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Essay on the Respiration in Humans
Essay Contents:
- Essay on the Definition of Respiration
- Essay on the Forms of Respiration
- Essay on the Respiratory Units
- Essay on the Functions of Respiration
Essay # 1. Definition of Respiration:
Respiration is the process by which oxygen from the lungs is carried by the blood to the tissues; and carbon dioxide formed in the tissues by metabolic activity is carried by the blood to the lungs and is expired out.
The process of respiration involves four stages:
i. Ventilation means the passage of air in and out of lungs during inspiration and expiration respectively.
ii. Intrapulmonary gas-mixing or distribution of oxygen-rich inspired air with the air already present in the lungs.
iii. Diffusion which means gas-transfer across the alveolo-capillary membrane due to tension gradient.
iv. Perfusion means flow of adequate quantity of blood through the lungs so that the diffused gases are carried away.
Throughout the body, the function of an organ is reflected in its structure, this is true of the lung in particular.
Essay # 2. Forms of Respiration:
There are two forms of respiration:
A. Aerobic Respiration:
This is the release of relatively large amounts of energy by using oxygen to break down foodstuffs (i.e. by ‘oxidising’ them).
Aerobic respiration usually takes the form of the oxidation of glucose in the cytoplasm of living cells. The process is controlled by enzymes. It unlocks the chemical energy in the glucose molecule, releasing it for metabolic activities, and releasing also the waste products carbon dioxide and water.
The energy that is released may be used in the following ways:
(i) For muscle contraction (bringing about movement and locomotion)
(ii) To help link together amino acids in order to manufacture proteins (such as enzymes and proteins used for growth and repair)
(iii) For cell division and therefore growth
(iv) For active transport of chemicals through cell membranes (e.g. the uptake of glucose through the villus walls)
(v) Conversion into electrical energy in the form of impulses along nerve cells
(vi) Conversion into heat energy to maintain a constant body temperature.
B. Anaerobic Respiration:
This is a form of respiration in which relatively small amounts of energy are released by the breakdown of food substances in the absence of oxygen.
Anaerobic respiration is the process by which yeast cells break down glucose during fermentation. The product is ethanol (‘alcohol’).
Less energy is released than in aerobic respiration because the alcohol molecule is relatively large and still contains a considerable amount of chemical energy.
In the Human Body:
Anaerobic respiration also occurs in the human body – especially in muscles. It happens when there is not enough oxygen reaching the cells to convert all the glucose into carbon dioxide and water. Instead, the glucose is partially broken down, without oxygen, to lactic acid, with only a limited amount of energy being released.
The circulatory system carries the lactic acid away from the muscle to the liver, where it is oxidised to carbon dioxide and water. But this happens after the period of exercise has finished. The oxygen being used for this process is said to be used to pay off the oxygen debt.
If the circulation is inefficient, high levels of lactic acid in the muscles may lead to cramp.
Essay # 3. Respiratory Units (Fig. 8.2):
Respiratory units consist of:
i. Respiratory bronchioles.
ii. Alveolar ducts.
iii. Respiratory atrium.
iv. Pulmonary alveoli, the walls of these being so thin that gas exchange (Fig. 8.2) may take place between the air contained in these tubules and the capillaries on their wall.
Each terminal bronchiole opens into a thin-walled respiratory bronchiole of equal diameter which communicates with some alveoli situated on its wall? However, for the most part each respiratory bronchiole opens into several alveolar ducts, these latter open into dilated spaces called (pulmonary) atrium which again communicate with many pulmonary alveoli.
Each alveolus is a thin-walled sac filled with air measuring form 75 to 300 µm in diameter. The capillaries of the pulmonary blood vessels ramify around the walls of the alveoli. The alveolar tissue (parenchyma) contains fibres of elastin and collagen and the fluid lining the alveoli has surface tension. As a result the lung is elastic and is held expanded by keeping the pressure around it (intrapleural pressure) lower than alveolar pressure.
Broncho-Pulmonary Anastomosis:
On the wall of the respiratory bronchioles the venous blood from the bronchial circulation via the bronchial arteries which arise from the aorta, drains directly into the arterial blood of pulmonary veins. Venous admixture also takes place by direct shunts between the branches of pulmonary artery and pulmonary vein.
Pulmonary Alveoli (Fig. 8.3):
Pulmonary alveoli are polygonal in shape and are packed so tightly that some have no distinct separate walls and communicate with adjacent alveoli by minute pores. They are lined by a thin layer of squamous epithelium which is separated from the endothelium of the pulmonary capillaries by a homogeneous basal lamina which together with small amount of connective tissue constitutes interalveolar septa.
Scattered in between the alveolar epithelial cells are found isolated cuboidal cells or great alveolar cells which are characterised by microvilli on their free surface. The cytoplasm of these cells contains rough-surfaced endoplasmic reticulum and characteristic multilamellar bodies which secrete a substance called surfactant. Surfactant (a layer of phospholipid) has got the unique property of reducing surface tension of intra-aveolarfluid and thus helps in keeping the alveoli open and prevents their collapse.
Essay # 4. Functions of Respiration:
i. Gas Transfer:
Transfer of O2 from the alveoli to the venous blood and CO2 in the opposite direction.
ii. Regulation of PCO2 of Blood:
The most important function of respiration is to keep the arterial PCO2 at 40 mm Hg which is essential for many vital functions of the body.
iii. Regulation pH of Blood:
By the reversible reaction H2CO3 ↔ H++HCO3.
iv. Excretion of Certain Volatile Gases:
Excretion of certain volatile gases, e.g., chloroform, ether, ammonia, etc.
v. Pumping Action:
The rhythmic movement of the diaphragm and chest wall causes rhythmic alteration of pressure in the abdomen and chest cavity. This assists in drawing blood from the lower part of the body to the abdomen and then to chest and thus helps in maintaining venous inflow to the heart.
Pleural Cavity and Intrapleural Pressure:
The lungs are covered by visceral pleura which are reflected over the inner aspect of the chest wall enclosing a potential cavity between its two layers known as pleural cavity. The two layers, parietal and visceral are kept moist and lubricated by a few millilitres of a mucopolysaccharide containing fluid in the interpleural space so that during respiration the lungs with visceral pleura surrounding it glide smoothly over the parietal pleura (Fig. 8.4).
Intrapleural Pressure:
Normal intrapleural pressure, that is pressure in the pleural cavity is negative and amounts to – 2.5 mm Hg at the end expiratory position. This means that the lungs are not completely collapsed and that alveoli remain partially inflated even after complete expiration.
The factors responsible for negative intrapleural pressure are:
i. Elastic Recoil:
Due to presence of elastic fibres the lung tissue has a continuous tendency to recoil away from the chest wall. The tendency naturally increases during inspiration with inflation of the alveoli.
ii. Surface Tension of the Intra-Alveolar Fluid:
Due to intra-molecular attraction of the surface-layer of the fluid lining the alveoli, they have got a tendency to collapse. This collapsing force of the millions of the alveoli produces a summated effect resulting in tendency of the whole lung to recoil away from the chest wall.
In fact about two-thirds of the recoil tendencies of the lungs are attributable to the surface tension phenomenon.
Surface Tension at the Fluid-Air Interface within the Alveoli and the Role of Surfactant:
It was observed by von Neergaard that the pressure required to inflate the air-filled lungs was higher than when it was filled with normal saline. Alveoli are minute spherical bodies, not necessarily of the equal size, lined by the thin layer of fluid and filled with air. The surface tension at the liquid air interface is high and prevents, its expansion whereas in lungs filled with physiological saline the surface tension is absent so that they expand readily.
In a spherical bubble the tension of its wall T tends to collapse the bubble whereas the pressure of air within (P) tends to expand it. The relationship between these two opposing forces in equilibrium is given by the equation P = 2 × T/r, where r is the radius of the bubble.
Naturally, large bubbles (alveoli in this context) have got lower tension than smaller ones and if communication exists between the two the smaller bubbles (alveoli) will empty into the larger ones. Further as the alveoli become smaller during expiration, the surface tension (T) increases and tends to collapse the alveoli. This is prevented by surfactant because of its surface tension-reducing properties.
Alveolar surfactant is a lipoprotein with dipalmityl lecithin as an important component. It is secreted by the lamellar bodies of the great alveolar cells lining the alveoli. This substance forms a lining for the interior of the alveoli and increases their surface tension during expansion, i.e. inspiration and decreases their surface tension during expiration.
Surfactant, therefore, not only prevents collapse of the alveoli during expiration but also prevents emptying of smaller alveoli into larger ones, thus ensuring stabilising effect on the respiratory process.
Pressure Changes in the Pleural Cavity and its Relation to Volume Changes in the Lungs:
Pressure changes in the pleural cavity and volume change in the lungs and the intrathoracic pressure (intrapleural pressure) at the resting stages is slightly negative – 2.5 mmHg. With the enlargement of the thoracic cage in all its diameters during inspiration the intrapleural pressure becomes still more negative and at the end of inspiration in quiet breathing becomes about – 6 mmHg.
This inspiratory increase in negative pressure in the pleural cavity is reflected in the pressure within the lungs (intrapulmonary pressure) which normally is ‘o’ at rest but falls to about – 2 mmHg at the end of inspiration. Air, therefore, rushes in from the atmosphere to the lungs causing inflation of the lungs during inspiration. Negative intrapleural pressure is thus primarily responsible for inspiratory inflow of air into the lungs. It is also responsible for keeping the patency of the airways.
Expiration is usually a passive process due to relaxation of the inspiratory muscles. The intrapleural pressure rises to its resting value with the diminution in size of the thoracic cage, the lung collapses and the intrapulmonary pressure rises above the atmospheric pressure till at the end-expiratory resting stage it becomes equal to the atmosphere.
In forced inspiration and expiration, the pressure variations in pleura and the lungs are considerably exaggerated. In forced expiration with closed glottis the intrapulmonary pressure may go up to + 40 mmHg. It is possible to record intrapleural pressure with a fine polythene tube attached to a thin-walled balloon lying in the lower third of the oesophagus. The diagram shows intrapleural pressure tracing synchronous with volume tracing during respiratory cycle in a normal subject (Fig. 8.5).
Note the time lag between volume tracing and pressure tracing. The pressure change occurs fraction of a second earlier than volume changes.
The undulation on the pressure tracing with intraoesophageal balloon is due to pressure variations resulting from heart-beat.