The Lungs

Lung Lobes.

    The lungs are the organs where gas exchange occurs, where we can extract oxygen of the air. Oxygen is essential to our body, it is used to produce energy in our cells. It is also in the lungs that we gets rid of carbon dioxide generated by cell metabolism.

    We have two lungs, left and right. Each lung is covered with a thin membrane called the pleura. This membrane is attached to the lung on one side and to the thoracic cage on the other side, maintaining 'air-sealed', like for the membrane of a balloon. Together, the two lungs have five lobes: three lobes on the right side (upper, middle and bottom lobes) and two lobes on the left side (top and bottom lobes).The heart takes a little more space on the left side.

    The air reaches the lungs through the bronchi and bronchioles, which we can see in transparency, on the figure. For asthmatics, smokers and during lung infection, it is often the smaller bronchioles that contract abnormally and clog with mucus.

Lung Vasculature.

Ventral view of the heart and lung (anterior face removed). An efficient network of veins and arteries.

    This figure shows the important interconnection between the heart and the lung, a vast network of 'highways' arteries and veins. Remember that the heart works in closed circuit: for each beat, the same blood flow passes into the lung, as the one going to the rest of the body. In the figure, most of the lung was removed, and the heart was displaced to the right. When different branches have the same name, they are distinguished according to the lobe, or section of lobe, they perfuse.

    This figure shows two radiographs of the lung. On the left, the arterial branches are contrasted, compared to the venous ramifications highlighted on the right side. The numbers on the radiographs correspond to the nomenclature shown in figure of the trachea and bronchi. Although one can find there a vast network of veins and arteries, this figure does not render justice to the actual density of vasculature.

Lobules.

Histology of a pulmonary bronchiole.

    Each lung lobe is subdivided into thousands of lobules. The bronchiole allows the air to reach lobules which further subdivide into multiple alveoli where gas exchange can takes place. The pulmonary arteries, carrying oxygen depleted blood, and the veins, bringing back the oxygenated blood, travel along the bronchioles and their ramifications to reach each lobule.

Alveoli.

Gas exchange between the alveolus and a capillary.

    There are at about 300 million alveoli that participate to gas exchange in our lungs. This provides an enormous surface area of 70-75 squared meters, for gas exchange. Following inspiration, the partial pressure of oxygen ("oxygen concentration) in the alveoli becomes higher than it is in the blood.

    This causes oxygen (O2) to diffuse toward the blood. Oxygen can then fix on the hemoglobin and displace carbon dioxide (CO2) which will diffuse out to the alveoli where it will be exhaled. Because air flow in the alveoli, the inside pressure inside varies from positive to negative. Small changes from 1 to -1 mmHg are sufficient to cause air movement. In extreme cases, the pressure change can go as high as 100 mmHg and as low as -80 mmHg.

    Normally, at low pressure, the lung could collapse, like a deflated balloon. The alveoli do not collapse because there is a fluid, the surfactant, secreted into the airways, which decreases the surface tension and keeps the alveoli open, like soap will help to keep open the bubbles.