3.1 The Basic Fabric of Creation

(continued from previous issue)

Thus, we find that sneezing is an important and necessary protective reflex. Sneezing becomes a problem in persons with hypersensitivity of the nasal lining triggering off the loop even for small non-dangerous irritants.

2. Pharynx
This is a common muscular tube that lies behind the nose and the mouth. The pharynx is guarded by tonsils at the sides, and adenoids at the top. Tonsils and adenoids are collections of white blood cells (lymphatic tissue) which act as army guards, defend by fighting away any dangerous germs which may contaminate the inhaled air. These white blood cells, in fact, surround the bacteria to digest them away and die themselves in the process, to be replenished soon. The muscles of the pharynx are chiefly useful in swallowing. The pharynx opens into the larynx (the voice box) in front and the oesophagus (the food tube) behind.

3. Larynx
Larynx is composed of pieces of cartilages to maintain its shape and the potency. Attached on top is a small piece of cartilage called epiglottis which closes the larynx like a lid during swallowing so that the food glides over it into the food canal (oesophagus) and not into the wind pipe (trachea).
The vocal chords are thin flaps projecting from the side walls of the larynx. The movement of these flaps are controlled by highly efficient groups of delicate laryngeal muscles. The vibration of these chords due to the air passing through the glottis, causes voice production.

4. Trachea
The trachea (wind pipe) about 10 cm long extends from the larynx to divide into right and left bronchi. There are incomplete rings of cartilages in the wall to maintain the potency under varying intra-thoracic pressure conditions occurring during breathing. The trachea divides into right and left bronchi which enter the corresponding lungs.

5. Bronchi
The right and left bronchi are formed by the bifurcation of the trachea. The right bronchus is shorter and divides into 3 branches to enter three lobes of the right lung and the left is longer and divides into 2 branches to enter two lobes of the left lung. These divide and redivide further into smaller bronchi and finally as bronchioles that enter the air sacs called alveoli.
The larger bronchi have imcomplete rings of cartilage in their walls. These rings disappear in the smaller bronchi. The inner lining of the bronchus (mucous membrane) has ciliated (hairy) surface layer of cells, the movement of the cilia being upwards to remove the finer pollutants that might have escaped the nose. The inner lining contains mucous secreting cells that secrete thin mucus to provide a lubricating layer of moisture, and helps to humidify the air. The most important of the bronchus is the thin circular layer of smooth muscle that runs all the way along the bronchial tree. The contraction of this thin circular bronchial muscle leads to its narrowing and relaxation to dilation of the air passages. The calibre of the bronchus varies with several factors and is controlled by a highly complex neuro-chemical mechanism.

6. Lungs
The lungs, two in number, are the principal organs of respiration. They fill the chest cavity, one on each side separated in the middle by the heart, its blood vessels and other structures. The lungs are cone-shaped with the apex above and the base resting on the floor of the chest cavity on the diaphragm. They are divided into lobes by fissures; the right has 3 and left has 2 lobes.

7. Alveoli or Air Sacs
Alveolus is lined by a single layer of flat cells. It is here that the blood comes into almost direct contact with the air, being separated only by exceedingly thin permeable membrane consisting of two layers of flat cells. Oxygen diffuses from the inhaled air into the air sac to the blood moving very slowly in the capillaries, and carbon dioxide moves from the blood to the air sac due to the differences in partial pressure of the two gases. Carbon dioxide moves more repidly than oxygen. Pulmonary artery and veins carry de-oxygenated and oxygenated blood from and to the heart respectively. The compositions of inhaled and exhaled air are given below :

(to be continued in the next issue)