What Are The Components Of The Respiratory Membrane?
Gaseous exchange takes place in our lungs through tiny tubes called bronchioles which branch off into smaller tubes known as bronchi. These bronchioles are lined by ciliated epithelium cells which create mucus (a thin fluid produced by these cells). This mucus serves as a protective barrier against foreign particles and pathogens. At the end of each bronchiole is a larger tube filled with pulmonary artery smooth muscle cells and surrounded by connective tissue. This large tube is known as the trachea. It branches off into several main bronchi which continue branching off until they eventually lead to the thousands of terminal bronchioles that make up the lung’s air sacs. Each air sac is separated from one another by a thick double-layered cell structure known as the pleura. The outermost layer of this structure is made of elastic fibers while the innermost layer is made of collagen fibers. Between them is a thin space referred to as interstitial tissue. The airspace within this space is where gas exchange occurs.
The pleura separates the chest cavity from the outside environment. The pleura itself consists of two layers; visceral pleura on the inside and parietal pleura on the outside. The visceral pleura lines the interior of the chest cavity while the parietal pleura covers the entire surface of the rib cage. The space enclosed by the pleura is known as the pleural space. On either side of the pleurae are small structures called costal cartilage plates which help give shape to the ribs. The middle part of the rib cage contains the sternum which has a slight curve to it.
Between the pleura and the cartilage there is a thin, flexible membrane called the thoracic diaphragm or simply the diaphragm for short. Surrounded by muscles, the diaphragm functions as a valve that helps keep the chest cavity closed. A network of ligaments attaches the diaphragm to the chest wall and gives it its characteristic dome-like appearance. Two other important features of the diaphragm include its ability to contract and expand during breathing and the presence of natural pleurae along its edges.
The next component of the respiratory membrane is the alveolus. Alveoli are the smallest unit of the lung and consist of millions of minute cup-shaped spaces. Within each alveolus there is a thin film of fluid called pulmonary surfactant that coats the internal surface of the alveolar walls. Pulmonary surfactant prevents the collapse of the alveolus when the pressure becomes too great. Without surfactant, the collapsed alveolus would be forced open like a popped balloon and allow gas exchange across the walls of the alveolus rather than allowing gas to flow freely through their porous surfaces. In addition to preventing alveolar collapse, surfactant also plays an active role in spreading oxygen molecules around the alveolar walls so that more can enter the bloodstream via diffusion. Surfactants are not present in all animals but are found in most mammals including humans, dogs, cats, mice, cows, pigs, horses, guinea pigs, rats, rabbits, guineas, chickens, and amphibians.
The third component of the respiratory membrane is the capillaries. Capillaries form the basic building blocks of the circulatory system. They serve as the primary means by which gases travel throughout the body. As such, the diameter of the capillaries ranges from less than 10 microns at the base of the eye to greater than 100 microns near the heart. Most capillaries have three layers. The first layer is formed by the endothelium and surrounds the lumen (the central channel of the capillary). The second layer is composed of a discontinuous monolayer of cells called pericytes. Pericytes are critical because they provide support for the capillary walls and prevent leakage of fluids out of the capillary lumen. The final layer is formed by the basal lamina. Basal laminar forms a continuous sheet over the endothelium and pericytes and provides structural integrity to the capillary wall. One interesting feature of the capillaries is that they do not contain any red blood cells. Instead, the blood plasma flows through the capillary lumen.
Capillaries are extremely fragile and prone to injury. Any disruption of the capillary lining will cause damage to the surrounding tissues resulting in hemorrhage. When capillaries become damaged, platelets clump together forming plugs that block the opening of the affected vessel. This process is known as thrombosis and causes serious problems if left unchecked. Fortunately, clotting factors released by injured capillaries attract white blood cells that remove the plug and restore normal circulation. If clotting fails to occur, however, then the capillary may rupture causing life threatening blood loss.
In order to avoid catastrophic bleeding, the capillary must maintain constant negative pressures ranging anywhere from -7 mmHg to -25 mmHg. This is accomplished by contraction of the vascular smooth muscles located just beneath the endothelium. Once the proper level of tension is set, the smooth muscles relax so that the capillary lumen can return to its resting state. This relaxation is dependent upon nitric oxide being released by the endothelium. Nitric oxide is a potent vasodilator molecule that increases the permeability of the capillary walls. Therefore, the release of nitric oxide creates a positive feedback loop that keeps capillary pressures low.
This concludes our discussion of the components of the respiratory membrane. Next time we’ll explore how the chemical properties of the various substances contained within these membranes enable their function.
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