A. Anatomy of the Human Respiratory System
The human respiratory system is typical of all mammals.
1. Air Flow
The air first passes through the nasal passages, important for filtering, warming, and moistening the air before it enters the lungs. The nostril hairs act as initial filters. The nasal cavity is lined with a mucous membrane. This thin layer of mucus is constantly being swept downwards towards the throat by ciliary action. The nose contrains many capillary bed s which warm the air before it enter the lungs. The nasal cavity is separated from the mouth by a hard plate, which is a bony shelf at the roof of the mouth that ends in a soft muscular region called the soft palate. The air moves from the nasal passages to the pharynx to the larynx and into the trachea. The laryngeal opening contains the vocal mechanisms. Below it is the trachea. The trachea is a tube that is composed of C shaped rings of stiff hyaline cartilage that holds the passage open. The trachea branches into the right and left primary bronchi in the chest. The bronchi branch again into bronchioles to from the respiratory tree. The trachea and other bronchiole structures have a lining similar to that of the nasal passages. The air is thus cleared of dust and debris.
2. Human Lung
Bronchioles are the smallest branches of the respiratory and end in grape-like clusters of air spaces called aveoli. Each of the alveolus is enclosed in a dense capillary bed. The atmosphere is only one membrane away from the blood. The alveoli provide an enormous surface area. The total surface area of the 300 million alveoli in our lungs is equal to 750 square feet, can could cover a tennis court. Lungs are in a triangular shape. The right lung has three lobes while the left lung ahs two lobes because of the space that the heart takes up. Two bag like membranes enclose the lungs. The inner pleura is attached to the spongy surface of the lung. The outer pleura froms the tough lining of the pleural cavity which houses the lungs. The pleural cavity is bounded by the muscular shelf, the diaphragm. Only mammals have diaphragms.
3. Gaseous Exchange and respiration Control
Breathing involves intercostals muscles and the diaphragm. The relaxed diaphragm protrudes into the pleural cavity. Inhalation is accomplished by contracting the diaphragm and contraction of the intercostals. The change increases the volume of the pleural cavity, creaeting a partial vacuum. The pressure of the atmosphere forces air down into the lungs, causing them in inflate. Exhalation is produced by the relaxation of the rib cage and diaphragm.
The volume or air an animal inhales and exhales with each breath is called a tidal volume. On average, humans have a tidal volume of 500 mL. The maximum volume of air that can be inhaled and exhaled during forced breathing is called vital capacity, in humans 4,000 to 5,000 mL. Lungs actually hold more air than their vital capacity. It is impossible to completely remove all air from the lungs. The air left behind is called the residual volume.
4. Partial Pressure
The amount of one particular gas in a system can be described in terms of partial pressure Air is about 21% oxygen. The partial pressure of oxygen at sea level is .21 atm and the partial pressure of carbon dioxide is .23 atm. The concept of partial pressure is important in understanding how gas is exchanged. Gases move form a region of higher partial pressure to a region of lower partial pressure. Blood arriving to the lungs has a lower P02 and a higher PCO2. The oxygen from the lungs diffuses into the blood and the carbon dioxide from the blood loosely diffuses into the lungs.
5. Exchange of Gases
The exchange takes place on the moist inner surfaces of the alveoli through simple diffusion because of differences in partial pressure. The blood enters the lung from the heart. There is low partial pressure of oxygen while the partial pressure of carbon dioxide is high. Some of the carbon dioxide is in the form of bicarbonate ions, some as dissolved carbon dioxide and some carbon dioxide loosely bound to hemoglobin.
The alveoli produce the enzyme carbonic anhydrase which converts the HCO3 to carbon dioxide. This conversion increases the partial pressure of carbon dioxide. The blood and air are in near contact on the opposite sides of the thin alveolar membrane. Near equilibrium occurs when molecules of oxygen go into the blood and carbon dioxide goes out. The equilibrated air is exhaled and fresh air is inhaled. The blood flow is continuous; oxygenated blood moves away from the lungs nd deoxygenated blood moves toward the lungs.
6. Respiratory Pigments: Transportation of oxygen
Oxygen is not very soluble in water. Most animals use respiratory pigments to carry oxygen. Respiratory pigments are proteins containing a metal atom. Hemoglobin, abbreviated Hb, is the pigment of most vertebrates. The protein contains iron which binds to the oxygen. Hemocyanin, which can be found in horse shoe crabs, has a copper atom. Hemoxyanin causes the blood to be blue and is dissolved in the plasma, not on the blood cell. Hemoglobin is highly specialized to associate and disassociate from oxygen. A molecule of hemoglobin contains four heme groups. Each heme group contains an iron atom. In hemoglobin there are four iron atoms in all. This allows the heme groups to bind reversibly to four molecules of oxygen. The binding of a oxygen to one subunit causes a slight shape
7. Carbon dioxide
Seven percent of the carbon dioxide released by cells is transported as carbon dioxide dissolved in plasma. Twenty three percent of the carbon dioxide binds to the amino groups of the amino groups of hemoglobin. The combination of carbon dioxide and Hb is called carbamino Hb.
Hb + 402 + CO2 ß—àHb-CO2 + 4O2
70% of the carbon dioxide is transported as bicarbonate ions.
Enzyme: carbonic anhydrase
CO2 + H2Oß–à H2CO3ß—à HCO3 + H
The release of H+ makes the blood more acidic. The higher the acidity affects the dissociation of Hb, inducing it to unload oxygen.
The control of respiration: The major respiration control centers are in the pons, a small sphere at the back of the brain which tapers into the spinal cord. The medulla also contains control centers. The best understood center is called the rhythmicity center which has two circuits of opposing neurons: one for inspiration and one for expiration. This coordination produces rhythm of breathing.
8. Breathing Sequence
a. The inspiratory circuit activates muscles for inhalation and inhibits the expiratory circuit.
b. The lungs fill, activating stretch receptors which fire the signal to inhibit inspiratory circuit which ceases to inhibit the expiratory circuit.
c. The expiratory circuit activates, inhibiting the inspiration center and exhalation occurs.
d. When the lungs are emptied, inhibition of the inspiration circuit ceases and the inspiration circuit activates.
Respiration centers respond to level of CO2 in the body. The body also responds to an increase in acidity level created when the CO2 forms H2CO3 which dissociates.
9. Hyperventilation
The oxygen sensor is the carotid body which is a group of nerves in the carotid artery ( in the neck). This carotid body monitors the partial pressure of oxygen moving toward the brain. If there is too much oxygen, the carotid body constricts the carotid artery which constricts the amout of oxygen to the brain. When one hyperventilates, the mechanism overreacts to the high level of oxygen in the body. The carotid body constricts the carotid artery so much that the brain is starved of oxygen and one passes out.