INTRODUCTION
Respiration is the gaseous exchange between an individual and its surrounding environment, through which O2 is transported from the atmosphere to cells and CO2 and H2O are transported from cells to the atmosphere.
17.1.1 Functions of Respiration
I) Primary or main function:
Through respiration, the living body obtains oxygen, which is used for the oxidation resulting in the liberation of energy and CO2. The formed CO2 is eliminated through respiration.
(UI T).. + O2 → CO2+ H2O + Energy.
II) Secondary functions:
1. Elimination of water; control water balance through excretion of water vapour.
2. Regulation of body temperature, especially in panting animals as dogs during summer.
3. Phonation or voice production by the larynx: The movement of air across the vocal cords causes their vibration which initiate the different sounds of speech.
4. Regulation of blood pH.
17.1.2 Classification of Respiration
I) External or pulmonary respiration:
In which is the gaseous exchange between the blood in pulmonary capillaries and the surrounding environment.
II) Internal respiration:
In which is the gaseous exchange between the blood in the systemic capillaries and the tissues.
Physiological anatomy of the respiratory system:
In animals, the respiratory system consists of two lungs, the air passages leading to them (nasal cavity, pharynx, larynx, trachea, bronchi and bronchioles), the thorax, pleural sac, and respiratory muscles, in addition to respiratory centres and the nerve supply controlling all the previous structures.
17.1.2.1 Nasal cavity
The mucus membrane of the nasal cavity is moist, highly vascular, and containing numerous glands and sensory endings of the olfactory nerve. The caudal part of the nasal cavity contains olfactory (neuro-) epithelium, which is involved in the sense of smell.
In common colds and influenza, the act of smelling is decreased due to the blockage of nasal receptors. Additionally, the nasal cavity acts as a buffer, warming excessively cold air, cooling excessively hot air, and moisten dry air. It also removes suspended particles from the inspired air.17.1.2.2 Pharynx
It is a common chamber for the passage of air and food. During swallowing, the respiratory movements are reflexively inhibited, the soft palate is elevated to close the posterior nares. The laryngeal opening (glottis) is closed by vagal reflex to prevent the escape of food into the trachea and lungs.
17.1.2.3 Larynx
It contains the vocal cords, the vibration of which produces the voice and controls the amount of air passing to the lungs.
17.1.2.4 Trachea
It is always kept open by the presence of an incomplete ring of cartilage in its wall. The mucous membrane is ciliated with the presence of many goblet cells (pseudostratified ciliated columnar epithelium containing many goblet cells). The cilia and the mucous secretion of the glands prevent the entrance of dust and foreign matters into the lungs.
17.1.2.5 Bronchi
Bronchi are similar in structure and function to the trachea. In addition, smooth muscle fibres in the wall of the bronchi show peristaltic movement, which helps in driving foreign materials towards larger air passages. The wall contract or relax to alter the calibre of smaller bronchi and bronchioles, leading to change resistance to air flow (inflow or outflow).
17.1.2.6 Lungs
The lungs are two elastic membranous sacs situated in the thoracic cavity. Each lung has numerous alveoli. The
alveolus is composed of a single layer of respiratory epithelium which is surrounded by blood capillaries that are composed of a single layer of endothelium. Across this layer of cells and the endothelium of capillaries, gaseous exchange between the air in the alveoli and the blood in the adjacent capillaries takes place.
17.1.3 Respiratory Muscles
1.
External and internal intercostal muscles.2. The diaphragm is a musculo-tendinuous organ separating the thoracic cavity from the abdominal cavity. Its thoracic surface is convex, while its abdominal one is concave. It plays an important role in respiration; its contraction increases the length of the thoracic cavity.
3. The Sternomastoid muscle is active during forced inspiration, i.e., inspiration with effort, leading to elevation of the sternum and ribs.
17.1.4 Protection of Lungs Against Damage
1. Previously mentioned in the functions of the air conducting part.
2. Cells of the reticuloendothelial system are known as “Alveolar phagocytes” or “Dust cells.” They engulf dust particles and bacteria entering the alveoli.
3. The presence of a thin layer of fluid between the parietal and visceral layers of the pleura acts as a lubricant, which decrease the friction between the thoracic wall and delicate lung tissue.
4. The tonsils have a protective function against bacteria.
17.1.5 Mechanical Respiration
The mechanism of respiration includes:
a) Mechanism of inspiration.
b) Mechanism of expiration.
17.1.6 Mechanism of Inspiration (Inhalation)
Inspiration means the enlargement of the thorax and lungs and accompanied by an inflow of air. This action is due to:-
a) Contraction of the diaphragm, which is the principal muscle of inspiration. During inspiration, the diaphragm contracts, leading to a reduction in its curvature and an increase the longitudinal diameter of the chest.
b) Movement of the ribs: during inspiration, the external intercostal muscle contractions lead to forward and outward movement of the ribs, resulting in an increase in the transverse diameter of the chest.
Due to a and b:
1) The size of thoracic cavity increases in all dimensions.
2) The intra-thoracic pressure decreases. Pressure between pleural surfaces (intrapleural pressure, already negative; subatmospheric) is reduced from -2 to -6 mmHg (i.e., an increased suction pull is exerted on lung tissue)
3) The air rushes in from the atmosphere through the air passages to fill the lungs (inflow of air).
c) Sternomastoid muscle active during forced inspiration. i.e., inspiration with effort → elevation of sternum → rib elevators.
17.1.7 Mechanism of Expiration (Exhalation)
Expiration means a decrease in the size of the thorax and lungs and accompanied by an outflow of air. This action is due to:
a) Relaxation of the diaphragm.
b) Relaxation of the external intercostal muscles.
Due to a and b:-
1) The size of thoracic cavity returns to its normal dimensions (resting position).
2) The intra-thoracic pressure increases. Pressure between pleural surfaces (intrapleural pressure) is reduced from -6 to -2 mmHg (i.e., less pull is exerted on lung tissue).
3) The air rushes in from the lungs to the atmosphere through the air passages (outflow of air).
17.1.8 Respiratory Cycle
At rest, a normal male adult breathes about 16 times (=cycles) per minute. Each cycle consists of:
1. Inspiration: Increase in the size of thoracic cavity and lungs leading to the inrush of air into the lungs.
2. Expiration: Decrease in the size of thoracic cavity and lungs leading to the outflow of air from the lungs. It is longer than inspiration.
3. Expiratory pause: Period of rest following the end of expiration.
N.B.: absent in rapid respiration as in muscular exercise.
17.1.9 Elasticity of the Lungs
When the lung is inflated (stretched), it tends to recoil (collapse) like an elastic rubber band.
(1) Elastic fibres of lungs: Lungs contain a network of elastic fibres responsible for 1/3 of lung recoil.
(2) Surface tension of fluid lining the alveoli: Molecules of fluid lining the alveoli attract each other and try to collapse the alveoli and consequently the whole lung. It is responsible for 2/3 of lung recoil.
17.1.10 Factors Preventing Lung Collapse (Recoil)
1. Lung surfactant:
Nature: Lipoprotein, present in the lumen of alveoli.
Source: Alveolar epithelium.
Action: Decrease the attraction between fluid molecules lining the alveoli (decrease surface tension), which prevent collapse of alveoli.
2. The faster growth of the thoracic cage compared to that of thoracic contents, including the lungs, causes the lungs to tend to distend to fill the thoracic cavity.
3. Lymph film in the pleural cavity pulls the lungs towards the rigid chest wall (hydraulic attraction).
4. The intrapleural pressure (sub-atmospheric) is always below the intrapulmonary pressure, helping suction of the lungs towards the rigid chest wall.
17.1.11 Pressure Changes During
Normal Respiratory Cycle
1. Intrapleural pressure
2. Intrapulmonary pressure.
3. Intra-abdominal pressure.
1) Intrapleural pressure (IPP)
Pressure inside the pleural sac is usually negative (sub- atmospheric; less than atmospheric pressure 760 mm Hg) during normal respiratory cycle.
17.1.12 Pleura
1. Thin, glistening serous membrane; sac (right and left).
2. Each sac encloses the corresponding lung.
3. Consists of: parietal pleura = outer wall lining the inner thoracic wall.
Visceral pleura = inner wall lining of the lungs.
Pleural cavity: Space between parietal and visceral pleura containing a thin layer of lymph (serous fluid) which acts as lubricant and keeps the lungs always distended.
17.1.13 Values of IPP
Normal inspiration= -6: -10 mm Hg.
Forced expiration = +40 or more.
Normal expiration = -3 mm Hg.
Forced inspiration = -30 mm Hg.
17.1.14 Importance of the Negativity of IPP
1) Help flow of:
• Venous blood flows from extrathoracic into intrathoracic veins.
• Lymph flows from extrathoracic into intratho- racic lymph vessels.
• Blood flows via pulmonary veins.
2) It is a measurement of the elastic recoil of the lung. This recoil is measured by the amount of negative pressure in the intrapleural spaces required to prevent lung collapse.
2) Intrapulmonary pressure
It is pressure inside lung alveoli.
Values of intrapulmonary pressure:
17.1.15 During Normal Inspiration
• Beginning = Zero mm Hg (=atmospheric).
• During = -2 mm Hg.(below atmospheric by 2 mm Hg.
), as the lung alveoli are distended before air reaches them.• End = Zero mm Hg again.
• During normal expiration:
• beginning = Zero mm Hg (=atmospheric).
• during = +2 mm Hg (above atmospheric by 2 mm Hg. ), as the lung alveoli collapse before air is expelled from them.
• end = Zero mm Hg again.
3) Intra-abdominal pressure (IAP).
Pressure inside the abdominal cavity.
Values of intrapulmonary pressure:
17.1.16 During Normal Inspiration
• Beginning = increases due to the descent of the diaphragm.
• End = return to its original level due to decreased tone of muscles of the abdominal wall and pelvic floor.
17.1.17 During Normal Expiration: The
Opposite Occurs During
17.1.17.1 Some Respiratory Terms
1. Respiratory rate: It is the number of respirations (inspiration and expiration) per minute.
2. Eupnea: It is a condition of normal breathing.
3. Dyspnea: It is a condition of difficult breathing.
4. Orthopnea = difficult respiration when lying down but not when standing or sitting. It is due to decreased V.C.
5. Hyperpnea: It is a condition of breathing in which the rate or depth, or both are increased.
6. Polypnea: It is a rapid, shallow, panting type of breathing.
7. Apnea: It means temporary cessation (stopping) of breathing.
17.2