Respiration and Gas Exchange

The diaphragm functions in breathing. During inhalation, the diaphragm contracts and moves in the inferior direction, thus enlarging the volume of the thoracic cavity (the external intercostal muscles also participate in this enlargement). This reduces intra-thoracic pressure.

Breathing involves two stages, inspiration during which atmospheric air is drawn in and expiration by which the alveolar air is released out. The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere. Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure. Similarly, expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure. The diaphragm and a specialised set of muscles external and internal intercostals between the ribs, help in generation of such gradients.

• A hiccup is an involuntary contraction of the diaphragm involving a reflex arc, that may repeat several times per minute. 
• Once triggered, the reflex causes a strong contraction of the diaphragm followed by about 0.25 seconds later by closure of the vocal cords, which results in the classic "hic" sound. 
• Hiccups may occur individually, or they may occur in bouts. 

Breathing or pulmonary ventilation is a process where the nasal and oral cavities inhaled the air and enter into the lungs through the pharynx, larynx and trachea. Air is also exhaled through the same pathway. While larynx, pharynx and trachea are involved the changes to the volume and air pressure in the lungs trigger  breathing. During the process of  inhalation, the diaphragm and external intercostal muscles contract thereby elevating the rib cage. With the increase in the volume of the lungs, air pressure drops letting the air rush in. Similarly, during exhalation, the muscles relax letting the lungs become smaller, which raises the air pressure and helps to expel the air.

Carbon dioxide is released in the process of exhalation in large amount.$CO _2$ dissolves in water and can easily and reversibly form compounds such as carbonic acid and bicarbonates. when it is released out, it is saturated with water vapour and this is the reason for water droplets in the mirror.

Forced deep breathing (hyperventilation) lowers the partial pressure of CO2 in the blood. Since CO2 is the primary chemical stimulus for breathing, low levels lead to a temporary cessation of breathing (apnea) until CO2 levels rise again.

Breathing involves the contraction and relaxation of the diaphragm and the external intercostal muscles, which change the volume of the thoracic cavity and create the pressure gradients necessary for air movement.

During breathing, 20% oxygen is taken in and 15% oxygen is exhaled out. Movement of diaphragm is very important during breathing process. Ribs will also move in and out with the diaphragm. It is not possible to breathe by moving the diaphragm along without moving ribs. Even after forceful exhalation, lungs are never fully emptied. Residual volume, that is minimum amount of air in lungs always remain at all times. Eustachian tube connects middle ear cavity with nasopharynx and hence, one can breathe out air through it by closing both nose and mouth.

A)Nasal chamber opens into nasopharynx which is a portion of pharynx – True because It also connects the nasal system of air chambers and the upper end of the trachea. The opening of the mouth into the pharynx is the oropharynx, and the nasal opening is the nasopharynx. The eustachian tube from the middle ear opens into the pharynx on each side.

B)Larynx produces sound during inspiration as well as expiration – True, because, the larynx commonly called the voice box, is an organ in the top of the neck of tetrapods involved in breathing, producing sound. They have skeletal muscle, cartilage of the upper larynx collapses inward during inhalation, causing airway obstruction, results in the creation of sounds.

C)During swallowing, gullet can be covered by a thin elastic cartilaginous flap called Epiglottis – false, because, the epiglottis is a flap in the throat that keeps food from entering the windpipe and the lungs. The flap is made of elastic cartilage covered with a mucous membrane, attached to the entrance of the larynx. It projects obliquely upwards behind the tongue and the hyoid bone, pointing dorsally.

D) The trachea is get divided at the level of 6th thoracic vertebra into right and left primary bronchi – False, because The trachea, commonly called the windpipe, is the main airway to the lungs It divides into the right and left bronchi at the level of the fifth thoracic vertebra, channeling air to the right or left lung.

E)The branching network of bronchi, bronchioles, and alveoli comprise the lungs – True.

So, the correct option is ‘2’.

During inhalation, the diaphragm contracts, thus enlarging the thoracic cavity. This reduces intra-thoracic pressure: enlarging the cavity creates suction that draws air into the lungs. When the diaphragm relaxes, air is exhaled by elastic recoil of the lung and the tissues lining the thoracic cavity in conjunction with the abdominal muscles, which act as an antagonist, paired with the diaphragm's contraction.

Compressive asphyxia (also called as chest compression) is mechanically limiting expansion of the lungs by compressing the torso, hence interfering with breathing. Compressive asphyxia occurs when the chest or abdomen is compressed posteriorly. In accidents, the term traumatic asphyxia or crush asphyxia usually refers to compressive asphyxia resulting from being crushed or pinned under a large weight or force. 

Hypoxia is the specific term for an inadequate supply of oxygen to the tissues. While related to asphyxia, hypoxia is the condition of the tissues themselves.

Deep breathing rises the $O _2$ level of the blood. By excess formation of oxyhaemoglobin the acidity of blood increases. To control it, more of free $CO _2$ changes into bicarbonates and the free $CO _2$ level in the blood decreases. So due to lack of stimulant for inspiratory centre cessation of breathing takes place, after some time $O _2$ is consumed by the tissues and rise in $CO _2$ stimulates breathing again. So, the correct answer is 'Both too much O$ _2$ and very little CO$ _2$ in the blood'.

• Regulation of Respiration is done by the nervous mechanism.
  
• A specialised centre present in the medulla region of the brain called the respiratory rhythm centre is primarily responsible for this regulation.
  
• Another centre is present in the Pons region in the brain called pneumotaxis centre can moderate the function of Respiratory Rhythm Centre.