Respiratory System

What is Respiratory System?

The biological system which is responsible for taking oxygen and expelling carbon dioxide is called respiratory system. The major organs of this system are lungs which carry out the exchangeable gases.

Parts of respiratory system

The respiratory system can be divided into 3 parts:-
1. Ventilation —the inflow and outflow of air in the lungs.
2. Exchange of gases —the taking in of oxygen and the throwing out of carbon dioxide.
3. Regulation of ventilation

The respiratory system

Ventilation

1. The inflow of air into the lungs is because of expansion of the lungs which sucks in atmospheric air through the respiratory passages—mouth, larynx, tracheav bronchi, bronchiole, air sac. This is called inspiraton. Expansion of lungs is mainly because of contraction of the diaphragm —a muscular sheet which separates the thorax (chest) from the abdomen.
2. Inspiration is an active process caused by contraction of the respiratory muscles.
3. After inspiration which draws in fresh air, the muscles of inspiration relax. As a result, the lungs collapse and “used up” or stale air is thrown out. This process is called expiration.
4. Expiration is a passive process. However when we wish to, we can breath out forcibly. This is called forced expiration.
5. For inspiration and expiration to be easy, the lungs are compliant —soft and strechable.
6. The maximum volume of air that can be forcibly breathed out after a maximum inspiration, is called the vital capacity. It is an indication of the respiratory capacity of the person.
7. The maximum volume of air that can be forcibly breathed out after a maximum inspiration in the first second is called FEV1, i.e. forced exe piratory volume in the 1st second. It is the most important indicator of the respiratory capacity. A good FEVI also indicates that there is no obstruction anywhere in the respiratory FEV1 falls in lung disease as also in narrowing of the respiratory passage at any point.
8. The volume of air normally breathed in and out at each breath is called the tidal volume
9. The air in the respiratory passage which is not exchanged without side air is called the dead space.

2. Bronchiole and Alveolus (Air Sac)

1. The terminal part of the respiratory tree is the bronchiole which ends in the alveolus or air sac.
2. The bronchiole has a ring of circular smooth muscle(“bronchiolar pinch-cock”) which can increase or diminish, die diameter of the bronchiole and thus control the volume of air that goes in and out of alveolus. If the bronchiole muscle goes into spasm (asthma), breathing becomes difficult.
3. The bronchiole also has a lining of the mucous membrane. If the mucous membrane swells up due to congestion or inflammation. the airway is narrowed and breathing become diffcult. Thick secretions (mucus) can also block the bronchiolar airway.

Bronchiole and alveolus
4. Although drugs are available for dilating the constricted bronchiolar muscle and shrinking the swollen mucous membrane, treatment of bronchial asthma is not very satisfactory.
5. Apart from foreign bodies in the upper respiratory tract, the most vul- nerable part of the respiratory airway is the bronchiole.
6. Most of the important diseases of the respiratory system involve the bronchiole.

3. Gas Exchange

1. The alveolus or air sac contains air. This is called alveolar air.
2. Pulmonary capillaries carrying blood are present in the walls of the alveoli.
3. The membrane between the alveolus and pulmonary capillaries is very thin and allows the diffusion of oxygen and carbon dioxide.

Exchange of gases in lungs
4. The pulmonary capillaries bring “used up” blood which has given up O2 to the body tissues and which has taken up CO2 from the body tissues.
5. The alveolus contains air which is rich in O2 and poor in CO2.
6. Diffusion of gases takes place between the blood in pulmonary capillaries and alveolar air.
7. CO2 diffuses from the al veolar air (high O2 tension) to pulmonary capillaries blood (low O2 tension).
8. CO2 diffuses from the pulmonary capillary (high CO2 tension), to the alveolar air (low CO2 tension).
9. The pulmonary capillary blood which is returned to the heart (left auricle) is now oxygenatd Most of its CO2 is removed. This blood is now ready to be pumped out by the heart to the body to supply oxygen to the tissues.
10. The alveolar air, the oxygen content of which has been diminished and the CO2 content of which has been increased, is breathed out in the atmosphere. Fresh air is breathed in from the atmosphere into the alveolus. This fresh air is rich in O2 and poor in CO2.
11. The cycle of breathing has been completed.
12. In disease the diffusion of gases may be hampered. This is seen in fibrosis (scar tissue) formation in the lungs, emphysema, etc.
13. When there is diminished ventilation, the supply of fresh air to the lungs is diminished e.g. respiratory depression due to drugs (e.g. morphine) or disease.

Disorders of Respiratory System

4. Respiratory Insufficiency

1. The laboratory method for assessing respiratory efficiency is a study of the blood gases and pH of blood. The oxygen tension is called the pO2 and carbon dioxide tension is called the pCO2.
2. The respiratory function tests include measurement of tidal volume, vital capacity and forced expiratory volume in the first second (FEV1).
3. The respiratory rate/min. is normally 15-20/min.
4. Some of the conditions which cause diminished ventillation are:
—- respiratory failure
— increased airway resistance caused by bronchiolar constriction.
— stiffness of lungs.
— congestion of lungs
— engorgement of lungs by blood
— destructive diseases of the lungs like tuberculosis and cancer.
5. When the oxygenation of the blood is poor, oxygen may be administered so that the concentration of oxygen in the inspired air in the alveolus is increased. The result is that the oxygen concentration in the blood increases. There are however many dangers of using oxygen to enrich the alveolar air.
6. Oxygen can be administered by a intranasal tube, or via a mask or by using an oxygen tent. In whichever way oxygen is administered, it should be first moistened by bubbling it through water so that the possible damage to the respiratory mucous membrane by breathing in excessively dry oxygen or air is prevented.

5. Regulation of Respiration

1. The most important function of the respiratory system is to maintain optimum oxygen and corbon dioxide concentration in the blood. It is therefore very important that respiration is regulated very precisely.
2. The centre for regulation of respiration lies in the brain. It is called the respiratory centre.
3. When inspiration takes place, the lung tissue is stretched. The stretching of the lung tissue sends nervous impulse to the respiratory so that at a certain stage, when the inspiration is adequate, the respiratory centresends out nervous impulse to end inspiration.
4. Inspiration is followed by expiration. When expiration takes place the stretched lungs revert to the resting, relaxd state. The “stretch” impulses from the lung which inhibit inspiration stop being transmitted to the respiratory centre. Automatically the next inspiration takes place This is called Adrian’s reflex.

6. Direct Effect of Carbon Dioxide on the Respiratory Centre

1. Carbon dioxide has a direct exciting action on the respiratory centre.
2. Rise in the blood concentration of carbon dioxide increases respiration resulting in increased elimination of corbon dioxide by the lungs. The carbon dioxide concentration is thus reduced normal levels

7. The Peripheral Chemoreceptor Systern

1. The large arteries in the neck contain a collection of very special tissues which are very sensitive to fall of the oxygen concentration in the blood. These are carotid and aortic bodies.
2. When oxygen concentration of blood falls, the card and aortic bodies are stimulated and nervous impulses are transmitted to the respiratory centre, resulting in increased respiration.
3. The oxygen concentration in alveolar air rises and the oxygen concentration in blood rises. Fall in oxygen concentration is thus prevented. The most important significance of these biofeedback mechanisms is that during exercise when oxygen concentration in the blood falls and carbon dioxide concentration rises, these regulatory mechanisms prevent a fall in the blood oxygen concentration or a rise in blood carbon dioxide concentration.
4. Respiration can also be controlled voluntarily but this is a special control which we normally do not undertake.

8. Abnormalities of Respiratory Control

1. Respiratory depression. Respiratory centre may be depressed by damage to the nervous system or depression of the nervous system by drugs. The most important drug which depresses respiration is morphine and its analogues.
2. Respiratory centre is also affected by many of the drugs used in anaes- thesia.
3. Abnormal activity of the respiratory centre is seen in kidney failure, where because of acidosis respiration is very rapid.
4. Rapid breathing may also be seen when the lungs are engorged with blood, e.g. in cardiac failure, where excessive fluid in the lungs increases the stiffness of the lungs.

Nervous System
The Kidneys

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