Insects have specialized structures called spiracles arranged in a series on the sides of its body. These are small openings through which the oxygen-rich air enters into the body. Spiracles are connected to a network of tubes called trachea which diffuses oxygen into the cells of the body. The transport of CO2 occurs in reverse direction, that is, from the trachea to the spiracles. Thus, the gaseous exchange occurs in the insects through the tracheal system.
(ii) pO2 higher, pCO2 lesser
In the atmosphere, O2 is present in large quantity and thus, its partial pressure is higher (159 mm Hg) than pO2 in alveolar air (104 mm Hg).
The pCO2 is lesser in atmosphere (0.3 mm Hg) as compared to pCO2 in alveoli (40 mm Hg).
As a person goes up a hill (that is, at higher altitude), the level of O2 in the atmosphere decreases. So, less O2 is available for breathing and O2 level in the blood starts to decline. It results in an increase in the respiratory rate so as to meet the O2 demands of the body. Heart beat also increases to increase the supply of blood to the tissues so that every body tissue gets sufficient O2.
Vital capacity refers to the volume of air a healthy person can inhale after a forced exhalation or the volume of air a person can exhale after a forced inhalation. It is the sum of Expiratory Reserve Volume (ERV), Tidal Volume (TV), and Inspiratory Reserve Volume (IRV). A normal adult person has a vital capacity of 3-5 litres.
Significance: Vital capacity is highest when physiological competence of the body is highest. It goes down as incompetence increases and becomes zero when respiration ceases. It implies that more the vital capacity, healthier is the body. Also, it helps in getting rid of the foul air and aids in supplying fresh air. Thus, the vital capacity enhances the exchange of gases between the body tissues and the environment.
Tidal volume is the volume of air that is inhaled or exhaled during normal respiration. It is about 500 mL. A healthy person breathes about 12-16 times in a minute.
The approximate value of tidal volume for a healthy human per hour can be calculated as follows:
The tidal volume of a healthy human in an hour = Tidal volume per breath X Number of times an adult human being breathes per minute X 60 minutes
= 500 mL X 12-16 times per minute X 60 minutes
= 3,60,000 – 4,80,000 mL
Hypoxia is a condition, wherein the tissues are not oxygenated adequately due to decreased supply of oxygen to the lungs. The oxygen deprivation can have severe adverse effects on various body cells and thus, several important biological processes get hampered. In general, hypoxia results in a pathological condition. There are various types of hypoxia depending upon the reason of inadequate oxygen supply to the body:
Oxygen binds to haemoglobin reversibly to form oxyhaemoglobin, which transports throughout the body to supply oxygen to different tissues. The binding of one molecule of haemoglobin with four molecules of oxygen depends on the partial pressure of oxygen (pO2). However, the partial pressure of CO2 also plays a significant role in the binding of oxygen with the haemoglobin molecule and its transport from lungs to tissues and vice-versa.
In alveoli, the pO2 is high but pCO2 is low. Thus, oxygen binds to haemoglobin and oxyhaemoglobin is formed. It transports the oxygen to body tissues. However, in tissues, the pCO2 is higher than pO2. Thus, O2 dissociates from haemoglobin and is released. Here, CO2 binds to haemoglobin to form carbamino-haemoglobin. CO2 Is thus transported to the lungs and is released in the lung cavity in exchange of O2. Thus, it can be stated that the affinity of haemoglobin for O2 increases as pCO2 decreases.
Respiration in human beings is a regulated process. It is regulated by the neural system according to the demands of the body tissues. It is mainly controlled by two regulatory regions present in the brain. These are:
(a) Respiratory rhythm centre: It is present in the medulla region of the brain and is mainly responsible for regulating the process of respiration. The areas adjacent to the respiratory rhythm centre are highly chemo-sensitive. They can sense the CO2 and H+ levels, whose increase activates the respiratory rhythm centre. Once activated, this centre adjusts the respiratory process, which helps in the elimination of CO2 and H+ from the body. Receptors associated with the aortic arch and carotid artery also sense the change in CO2 and H+ concentration and send signals to the respiratory rhythm centre to adjust the respiratory process.
(b) Pneumotaxic centre: It is present in the pons region of the brain. This centre can moderate the function of respiratory rhythm centre and reduces the duration of inspiration, which in turn changes the respiratory rate.
Blood is the medium for transport of CO2. There are three main ways by which CO2 is transported in the body:
The gaseous exchange (exchange of O2 and CO2) occurs in our body by simple diffusion between the blood capillaries around the alveoli and gases present in the alveoli. Alveoli are made up of thin and highly permeable layers of squamous epithelial cells. The reasons for gaseous diffusion in the alveolar region are as follows:
The volume of air that remains in the lungs after normal breathing is called FRC (functional residual capacity). This includes expiratory reserve volume (ERV, additional volume of air, a person can exhale by a forcible exhalation) and residual volume (RV, the volume of air remaining in the lungs even after a forcible exhalation). In a normal individual, ERV is about 1000 to 1100 mL while RV is about 1100 to 1200 mL.
Since, FRC = ERV + RV
FRC = 1000 (or 1100 mL) + 1100 (or 1200) mL
= 2100 or 2300 mL.
Thus, the volume of air remaining in the lungs after normal breathing is around 2100 mL to 2300 mL.
(a) IRV and ERV:
IRV |
ERV |
IRV (Inspiratory Reserve Volume) is the additional volume of air that a person can inhale by a forceful inspiration. | ERV (Expiratory Reserve Volume) is the additional volume of air that a person can exhale by a forceful expiration. |
It is approximately 2500-3500 mL. |
It is approximately 1000-1100 mL. |
(b) Inspiratory capacity and expiratory capacity:
Inspiratory capacity |
Expiratory capacity |
Inspiratory capacity is the volume of air that can be inhaled after a normal expiration. | Expiratory capacity is the volume of air that can be exhaled after a normal inspiration. |
It includes tidal volume and IRV. | It includes tidal volume and ERV. |
(c) Vital capacity and Total lung capacity:
Vital capacity |
Total lung capacity |
Vital capacity refers to the volume of air a healthy person can inhale after a forced exhalation or the volume of air a person can exhale after a forced inhalation. | Total lung capacity is the total volume of air present in the lungs after a forced inhalation. |
It includes ERV, IRV, and tidal volume. |
This includes ERV, IRV, residual volume and tidal volume. |
It is about 4000 mL. |
It is about 5000-6000 mL. |
Oxygen is transported in the blood by haemoglobin in the form of oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four O2 molecules. Binding of oxygen to haemoglobin is primarily affected by the partial pressure of O2 (pO2). However, the partial pressure of carbon dioxide (pCO2), H+ concentration and temperature also affect the binding of oxygen to haemoglobin. When the partial pressure of oxygen is high like in alveoli, haemoglobin binds with oxygen and forms oxyhaemoglobin. However, in tissues where pO2 is low, oxygen dissociates from oxyhaemoglobin. The per cent saturation of haemoglobin with oxygen at various pO2 can be studied using the oxygen dissociation curve. The oxygen dissociation curve is a sigmoid curve which is obtained by plotting the percentage saturation of haemoglobin with O2 against pO2.
The oxygen dissociation curve is sigmoid in shape because the binding of oxygen molecules to haemoglobin is co-operative. It means that binding of first O2 molecule to the haemoglobin increases the affinity of haemoglobin for another oxygen molecule. As a result, the haemoglobin attracts more oxygen and thus, the graph showing the percentage saturation of haemoglobin with O2 against pO2 appears sigmoid.
The process of taking the air from outside the body into the lungs is called inhalation or inspiration. The process of inspiration involves the generation of the pressure gradient between the atmosphere and the lungs. Inspiration takes place when the pressure within the lungs is less (negative pressure) than the atmospheric pressure. The process of inspiration involves the following steps:
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