Respiratory Physiology
Functional Anatomy: Two lungs taken together can be viewed as a trumpet having two separate zones-1.Conducting
2.Respiratory zone.
Gaseous exchange occurs in the respiratory zone but not in the conducting zones (also called, dead space).Accordingly the respiratory zone presents a typical histological pattern conducive to the exchange. Mast cells of mucosa secrete histamine (H) when their membrane perturbed and H1 receptors are occupied by H bronchospasm.Mast cells also produce SRS-A, which causes bronchospasm.Bronchial muscles supplied by sympathetic and parasympathetic.Besides VIP secreting neurons also supply. Blood in pulmonary capillary is separated from air within alveolus by alveolo-capillary membrane.
Respiratory Tract:The organs which allow the entrans of air into the lungs and exchanges of gases with the blood in the air passages from the nose to the pulmonary alveoli.
Division of respiratory tract:
a.Upper respiratory tract: From nose (anterior nares) to the vocal fold.It consists of-
1.Nose.2.Nasopharynx.3.Oropharynx.4.Larynx upto vocal folds.
b.Lower respiratory tract:From vocal folds to the alveoli of the lungs.It consists of-
1.Larynx below the vocal folds.2.Trachea.3.Two bronchi.4.Bronchioles.5.Terminal bronchioles.6.Respiratory bronchioles.7.Alveolar duct.8.Atria.9.Air sac.10.Alveoli.
Linning epithelium of respiratory system:
a.Trachea and major bronchi: Pseudostratified ciliated columnar epithelium.
b.Bronchioles: Ciliated columnar epithelium.Onwards from respiratory bronchioles:The cilia disappear and the epithelium
becomes cubical.
c.Alveolar wall: Thin,simple squamous type. The linning epithelial cells are of two types.
1.Type I cells:Squamous cells.
2. Type II celles (also called granular pneumocytes ) are interminged with the type I cells.They secrets surfactant a substance which reduce the surface tension in the alveoli.They constitute about 10 percent of the surface area of alveoli.
Alveoli:
a.Total numbers :300 million
b.Total area of the alveolar walls in
contact with capillaries in both lungs is about :70 m2
c.Space between air and blood :0.5 micro meter.
Alveoli communicate with each other by small pores called pores of khan.Accessory communation sometimes occur between fine bronchioles and their adjacent alveoli and known as Lamberts sinuses.
Conducting and respiratory zone:
The whole lung can be divided into two major zones, conducting zone and respiratory zone.Weibel numbered each generation of tracheobronchial tree.Thus the trachea is generation 0(zero).The two major divisions of the trachea , viz, the right and left bronchi,constitute the first generation and so on.In the 16th generation,the bronchi are called the terminal bronchiole.The 17th generation bronchioles are the respiratory bronchioles.There are three generations of respiratory bronchiole.The alveolar sac is the 23rd and the last generation.On and from the 17th generation, few alveoli can be found on bronchioles.Although, the major portion of the O2 and CO2 exchanges occurs in the alveoli (the 23rd generation), some exchanges begin to occur from the 17th generation.Therefore the portion upto the 16th generation,is called the conducting zone, whereas on and from the 17th generation begins the respiratory zone.The two lungs taken together,can be represented as a trumpet shaped organ.The total capacity of the conducting zone is only about 150 ml whereas that of respiratory zone is about 3 liters.In short,the walls of the tracheobronchial tree in the conducting zone are thick, contain-cartilage and heavy amount of smooth muscles.Also there are glands in the submucous coat draining into the lumen.The epithelia are ciliated columnar type.No alveolus sprouts from these areas and no gaseous exchange occurs here.On the other hand, respiratory zone is a big area containing large number of exceedingly thin walled alveoli, without any mucous secretion.Gaseous exchanges occur through the alveoli of this zone.It will be seen afterwards that velocity of air flow is high in the conducting zone, whereas it is low in the respiratory zone.These multiple divisions greatly increase the total croos-sectional areaof the air ways-
1.Trachea : 2.5 cm2
2.Alveoli : 11,800 cm2.
Pleura:
The lungs are inveested by pleura which has tow layers , parietal and viceral.The parietal pleura is the outer and viceral pleura is the inner of the double layer.In between the two layers, there is therefore a potential space,called the pleural cavity.The parietal layer is adherent to the parieties, inner side of the chest wall and thoracic side of the diaphragm.Therefore, when these structures move, the parietal pleura has to move.The viceral pleura is adherent to the underlying viscus, the lung itself, therefore when the viscus (the lung) moves, it has to follow the viscus.The parietal pleura is supplied by vessels which are systemic,(that is here the blood pressure is higher) whereas the viceral pleura is supplied by vessels of pulmonary circulation and so the blood pressure in these vessels is low.In between the two layers of pleura there is a very small amount (say about 2 ml) of fluid, called pleural fluid. It is dispersed throughout the pleural cavity.
Pulmonary circulation:
pulmonary blood vessels:Anatomical peculiarities: The pulmonary vascular bed resembles the systemic, except that-
1.The walls of the pulmonary artery and its large branches are about 30% as thick as the wall of the aorta.
2.The small arterial vessels,unlike the systemic arterioles ,are endothelial tubes with relatively little muscle in their walls.
3.There are also some smooth muscle in the walls of the postcapillary vessels.
4.The pulmonary capillaries are large, and there are multiple anastomoses, so that each alveolus sits in a capillary basket.
Pulmonary blood flow:
With two quantitatively minor exceptions, the blood put out by the left ventricle returns to the right atrium and is ejected by the right ventricle, making the pulmonary vasculature unique in that it accommodates a blood flow that is almost equal to that of all the other organs in the body.One of the exceptions is part of the bronchial blood flow.As noted above there are anastomoses between the bronchial capillaries and the pulmonary capillaries and veins, and although some of the bronchial blood enters the bronchial veins, some enters the pulmonary capillaries and veins, by passing the right ventricle.The other exception is blood that
flows from the coronary arteries into the chambers of the left side of the heart.Because of the small physiologic shunt created by those two exceptions, the blood in systemic arteries has a PO2 about 2 mm Hg iower than that of blood that has equilibrated with alveolar air, and the saturation of hemoglobin is 0.5% less.
Pulmonary edema:
pulmonary edema occurs in the same way that edema occurs elsewhere in the body.Any factor that causes the pulmonary interstitial fluid pressure to rise from the negative range into the positive range will cause sudden filling og the pulmonary interstitial spaces and alveoli with large amounts of free fluid.
The most common causes of pulmonary edema are as follows:
1.Left sided heart failure or mitral valvular disease with consequent great increases in pulmonary venous pressure and pulmonary capillary pressure and flooding of the interstitial spaces and alveoli.
2.Damage to the pulmonary capillary membrane caused by infections such as pneumonia or by breathing noxious substances such as chlorine gas or sulfur dioxide gas.
Each of these causes rapid leakage of both plasma proteins and fluid out of the capillaries and into both the lung interstitial spaces and the alveoli.
Broncho pulmonary segment:
Each tertiary bronchus and its ramifications and the alveoli connected with them, constitute a broncho pulmonary segment, which is self contained, functionally independent of lung tissue. It has its own blood , lymph and nerve supply and connective tissue invesment. This is of surgical importance in dissection of a portion of the lung in diseased condition.There are ten bronchopulmonary segments in the right lung and nine in the left lung. The medial basal or cardiac segment is absent in the
left lung.
Function of the lungs:(Respiratory function):
1.Gaseous exchange:Carriage of oxygen from the lungs to the site of tissue respiration for sub-sequent utilization and also carrige of CO2 from that site to the lung alveoli for elimination. 2.Metabolic function:Oxygen is essential for maintenance of metabolism in the tissue.Aerobic metabolism cannot take place in the abscence of oxygen.
3.Excretion: It excretes volatile substancess like ammonia, keton bodies, essential oils etc.
4.Maintenance of acid-base balance:This is done chiefly by adjusting the amount of CO2 elimination, the normal PH of the body fluid is 7.4 any change in the PH causes alteration in the rate and depth of breathing.
5.Maintenance of water balance:600-800 ml of water per day is lost as water vapour during expiration.
6.Maintenance of temperature balance: When water is transformed into gaseous form , heat is absorbed, so heat is lost through water vapours. About 10% of body heat is changed in this way.
7.Homeostatic function: It helps in maintenance of homeostatic of the internal environment of the body.
b.Non-respiratory function of lungs:
1.Metabolic function.
2.Excretory function.
3.Maintenance of acid base balance.
4.Maintenace of temprature balance.
5.Maintenance of water balance.
6.Role on circulation.
7.Maintenance of homeostatic of the internal environment of the body.
Respiration:
Defnition:Respiration is a physiological process which means the transport of O2 from atmosphere to the body cell for oxydation of the ingested food materials and elemination of CO2 and other volatile metabolic end products from the cell to the atmosphere.
Types of respiration:Respiration are of two types-
1.External respiration: Intake of O2 and removal of CO2 from body is called external respiration.
2.Internal respiration:The utilization of O2 and production of CO2 by cells and the gaseous exchange between the cells and their
fluid medium.
Rate of respiration:
It is the total number of respiration per unite time.It is counted in minute. The normal rate of respiration varies in accordance with age, sex, size ,work, rest and sleep.
Respiratory rate at different age groups:
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: Age in years : Respiratory rate per minute :
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: At birth : 14-60 :
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: First years : 25-35 :
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: 2-4 years : 20-30 :
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: 5-14 years : 20-25 :
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: Adult (male) : 10-18 :
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: Adult (female) : 10-18 :
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