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Biology - Human Physiology

Let us start with breathing. Breathing is involuntary. The body gets ordered to breath. The signal to breath originates in the medulla oblongata, which is a primitive part of the brain. The signal originates in a part of the medulla that is called the respiratory center. It travels to the diaphragm via a nerve called the phrenic nerve. 

The diaphragm is a muscle and when stimulated by the phrenic nerve, it contracts. Contraction of the diaphragm begins the process of breathing in, which is called inspiration. Before the diaphragm contracts, it is shaped like a dome, then when it contracts, it flattens out and creates empty space within the thoracic cavity. The empty space creates negative pressure. The rib cage expands as the diaphragm contracts, which increases the negative pressure. In order to fill the space and eliminate the negative pressure, the lungs expand. 

The lungs have a natural elasticity tending to keep them from expanding. When the lungs expand, they create a negative pressure inside themselves. In order to eliminate that negative pressure, air rushes into the lungs to fill the empty space that completes the process of inspiration. 

First, air enters the nose and flows into the trachea. Next, it reaches the bronchi. The bronchi break up into smaller branches and these smaller branches splits several times more. The tiny twig-like branches are called bronchioles. The trachea and bronchi have rings that help keep them open. 

The respiratory tract begins with the nose. The nose cleans, warms, and moistens incoming air. Large particles are trapped in hairs lining the nostrils. Particles that make it pass the nose get stuck in mucus, which lines the lower parts of the respiratory tract. Ciliated cells sweep the dirty mucus back out of the system. Very small particles can make it all the way to the alveoli, which are at the end of the bronchioles and these particles are eaten by phagocytic cells lining the alveoli. 

Each alveolus is a hollow sphere situated at the end of a tiny bronchiole. Alveoli are the terminal points of the respiratory tract. When air is inspired and drawn through the respiratory tract, it ends up in the alveoli. The alveoli take oxygen from the air and supply it to the blood. They also take carbon dioxide from the blood and send it out into the atmosphere. 

Atmospheric air is richer in oxygen than it is carbon dioxide. On inspiration, the inner surface of the alveolus naturally comes in contact with the atmospheric air the lungs have inspired. The outer surface of every alveolus is in contact with the capillary carrying blood whose relative composition of oxygen and carbon dioxide is opposite to that of atmospheric air. 

The alveolar wall and capillary wall are both permeable to oxygen and carbon dioxide. Oxygen, therefore, moves by passive diffusion along its concentration gradient. It passes from the alveolus into the blood. Carbon dioxide moves by passive diffusion along its concentration gradient. It passes from the blood into the alveolus. Gas exchange can continue only if the respiratory system somehow reestablishes oxygen and carbon dioxide gradients between the alveoli and the capillaries that surround them. 

The system must rid each alveolus of the air it contains and then afford it a new supply that is rich in oxygen and poor in carbon dioxide. In order to empty the alveoli, the medulla oblongata stops sending its signal to the diaphragm and the diaphragm stops contracting. 

The lungs under the force of their own elasticity then recoil. That forces air out of the alveoli upward through the respiratory tract and out into the atmosphere that is called expiration. 

The alveolus has a film of fluid covering its inner surface. The fluid produces a force called surface tension and because of surface tension, the alveolus has a tendency to collapse. In order to avoid collapsing, the alveolus needs something to reduce its surface tension. There is a substance that does that, called surfactant. 

Carbon dioxide is byproduct of cellular metabolism. Cells are perpetually dumping carbon dioxide into the blood. The carbon dioxide travels through the bloodstream and ultimately reaches the capillaries that surround alveoli. When carbon dioxide leaves the cells and enters the bloodstream, it usually combines with water and forms carbonic acid. Most of this carbonic acid then dissociates into hydrogen ions and bicarbonate. Carbon dioxide does not always combine with water to form carbonic acid, sometimes it combines with hemoglobin. 

In the bloodstream, carbon dioxide travels as carbonic acid or bicarbonate ion or it travels within a hemoglobin molecule. When any of these molecules reach a capillary that surrounds an alveolus, the molecular carbon dioxide reemerges. 

The respiratory center is sensitive to the blood’s oxygen content and to its carbon dioxide content. When the oxygen content decreases, the respiratory center signals the diaphragm to increase the rate of respiration. Similarly, when the blood’s carbon dioxide content increases, the respiratory center signals the diaphragm to increase the rate of respiration. 

Remember that when carbon dioxide enters the blood from the cells, it combines with water to form carbonic acid and the carbonic acid in return dissociates into hydrogen and hydrogen carbonate. That means that if the cell starts to produce some greater amounts of carbon dioxide, the blood pH tends to fall. Remember that these things make the respiratory center raise the respiratory rate, lower the blood oxygen concentration, increase blood carbon dioxide concentration, increase blood hydrogen ion concentration which means lower the pH.