Respiratory organs are those which are concerned with the passage of the air to and from the lungs. The latter are, of course, mot important of all. The passage is also called respiratory tract. For the sake of convenience, the respiratory tract can be divided into upper and lower parts. The upper part extends from the external nostrils (external openings of nose) to the vocal sac present in the neck. The lower part extends from the vocal sac to the lungs. The nose has a passage called nasal passage which is lined by ciliated epithelium rich in blood capillaries. This passage is hairy.
When the air passes through the nose, certain amount of dust of the air is trapped and the air is warmed up. Behind the nasal passage is the vocal sac or the larynx. It is called Adam’s apple. Behind the larynx is a wind pipe called the trachea. It is a 12-cm long tube whose diameter is about 16mm. it is surrounded by incomplete rings of cartilages. As the air passes down the trachea, it becomes humidified, because some moisture from the mucus gets mixed up in it. The trachea divides at the lowe4r end into two bronchi. Each bronchus divides in turn to form thinner tubes called bronchioles.
The respiratory tract from the nose to the bronchioles is lined by ciliated epithelium. Bronchioles enter the lung and divide further into finer branches, the alveolar ducts, which open into the air sacs (alveoli). Mechanism of Respiration in Man Here, we are not going to discuss the biochemical mechanism of respiration. We shall discuss only the process of renewal of atmospheric air in the lung. In other words, we will discuss how we take the air into our lungs and how we expel it. The lungs occupy most of the chest cavity. This cavity is lined with a serous membrane, the pleura.
There is a small amount of serous fluid between the lungs and the pleura. This fluid lessens the friction between the membrane and the lungs. Internally, the cavity of the lung has very small (microscopic) air spaces, the alveoli. The number of alveoli in human lung may be 70,000,000. Each alveolus is lined by a layer of flattened polygonal (many-angled) cells called squamous cells. The air that we inhale comes to these air spaces where oxygen is taken up by haemoglobin and carbon dioxide is given off. Inspiration: The manner in which the air is brought into the lungs is called inspiration.
In this process, at least three things happen. First, the muscles between the ribs contracted. Press your chest and you will feel the rod-like bones. There is twelve pair of these bones called ribs. When external muscles between these ribs contract, the ribs are pulled out and the chest cavity expands, second, a muscular partition wall between the chest cavity and the lower abdominal cavity, called diaphragm, contracts in such a way that it is brought down a little bit. This also expands the chest cavity, called diaphragm, contracts in such a way that it is brought down a little bit.
This also expands the chest cavity. Third, there is a contraction of muscles of the belly. All these there events create a partial vacuum in the chest cavity and the atmospheric air rushes in the lung, as if the lungs were a suction pump. Expiration After the exchange of gases in the lungs, the air has to be expelled. Expulsion of the air from the lungs is called expiration. Reversal of the three events that had occurred in inspiration, results in expiration. Gas Exchange in Lungs of Man Atmospheric air inhaled finally come to these alveoli.
All around the alveoli there is a network of blood capillaries. These capillaries are extremely fine tubes with only one layered wall. The deoxygenated blood collected from different parts of the body is at first brought to the heart, and from here pumped out to the lungs. This blood which may also be called venous blood is sent to the lungs where it has to pass through the network of capillaries around the alveoli. In the alveoli, the inhaled air has about 21% oxygen whereas in the blood capillary the venous blood has less % of oxygen in the alveolus is higher than in the venous blood.
Therefore, oxygen from the alveolus diffuses out into the blood capillary and is picked up by the hemoglobin molecules present inside the red blood corpuscles. By the same principle, carbon dioxide, which is in greater amount in the venous blood, comes from the capillary into the alveolus. Breathing Breathing is a term which is applied to inhalation and exhalation. In other words, the act of intake of air into the lungs and expulsion of the air from the lungs is called breathing. Coming in and going out of the air ventilates the lung. Breathing rate in a normal person is 17 per minute.
The volume of air breathed in and out in a single quiet respiration is 500 milliliter, which is also called the tidal volume. Breathing goes on automatically. But if you want to take a deep breath, you can do so. Therefore, we say that respiration is partly involuntary (automatic), and partly voluntary (by will). Respiration is controlled by a small centre located in the posterior part of the brain. This centre is called respiratory centre. A healthy person can inhale up to 3 liters of air in one breath. This is called his vital capacity.
If you try to expel all the air contained in your lung, you will not be able to empty your lungs as some amount (~1200 cc) of air will remain in the lung no matter how hard you try to expel it. The air which remains in the lungs is called the residual air. Factors affecting the rate of respiration The rate of respiration has to do with the supply of oxygen to the tissues and cells of the body. Cells need more oxygen if they have to produce more energy. When you perform exercise or start running your body needs more energy for which more oxygen would have to be supplied, and this would call for an increase in the rate of respiration.
On the other hand, if the demand of oxygen is lessened, the rate of respiration must drop accordingly. (i) If you go the higher altitudes, say, to a hill station, your rate of respiration will increase. Why so? It is because; at a higher altitude the amount of oxygen in the atmosphere is less. But your body demands the usual amount of oxygen. Therefore, you have to compensate for it by increasing the rate of breathing. (ii) If hydrogen ion concentration is increased in the blood, this will pose a problem to the hemoglobin and it will not be able to effectively bind oxygen and transport it to the tissues.
Again, to compensate for this loss, the rate of respiration has to be increased. (iii) Similarly, if carbon dioxide concentration is increased either in the body or outside, it will increase the rate of respiration. (iv) Environmental temperature influences the rate of respiration. In extreme cold, the body heat is dissipated in the atmosphere. As a result, the body temperature beings to drop. In order to maintain the body temperature, the rate of respiration is increased. (v) Oxygen consumption is related to metabolic rate of the organism.
Metabolism in a given time is called the metabolic rate. Metabolic rate can be estimated, among other things, by food consumption and the energy release. Metabolic rate is supposed to be inversely proportional to the body size of a particular group of animals. For example, the rate of respiration in a mouse will be greater than that in an elephant. (vi) Rate of respiration also depends upon the age. Very old persons have low metabolic rate, and, therefore, the rate of respiration in them will be less than those in younger persons.