Diaphragm and the intercostal muscles plays important role in creating the pressure gradient between lungs and the atmosphere during normal respiration, by showing the inward and upward or contraction and enlargement of these Intercostal muscles and the diaphragm respectively.
Breathing is brought about by alternate expansion and contraction of the thoracic cavity wherein the lungs lie. This leads to the intake of fresh air called inspiration (inhalation, breathing in) and elimination of the foul or contaminated air is called expiration (exhalation, breathing out). These both inspiration and expiration together referred as respiratory movements.
Inspiration: It is an active process which is brought about by diaphragm muscles and external intercostal muscles which are referred as Inspiratory muscles. Movement of the diaphragm and the thorax is shown in the diagram.
Expiration: It is normally a passive process as it simply involves relaxation of the inspiratory muscles, peripheral muscles of diaphragm and external intercostal muscles. Movement of the diaphragm and the thorax is shown in the diagram.
Q:
Regulation of respiration occurs under the dual control: Nervous and Chemical. Here, we will discuss about the nervous control.
Nervous control: Normal breathing is an involuntary process. Human breathe about 12 times in a minute, but infants breathe about 44 times in a minute. This steady rate is controlled by group of neurons located in the medulla oblongata and pons varolii.
The respiratory centre, regulates the rate and the depth of breathing which is divided into three major group of neurons: Dorsal respiratory group (this causes inspiration), ventral respiratory group (this causes expiration) and pneumotaxic centre (sends signal to all the neurons of dorsal respiratory group and only to inspiratory neurons of ventral respiratory group, it limits inspiration).
(a) Inspiratory Reserve volume (IRV): It is the extra amount of air which can be inhaled forcibly after a normal inspiration. It is about 2000 to 2500ml.
Expiratory Reserve volume (ERV): It is the extra amount of air which can be exhaled forcibly after a normal expiration. It is about 1000 to 1500ml.
(b) Vital capacity: It is the amount of air which one can inhale with maximum effort and also exhale with maximum efforts. It is about 3.5 to 4.5 litres in a normal adult person. It is equal to the TV, IRV, and ERV of air (500 + 2000 + 1500 = 4000 ml).
Total lung capacity: It is the sum of the Vital capacity and the residual volume. It is about 5000- 6000 ml.
(c) Emphysema: It is the respiratory disorder which occurs due to the inflation or abnormal distension of the bronchiole or alveolar sac resulting in the loss of their elasticity.
Occupational respiratory disorder: These diseases are common in persons who works in an environment where they are constantly exposed to potentially harmful substances like gas, fumes etc. Person working in Industries involving grinding or stone breaking etc. suffers from these diseases.
About 5ml oxygen is sent to the tissues through every 100ml of oxygenated blood under the normal physiological conditions.
In simple solution, forms through plasma.
These volumes when arranged in the ascending order, the order would be:
Tidal volume < Residual volume < Expiratory Volume < Inspiratory Reserve volume
In which, tidal volume contains 350ml alveolar and 150ml dead space volume. Residual volume contains about 1500ml. Expiratory volume consists about 1000- 1500ml and Inspiratory reserve volume contains 2000- 2500 ml.
Tidal volume is the volume of air normally inspired or expired in one breath without any effort which is about 500 ml. Residual volume is some air always remains in the lungs even after forcible respiration, this amount of air is known as residual volume which is about 1500ml. IC is the total volume of air which can be inhaled after a normal expiration which is about 2500 to 3000ml. FRC is about 2500 to 3000ml. ERV is the extra amount of air which can be exhaled forcibly after a normal expiration which is about 1000- 1500 ml.
Vital capacity is the maximum volume of air that can be exhaled after a maximum inspiration. It is about 3.5 – 4.5 litres in the human body. It promotes the act of supplying fresh air and getting rid of foul air, thereby increasing the gaseous exchange between the tissues and the environment.
In insects, gaseous exchange occurs through a network of tubes collectively known as the tracheal system. The small openings on the sides of an insect’s body are known as spiracles. Oxygen-rich air enters through the spiracles. The spiracles are connected to the network of tubes. From the spiracles, oxygen enters the tracheae. From here, oxygen diffuses into the cells of the body.
The movement of carbon dioxide follows the reverse path. The CO2 from the cells of the body first enters the tracheae and then leaves the body through the spiracles.
The oxygen dissociation curve is a graph showing the percentage saturation of oxyhaemoglobin at various partial pressures of oxygen.
The curve shows the equilibrium of oxyhaemoglobin and haemoglobin at various partial pressures.
In the lungs, the partial pressure of oxygen is high. Hence, haemoglobin binds to oxygen and forms oxyhaemoglobin.
Tissues have a low oxygen concentration. Therefore, at the tissues, oxyhaemoglobin releases oxygen to form haemoglobin.
The sigmoid shape of the dissociation curve is because of the co-operative binding of oxygen to haemoglobin. As the first oxygen molecule binds to haemoglobin, it increases the affinity for the second molecule of oxygen to bind. Subsequently, haemoglobin attracts more oxygen.Hence Haemoglobin is a positively co-operative protein.
Hypoxia is a condition characterised by an inadequate or decreased supply of oxygen to the lungs. It is caused by several extrinsic factors such as reduction in pO2, inadequate oxygen, etc. The different types of hypoxia are discussed below.
Hypoxemic hypoxia
In this condition, there is a reduction in the oxygen content of blood as a result of the low partial pressure of oxygen in the arterial blood.
Anaemic hypoxia
In this condition, there is a reduction in the concentration of haemoglobin.
Stagnant or ischemic hypoxia
In this condition, there is a deficiency in the oxygen content of blood because of poor blood circulation. It occurs when a person is exposed to cold temperature for a prolonged period of time.
Histotoxic hypoxia
In this condition, tissues are unable to use oxygen. This occurs during carbon monoxide or cyanide poisoning.
(a)
|
Inspiratory reserve volume (IRV) |
Expiratory reserve volume (ERV) |
|
1. It is the maximum volume of air that can be inhaled after a normal inspiration. 2. It is about 2500 – 3500 mL in the human lungs. |
1. It is the maximum volume of air that can be exhaled after a normal expiration. 2. It is about 1000 – 1500 mL in the human lungs. |
(b)
|
Inspiratory capacity (IC) |
Expiratory capacity (EC) |
|
1. It is the volume of air that can be inhaled after a normal expiration. 2. It includes tidal volume and inspiratory reserve volume. IC = TV + IRV |
1. It is the volume of air that can be exhaled after a normal inspiration. 2. It includes tidal volume and expiratory reserve volume. EC = TV + ERV |
(c)
|
Vital capacity (VC) |
Total lung capacity (TLC) |
|
1. It is the maximum volume of air that can be exhaled after a maximum inspiration. It includes IC and ERV. 2. It is about 4000 mL in the human lungs. |
1. It is the volume of air in the lungs after maximum inspiration. It includes IC, ERV, and residual volume. 2. It is about 5000 – 6000 mL in the human lungs. |
Tidal volume is the volume of air inspired or expired during normal respiration.
It is about 6000 to 8000 mL of air per minute.
The hourly tidal volume for a healthy human can be calculated as:
Tidal volume = 6000 to 8000 mL/minute
Tidal volume in an hour = 6000 to 8000 mL × (60 min)
= 3.6 × 105 mL to 4.8 × 105 mL
Therefore, the hourly tidal volume for a healthy human is approximately 3.6 × 105 mL to 4.8 × 105 mL.
The volume of air remaining in the lungs after a normal expiration is known as functional residual capacity (FRC). It includes expiratory reserve volume (ERV) and residual volume (RV). ERV is the maximum volume of air that can be exhaled after a normal expiration. It is about 1000 mL to 1500 mL. RV is the volume of air remaining in the lungs after maximum expiration. It is about 1100 mL to 1500 mL.
∴FRC = ERV + RV
≅ 1500 + 1500
≅ 3000 mL
Functional residual capacity of the human lungs is about 2500 – 3000 mL.
Each alveolus is made up of highly-permeable and thin layers of squamous epithelial cells. Similarly, the blood capillaries have layers of squamous epithelial cells. Oxygen-rich air enters the body through the nose and reaches the alveoli. The deoxygenated (carbon dioxide-rich) blood from the body is brought to the heart by the veins. The heart pumps it to the lungs for oxygenation. The exchange of O2 and CO2 takes place between the blood capillaries surrounding the alveoli and the gases present in the alveoli.
Thus, the alveoli are the sites for gaseous exchange. The exchange of gases takes place by simple diffusion because of pressure or concentration differences. The barrier between the alveoli and the capillaries is thin and the diffusion of gases takes place from higher partial pressure to lower partial pressure. The venous blood that reaches the alveoli has lower partial pressure of O2 and higher partial pressure of CO2 as compared to alveolar air. Hence, oxygen diffuses into blood. Simultaneously, carbon dioxide diffuses out of blood and into the alveoli.
Plasma and red blood cells transport carbon dioxide. This is because they are readily soluble in water.
(1) Through plasma:
About 7% of CO2 is carried in a dissolved state through plasma. Carbon dioxide combines with water and forms carbonic acid.
Since the process of forming carbonic acid is slow, only a small amount of carbon dioxide is carried this way.
(2) Through RBCs:
About 20 – 25% of CO2 is transported by the red blood cells as carbaminohaemoglobin. Carbon dioxide binds to the amino groups on the polypeptide chains of haemoglobin and forms a compound known as carbaminohaemoglobin.
(3) Through sodium bicarbonate:
About 70% of carbon dioxide is transported as sodium bicarbonate. As CO2 diffuses into the blood plasma, a large part of it combines with water to form carbonic acid in the presence of the enzyme carbonic anhydrase. Carbonic anhydrase is a zinc enzyme that speeds up the formation of carbonic acid. This carbonic acid dissociates into bicarbonate (HCO3–) and hydrogen ions (H+).
(ii) pO2 higher, pCO2 lesser
The partial pressure of oxygen in atmospheric air is higher than that of oxygen in alveolar air. In atmospheric air, pO2 is about 159 mm Hg. In alveolar air, it is about 104 mm Hg.
The partial pressure of carbon dioxide in atmospheric air is lesser than that of carbon dioxide in alveolar air. In atmospheric air, pCO2 is about 0.3 mmHg. In alveolar air, it is about 40 mm Hg.
Inspiration or inhalation is the process of bringing air from outside the body into the lungs. It is carried out by creating a pressure gradient between the lungs and the atmosphere. When air enters the lungs, the diaphragm contracts toward the abdominal cavity, thereby increasing the space in the thoracic cavity for accommodating the inhaled air.
The volume of the thoracic chamber in the anteroposterior axis increases with the simultaneous contraction of the external intercostal muscles. This causes the ribs and the sternum to move out, thereby increasing the volume of the thoracic chamber in the dorsoventral axis.
The overall increase in the thoracic volume leads to a similar increase in the pulmonary volume. Now, as a result of this increase, the intra-pulmonary pressure becomes lesser than the atmospheric pressure. This causes the air from outside the body to move into the lungs.
The respiratory rhythm centre present in the medulla region of the brain is primarily responsible for the regulation of respiration. The pneumotaxic centre can alter the function performed by the respiratory rhythm centre by signalling to reduce the inspiration rate.
The chemosensitive region present near the respiratory centre is sensitive to carbon dioxide and hydrogen ions. This region then signals to change the rate of expiration for eliminating the compounds.
The receptors present in the carotid artery and aorta detect the levels of carbon dioxide and hydrogen ions in blood. As the level of carbon dioxide increases, the respiratory centre sends nerve impulses for the necessary changes.
pCO2 plays an important role in the transportation of oxygen. At the alveolus, the low pCO2 and high pO2 favours the formation of oxyhaemoglobin. At the tissues, the high pCO2 and low pO2 favours the dissociation of oxygen from oxyhaemoglobin. Hence, the affinity of haemoglobin for oxygen is enhanced by the decrease of pCO2 in blood. Therefore, oxygen is transported in blood as oxyhaemoglobin and oxygen dissociates from it at the tissues.
As altitude increases, the oxygen level in the atmosphere decreases. Therefore, as a man goes uphill, he gets less oxygen with each breath. This causes the amount of oxygen in the blood to decline. The respiratory rate increases in response to the decrease in the oxygen content of blood. Simultaneously, the rate of heart beat increases to increase the supply of oxygen to blood.