On entering the lung, each bronchus divides and subdivides repeatedly, spreading to each part of the lung. The tiniest sub-divisions, supplying oxygen to air sacs in the lung, are called bronchioles, and even these are held open by minute areas of cartilage. This branching arrangement is often called the bronchial tree.
The inner lining of the trachea and bronchi is made up of mucus-secreting and ciliated, columnar epithelium cells. Music is the sticky white gel which traps dust particles that may cause infection.
Each lung is divided ito a few lobes with a hilum, or root, that makes the entry of the bronchus, blood vessels and nerves on the inner side.
The outside of the lungs are lined with a moist membrane known as the pleura. The pleura continues around the inner thoracic cavity so that the two pleural layers slide over oe another with ease and without friction. The surface tension of the thin film of moisture does not allow the two layer to pull apart but does allow them to slide. This means that when the chest wall moves in breathing, the lungs move with it.
Each bronchus after repeatedly dividing ends in a group of ingle-layered globe-shaped structures called alveoli. The wall of the alveoli consist o very thin, flat simple squalor epithelium, and each alveolus is surrounded by the smallest blood vessels known as capillaries. The walls of the capillaries are also composed of simple squamous epithelium, in a single layer. This means that the air entering the alveoli during breathing is separated from the blood by only two single-layered, very thin wall.
There are elastic fibres around the alveoli enabling them to expnd and recoil with inspiration and expiration. A film of moisture lines the inside of each alveolus to enable the air gases to pass into solution. As the 2 layers of epithelium are very thin and semi-permeable, the dissolved gases can eaily ad rapidly pass through, in the process called gaseous exchange.
Ventilation is the movement of air in and out of the thorax to replenish the oxygen supply and remove surplus waste products (carbon dioxide + water). Ventilation has 2 phases, inspiration (or inhalation) and expiration (or exhalation).
The movements are effected by respiratory muscles attached to the skeleton. Two sets of intercostal muscles run obliquely at right angles to each other between the ribs, and the diaphragm is a dome-shaped muscle attached to the lower ribs and separatig the thorax from the abdomen.
When the intercostal muscles contract, the ribs moe upwards and outwards and at the same tie the contraction of the diaphragm causes it to flatten. All these movements serve to increase the volume of the thorax and the lungs and thus reduce the pressure inside the lungs, causing air to rush in from the environment. This is known as inspired, or inhaled air.
The main force in expiration during quiet breathing is the elastic recoil of the fibres around he alveoli and the relaxation of the diaphragm. However, during exertion, more forcible expiration can occur with the assistance of the other set of intercostal muscles contracting to move the ribs downwards and inwards. The volume o the thorax decrease, the pressure increases above that of the environmental air and air rushes out. Although the largest component of air is nitrogen and this too passes into solution, it takes no part in the process of respiration.
Breathing in fresh air replenishes the high concentration of dissolved oxygen molecules in the lung alveoli, and the removal of diffused oxygen by the bloodstream maintains the low concentration. With carbon dioxide, the situation is reversed- the high concentration is in the blood and the low concentrtion is in the refreshed air, so diffusion removes dissolved carbon dioxide from the bloo into the expired air from the lungs. Carbon dioxide and water are waste products from internal respiration in cells.
Diffusion occurs in liquids or gases because the molecules are in costant random motion, and dissusion is an overall ‘equalling up’ of a situation where you have a lot of molecules meeting a few molecules. Diffusion will stop in time, as the numbers o molecules become more evenly distributed.
In the human body, where diffusion is a common of transport, the state of equilibrium is not desirable as it means overall transport would cease. To prevent equilibrium being attained, the high concentration must be continually kept high and the low concentration must also be maintained.
Diffusion can only occur here there is no barrier at all to the molecules or where the barrier is thin. The rate of diffusion is enhanced with an increased surface area- usually by folds or similar structures to alveoli, and with temperature, since warmth increases the random motion of molecules.
The oxygen in the air we breathe combines with the haemoglobin in red cells through the alveolar-capillary membrane. This O2 is delivered to the cellular capillaries via the circulatory system. There the O2 is utilized in glucogenesis. The waste product is CO2 which is combined in the blood with H2O making H2CO3 (carbonic acid). The CO2 disassociates into CO2 and H2O in the pulmonary capillary bed and the CO2 diffuses into the alveoli and is exhaled into the air.
