NEET Biology Breathing and Exchange of Gases Mechanism of Breathing
Mechanism of Breathing
Mechanism of Breathing
The process of respiration mainly includes two phases:
- External Respiration
It is the process by which gases are exchanged between the blood and the air. It is a physical process. It is simply the intake of oxygen and giving out of carbon dioxide.
Respiratory movement involve two phases:
It is a process, by which fresh air enters in the lungs. In this, diaphragm becomes flat and gets lowered by the contraction of its muscle fibres and increases the volume of thoracic cavity. The external intercostal muscles contract and pull the ribs and sternum upwards and outwards direction and thus the volume of thoracic cavity is increased.
It is a process, by which C02 is expelled out from the lungs. Muscle fibres of the diaphragm relax make it convex, decreasing the volume of the thoracic cavity. External intercostal muscles relax and pull the ribs downwards and inwards direction, thus decreasing the size of the thoracic cavity.
- Internal Respiration
It is the process, in which gases are exchanged between blood and tissue fluid and between tissue fluid and cells.
Cellular respiration is the process, by which cells use oxygen (02) for metabolism and give off carbon diodide (C02) as a waste.
Respiratory Volumes and Capacities
The quantities of air, that lung can receive, hold or expel under different conditions are called pulmonary or respiratory volumes and combination of two or more pulmonary volumes are called pulmonary capacities.
The apparatus commonly used to measure the pulmonary volumes is a spirometer or respirometer.
The recording of breathing is known as spirogram. There are four respiratory volumes, i.e. Tidal Volume (TV), Inspiratory Reserve Volume (IRV), Expiratory Reserve Volume (ERV), Residual Volume (RV) and four respiratory capacities, i.e. Inspiratory Capacity (IC), Functional Residual Capacity (FRC), Vital Capacity (VC) and Total Lung Capacity (TLC).
Respiratory quotient is the ratio of volume of C02 produced to volume of oxygen consumed.
volume of CO, evolved
volume of 02 absorbed
It is determined by Ganong’s respirometer
Exchange of Gases
Breathing (pulmonary ventilation) is the process of moving air into and out of the lungs.
Pulmonary Gas Exchange
Alveoli are the primary sites of exchange of gases. Exchange of gases also occur between blood and tissues. Solubility of gases as well as the thickness of the membranes involved in diffusion are important factors that can affect the rate of diffusion. Partial pressure of 02 and C02 in the atmospheric air and the two sites of diffusion are given ahead.
Partial Pressure (in mmHg) of Oxygen and Carbon Dioxide.
As the solubility of C02 in blood is 20-25 times higher than that of 02. The amount of C02 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher as compared to that of 02. The diffusion membrane is made up of three major layers, i.e. thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance in between them. All these factors are favourable for diffusion of 02 from alveoli to tissues and that of C02 from tissues to alveoli.
Transport of Gases in Blood
The respiratory membrane has a limit of gaseous exchange between alveoli and pulmonary blood. It is called diffusing capacity.
Blood transport oxygen from the respiratory organs to the tissue cells and also transports carbon dioxide from the tissue cells to the respiratory membrane.
- Oxygen Transport
- 98.5% of oxygen (02) is transported by blood with the help of the respiratory pigment haemoglobin present in erythrocytes (RBCs).
- One molecule of haemoglobin can carry as much as 4 oxygen molecules.
- Gram of haemoglobin binds about 1.34 mL of oxygen (02). Thus, 100 mL of pure blood carries about 20 mL of oxygen.
Oxygen-Haemoglobin Dissociation Curve
When a graph is plotted between per cent saturation of haemoglobin and oxygen tension, a curve is obtained, which is termed as Hb-02 dissociation curve.
At normal condition that is on pC02 of 40 mmHg concentration, this curve is sigmoid and normal. Due to increase in concentration of COz, curve is shifted towards right side. Due to decrease in concentration of CO 2, curve is shifted towards left side.
- Carbon Dioxide Transport
Transport of carbon dioxide by blood is much easier than that of the oxygen due to the high solubility of C02 in water. About 7% of C02 is transported as dissolved in plasma, 23% as carbaminohaemoglobin and 70% as bicarbonates. Most of the C02 is transported by blood in the form of sodium bicarbonate in plasma.
- Bohr’s Effect
Hb-02 dissociation curve shifts to right, when C02 tension in the blood is high. Bohr’s effect discovered by Bohr in 1904. Deoxygenation of oxyhaemoglobin is directly proportional to the blood pC02. C02 of tissue fluid and alveoli does not exert Bohr’s effect.
- Haldane’s Effect
It is important for promoting C02 transport, Oxyhaemoglobin behaves as a strong acid. As more and more oxyhaemoglobin forms in the lungs, it releases more and more H+ ions incicasing the acidity of blood.. This is known as Haldane’s effect.
- Chloride Shift or Hamburger’s Phenomenon
To maintain electrostatic neutrality of plasma, many chloride ions diffuse from plasma into RBCs and bicarbonate ions pass out. The chloride content of RBCs increases, when oxygenated blood becomes deoxygenated. This is termed as chloride shift or Hamburger shift.
Entering of Cl- ions into RBCs is known as positive chloride shift. Shifting of Cl“ ions from RBCs into plasma is known as negative chloride shift.
Regulation of Respiration
Respiratory movements are under the control of medulla oblongata.
- The basic rhythm of respiration is controlled by the respiratory centre present in brain stem (medulla and pons). The medullary rhythmic area can be divided into a Dorsal Respiratory Group (DRG) and a Ventral Respiratory Group (VRG).
- The receptors for chemical regulation are located in carotid bodies, aortic bodies and in brain.
- The carotid and aortic bodies are situated in association with heart. The carotid and aortic bodies are responsible for the detection of 02 concentration in blood and its effect on breathing. These are considered as peripheral chemoreceptors. Thus, the blood oxygen does not have direct effect on the respiratory centre of brain.
- Along with nervous and chemical coordination, temperature also affects breathing rate. Increased temperature up to certain extent, increases metabolic activities, thus increase the breathing rate.
- Artificial respiration is required when person, drowns, comes under electric shock, met on accident, is exposed to gas poisoning or anesthesia.
- Two methods, manual respiration and mechanical respiration.
- Manual method include mouth to mouth breathing and mechanical respiration involves, drinkers method use of iron long and ventilation method involves use of rubber tube.
Disorders of Respiratory System
- Bronchitis It is caused by the permanent swelling in bronchi. As a result of bronchitis, cough is caused and thick mucous with pus cells is spitted out.
- Bronchial asthma It is an allergic attack of breathlessness associated with bronchial obstruction, characterised by coughing and difficult breathing.
- Emphysema A condition, in which the walls separating the alveoli breaks resulting in the reduction of surface area for the exchange of respiratory gases. Heavy cigarette smoking leads to emphysema.
- Pneumonia It is an acute infection or inflammation of the alveoli. The most common cause of pneumonia is the pneumococcal bacterium Streptococcus pneumoniae.
- Lung cancer It is caused by excessive smoking. The tissue increases limitlessly, which is called malignancy. The blood capillaries are ruptured, blood starts flowing and death is caused by excessive bleeding.
- Hay fever It is an allergic disorder of nasal lining. It develops due to hypersensitivity of the lining to pollen grains or other foreign particles. There is an nasal episode of sneezing that begins following allergy
Some of the other diseases effecting respiratory system are pulmonary TB caused by Mycobacterium tuberculosis, diphtheria, caused by Corynebacterium diphtheriae, Coryza caused by rhinovirus, influenza by influenza virus and SARs caused by HCV.
Occupational Respiratory Disorders
- Anthracosis It is pneumoconiosis of coal workers deposition of fibrous tissue in the lungs causing bronchitis and emphysema.
- Silicosis It is due to long exposure to dust containing silicon compounds. Workers of glass industry, potters, gold and copper miners develop progressive fibrosis in the liver.