Human Respiratory System - Class XII

• The pharynx, or throat, is the passageway leading from the mouth and nose to the oesophagus and larynx. 
• The pharynx permits the passage of swallowed solids and liquids into the oesophagus, and conducts air to and from the trachea, or windpipe, during respiration. 
• The pharynx also connects on either side with the cavity of the middle ear by way of the Eustachian tube and provides for equalization of air pressure on the eardrum membrane, which separates the cavity of the middle ear from the external ear canal. Hence, The short tube leading from the nose is pharynx.

• The diaphragm is a part of the abdominopelvic cavity.
•  It is located below the thoracic cavity and above the pelvic cavity.
•  Its dome-shaped roof is the thoracic diaphragm, a thin sheet of muscle under the lungs, and its floor is the pelvic inlet, opening into the pelvis. Hence, the diaphragm is a dome-shaped muscle.

• The nasal cavity and the mouth meet at the pharynx, or throat, at the back of the nose and mouth. 
• From there, air quickly enters the second part of your respiratory system, the trachea or windpipe. 
• The trachea is a tube that delivers air to the lungs, the third and most important part of your respiratory system. Hence, Air goes from the pharynx to. trachea.

• The bronchi (or bronchus) are the air passages into the lungs that begin at the end of the trachea. 
• There are two bronchi, one for each lung. 
• The bronchus divides into smaller branches known as segmental bronchi, which divide again into bronchioles, Hence, Bronchi divide into bronchioles.

Most carbon dioxide (about 70%) is transported in the blood as bicarbonate ions (HCO3-) dissolved in the plasma.

The respiratory system, including the lungs and bronchial tree, is derived from the endoderm germ layer.

The pneumotaxic center in the pons regulates respiratory rhythm by sending neural signals that can reduce the duration of inspiration and increase the respiratory rate. This is a key respiratory control mechanism. Option A is incorrect because about 70% of CO₂ is transported as bicarbonate, not by hemoglobin. Option C describes silicosis, not lung fibrosis. Option D - cigarette smoking causes bronchitis, not primarily inflammation of bronchi.

Insects respire via a tracheal system, which is a network of air tubes that brings oxygen directly to the body cells without the need for blood transport, known as direct respiration. Human beings, prawn, and fish rely on indirect respiration via blood and specialized respiratory organs.

The conducting part of the respiratory system includes the nose, pharynx, larynx, trachea, bronchi, and bronchioles. Its primary functions are to transport, filter, warm, and humidify the air. The alveoli are the specific site for gas exchange, which is part of the respiratory zone, not the conducting zone.

Statement (c) is correct as functional residual capacity is the volume of air remaining in the lungs after a normal expiration. Statement (d) is correct as total lung capacity is the maximum volume of air the lungs can hold after a forced inspiration. Statement (a) is incorrect because inspiration is primarily facilitated by the diaphragm and external intercostal muscles, not internal ones. Statement (b) is incorrect because a healthy human typically breathes about 6000-8000 ml of air per minute.

Respiratory quotient if the protein is 0.8-0.9