Structure and function of human respiratory system

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The human respiratory system is responsible for delivering atmospheric oxygen to the blood stream (human heart and circulatory system) and eliminating carbon dioxide from the blood. It starts with the nose and ends at the lungs.

Nose: The nose is not just a passage for air but has important functions. It humidifies the inhaled air as dry air is irritating to the airways. It also acts as a filter and prevents dust from entering the lower airways.

From the nose air enters a muscular tube called pharynx which starts at the roof of the nose. Through the pharynx air enters the voice box (larynx) and then the wind pipe (trachea). The entry to the larynx is guarded by a lid like structure called epiglottis which closes the entry to larynx when we swallow food or liquid and prevents choking.

The wind pipe (trachea) divides into two tubes called right and left bronchi (singular=bronchus) which enter the corresponding lung.

The lungs are two is number and are situated in the cavity of thorax on either side of the heart. They rest on the diaphragm. The lungs are sponge like structures. The right lung has three lobes and the left two. The left lung is slightly smaller than the right. The total weight of the two lungs is about 1000 grams.

Within the lungs the bronchi go on dividing and sub dividing into progressively smaller tubes till they end up in minute sac like structures called alveoli. Gas exchange takes place inside the alveoli.

respiratory system

Structure of bronchi: The bronchi are made up of cartilage, muscles and mucous membrane. Mucous membrane lines the inner surface of the bronchi. It is some what like the inner lining of the mouth. The cartilage gives strength to the tube and prevents it from completely collapsing. The muscular action causes narrowing of airways. When they relax the airways become wider.

normal bronchus

The total surface area of the alveoli is approximately equal to that of a tennis court. The bronchi and their divisions are accompanied by branches of pulmonary artery (carrying impure blood) and branches of pulmonary veins (carrying pure blood).

As blood passes through the alveoli carbon dioxide from the blood diffuses into the alveolar space and oxygen is taken up from the air in the alveolar space by the hemoglobin inside the red blood cells.

Mechanics of breathing: In the beginning of inhalation the diaphragm contracts. This brings down the dome of the diaphragm increasing the thoracic volume. At the same time the muscles between the ribs (external inter-costal muscles) also contract elevating and expanding the thorax.

Thus negative pressure is generated inside the thorax and lungs. This negative pressure sucks air into the lungs. The lungs are elastic in nature and recoil generating positive pressure inside which pushes air out. The relaxation of the diaphragm pushes the dome up contributing to the positive pressure.

The normal rate of breathing varies depending on age. In adults it is 16-20 per minute. Newborn babies breathe faster and a quiet baby may breathe up to 40 times per minute.

The chest wall is very soft in babies and when there in some difficulty in breathing the chest wall caves in during inspiration. This is called chest retraction and is a sign of respiratory distress in babies and small children.

As the airways in children are very small even slight swelling of the cells lining the airways can cause serious obstruction to airflow. Hence respiratory infection in children can lead to severe disease.

Why don’t the alveoli collapse completely when there is negative pressure inside? The cells of lungs secrete a substance called surfactant which lowers the surface tension of alveoli preventing collapse. In premature babies the lungs do not have adequate surfactant and hence such babies suffer from breathing difficulty (Hyaline membrane disease) such babies may even die.

The lungs play an important role in maintaining acid base balance in the body. The normal blood is slightly alkaline with a pH of 7.4. The normal range of pH is very narrow unlike many other parameters. Even though the adult hemoglobin level is about 13-15G per deciliter even 50 percent of this value is not incompatible with life. Nothing of that sort can happen with pH. Even a 0.1 unit change in pH can have disastrous consequences.

If blood tends to become acidic due to some metabolic derangement (for example uncontrolled diabetes can result in a condition called keto-acidosis which tends to make the blood acidic) the lungs breathe faster leading to carbon dioxide wash out. That is why a person in diabetic coma breathes deeply and rapidly.

If on the other hand any disease tends to cause the blood to be more alkaline (for example severe vomiting leading to loss of gastric juice which in highly acidic) the lungs breathe slower retaining more carbon dioxide.

Pulmonary function tests: Using an instrument called spirometer it is possible to assess the function of the lungs. The person undergoing the test is asked to breathe through a mouth piece into the spirometer. The instrument records the rate of air flow, the amount of air exhaled and hence the lung capacity. It is possible to detect the presence and severity of obstructive airway diseases like asthma.

Take care of your lungs: Now that you know how fascinating the structure and function of lungs are you should not do anything to damage them. Regular aerobic exercises keep the lungs healthy (Fitness and health) . Cigarette smoking causes considerable damage to the lining of the airways and predisposes to chronic cough and even cancer. Those working in industries should wear protective masks.

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Common respiratory problems


Bronchial asthma in children

Acute respiratory infections in children

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Page last reviewed on 2nd January 2011

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