Gland: Lung

THE LUNGS

The mechanics of pulmonary ventilation are performed by the diaphragm, which increases the superior-inferior length of the lungs and the expansion of the rib cage, increasing the circumference of the lungs. The major muscles that increase the size of the rib cage by raising the rib cage are the intercostals, and secondarily by the sternocleidomastoideus, scalene, and anterior serratus muscles. The muscles that depress and pull the ribs downward and aid in exhalation are the rectus abdominal and the internal intercostals.
The lungs are suspended from the mediastinum via the hilus and literally float in the thoracic cavity and use pleural fluid to act as a lubricator for movement. This creates a pleural pressure between the lung and chest wall causing at rest a -5-cm of water pressure (sucking pressure.) On inhalation, this negative pressure builds to -7.5-cm of water pressure, and then it reverses on exhalation. The alveolar pressure in the lungs is equal to the atmospheric pressure (0 centimeters of water pressure) with your glottis open. The alveoli require very little negative pressure (-1 centimeter of water pressure to bring in ½ liter of air into the lungs.)
The trans pulmonary pressure is the pressure between the alveoli and the pleural pressure, which measures the elastic ability of the lungs. The epithelial lining from the nose to the bronchioles contains cilia, sub mucosa and goblet cells that release mucous as the cilia remove mucous at a speed of 1 centimeter per minute. The cilia sweep toward the pharynx in the nose, as the cilia in the bronchioles, sweep up toward the pharynx, either to spit up or ingest the mucus. The turbinates in the nostrils also expedite the movement of mucous.

BLOOD FLOW IN THE LUNGS
The pulmonary arteries bring blood from the right ventricle of the heart that is actually unoxygenated venous blood. These arteries have very thin walls and large lumens, giving great flexibility and blood supply to the lungs. The pulmonary veins resemble the arteries, but are shorter, and bring oxygenated blood back to the left atrium.
The bronchial arteries bring oxygenated blood to the lungs themselves, and the bronchial veins bring the blood back to the left atrium instead of the right, making the output of the left ventricle greater than that of the right. The lymphatics extend into the lungs, connective tissue, the spaces and the terminal bronchioles, and drain back into the right thoracic duct.

Capillary exchange dynamics:
• Pulmonary capillary pressure is low, 7-millimeters hg, as compared to the 17-millimeters hg in peripheral tissues.
• The interstitial fluid pressure is slightly more negative than peripheral tissue.
• The pulmonary capillaries leak protein, which increase colloidal osmotic pressure to 17-millimeters hg instead of the 7-millimeters hg in peripheral tissues.
• The alveolar walls are extremely thin and weak, and can rupture due to any negative pressure causing fluid to build up in the alveoli.

This all creates a negative net diffusion of fluid out of the capillaries into the interstitial fluid and back into the circulatory system, keeping the alveoli dry. The interstitial fluid also creates pleural fluid via the mesenchymal membrane of the pleura, which it exudes and uses to line the potential space between the visceral and parietal pleura. The pleural fluid is then collected via the lymphatic vessels in the pleural cavity, then sucked away via a negative pressure in the lymphatic system. This negative pressure is what keeps the lungs inflated.

THE RESPIRATORY UNIT
The respiratory unit is composed of the bronchiole, alveolar duct, atria, and alveoli. The alveoli walls are thin and literally contain a sheet of capillaries on them. This membranous system is known as the respiratory membrane, and is composed of the following layers:
1. A layer of fluid that reduces the surface surfactant of the alveolar fluid
2. Alveoli epithelium
3. An epithelial basement membrane
4. A thin interstitial space between the epithelium and the capillary membrane
5. A capillary basement membrane that sometimes fuses with the basement membrane of the alveoli
6. A capillary endothelial membrane
The surface area of this membrane is said to be 50-100 sq. meters, with the amount of blood present in the lungs at any one time to be between 60-140mls, making the exchange a breeze.

THE TRANSPORT OF OXYGEN AND CO2
Oxygen and carbon dioxide diffuse in and out of the lungs. For example, the oxygen content in the alveoli is higher than that of the blood, 104 millimeters hg to 40 millimeters hg. Therefore, oxygen crosses into the capillaries. Diffusion capacity can be increased 3 fold through exercise, due to increased capillary participation, and the blood remaining in the capillaries longer for full oxygenation. Please note that during strenuous exercise your oxygen demands can increase 20 times normal. When this oxygenated blood reaches the peripheral interstitial tissues, the oxygen is at 94-millimeters hg and the interstitial tissue is 40-millimeters hg. Therefore, oxygen enters the peripheral interstitial spaces. Since the oxygen in cells is between 4-40 millimeters hg, oxygen enters the cells rapidly. Carbon dioxide goes in the opposite direction equal exactly to the number of oxygen molecules coming in. The only difference is that carbon dioxide diffuses 20 times faster.

The following tests are used to determine a lung condition:
1. SODIUM-Sodium is alkaline and causes swelling of membranes, which reduce exchange across a cell membrane. It can also affect tissue exchange of oxygen.
2. CARBON DIOXIDE-80-90% of the carbon dioxide found in plasma is bound to protein and is represented as bicarbonate ions (HCO3-.) The other 10 to 20% is bound to protein as CO3 (2) and carbonic acid (H2CO3.) This occurs as follows: Carbon dioxide in the blood reacts with water in the red blood cells to form carbonic acid. There is an enzyme in the blood cells called carbonic anhydrase, which catalyzes water and carbon dioxide 5,000 fold, creating a rapid exchange into carbonic acid. This allows tremendous amounts of carbon dioxide to react with the water in the red blood cells, even before the blood leaves the tissue capillaries. So, a red blood cell creates carbonic acid from water and carbon dioxide. In another small fraction of a second, the carbonic acid formed in the red blood cell disassociates into hydrogen and bicarbonate ions. The hydrogen ions combine with hemoglobin as the bicarbonate ions diffuse out into the plasma while chloride ions diffuse into the red blood cells taking the place of the bicarbonate ions. This is made possible by the presence of a bicarbonate/chloride carrier protein in the red blood cell membrane, which acts as a shuttle for these two ions. Thus, the chloride content of venous red blood cells is greater than that of arterial cells. This is known as the chloride shift. In addition, carbon dioxide can react directly with hemoglobin to form the compound carbaminohemoglobin.
This carbaminohemoglobin creates the reversible reaction releasing carbon dioxide into the alveoli of the lungs. Carbon dioxide, then, is the venous capillary exchange product after diffusion takes place. The active pressure intake of oxygen relies upon this diffusion. This active process realizes the ability of forced metabolism and is the ash by-product. Carbon dioxide is vital to pH balance and is controlled by the respiratory, renal and adrenal axis. When there is an accumulation of carbon dioxide in the blood, it causes acid blood.