of grapes. The lungs have over 400 million alveoli, along with blood and lymph vessels, nerves, and connective tissue.
Lungs - The lungs are two cone-shaped organs which fill the chest cavity. They are separated by the heart. The top of each cone is called the apex; the base is the lower, wider portion above the diaphragm. This is where the blood exchanges carbon dioxide for oxygen. The lungs have lobes infection in only one lobe is called lobar pneumonia.
Pleura - The chest cavity is lined with a membrane, the pleura, that folds back against the lungs. The surface is moist, so the lungs can move smoothly without friction when you breathe. Infection of the pleura is pleurisy.
Breathing - There are two kinds of breathing: breathing which occurs within body cells and lung breathing. The lungs hold about 3 1/2 quarts of air. You normally take in and let out about a pint with every breath. The diaphragm, that dome-shaped muscle between your breathing and digestive apparatus, helps push in and let out the air.