Respiratory System in Humans

Introduction

The main organs of the human respiratory system are the nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, and lungs. These organs work in coordination to assist in breathing. Breathing and respiration are different from each other. Breathing comprises inhalation and exhalation. When we take the air in through the nostrils, it is called inhalation or inspiration. Breathing out air is called exhalation or expiration. Respiration is the process of breaking down simple molecules like glucose to produce energy. Thus, respiration is a crucial process to produce energy for the proper functioning of the human body. 

Organs of the Human Respiratory System

• Nostrils: A pair of openings in the nose which lead to the nasal passage

• Nasal cavity: It is divided by the nasal septum into two nasal passages. The internal walls of the nasal passages are lined with numerous minute hair and mucus. This helps to trap dust and other particles and prevent them from entering the nasal chamber

• Pharynx: Pharynx is a hollow tube that connects to the top of the trachea and the oesophagus. It is the common passageway for food and air

• Larynx: Larynx is a cartilaginous box-like structure which contains vocal cords. It produces sound so it is also known as the sound  or voice box. It is situated right at the junction of the pharynx and trachea

• Trachea: Trachea is a tube-like structure that goes up to the middle of the thoracic cavity where it bifurcates into a pair of bronchi. On the walls of trachea, there are C-shaped cartilaginous rings called tracheal rings. These rings give support to the wall of trachea and help to keep the airway open

• Bronchi: The two branches of the trachea, right and left bronchi enter the lungs from their respective sides where they keep dividing till they form numerous minute branches called bronchioles

• Lungs: Lungs are large, conical, air-filled structures situated in the chest (thoracic) cavity on either side of the chest. Each lung is covered by a two-layered membrane called pleura. A pleural liquid is filled between the two layers. This liquid reduces the friction on the lung’s surface. The lungs’ purpose in the human body is to provide surface for gaseous exchange. Lungs are filled up with a large number of air sacs that are present at the end of each bronchiole. These air sacs are called alveoli. Alveoli are small spherical air pockets with a rich supply of blood capillaries in their thin walls. Alveoli provide increased surface area for the exchange of gases

The Passage of Air in the Respiratory System

When we inhale air, it enters the nostrils and reaches the nasal chamber through the nasal passage. From the nasal chamber, it passes into the pharynx. The pharynx opens into the larynx, and from there, the air enters the trachea and reaches the lungs through bronchi. In the lungs, it goes through bronchioles and finally reaches into alveoli where the oxygen from the air is absorbed into the blood and carbon dioxide is released from the blood into the alveoli. This carbon dioxide is then removed from the body through exhalation.

The Mechanism of Breathing

The lungs are situated in the thoracic cavity which is formed by the vertebral column on the posterior side, sternum on the anterior side, ribs on lateral sides, and diaphragm on the lower side. These anatomical structures together form a sort of air-tight chamber. The result is that whenever the volume of the chest cavity changes, it is reciprocated in the volume of the lung cavity or pulmonary cavity. 

As mentioned previously, breathing consists of two stages: inhalation or inspiration (breathing in) and exhalation or expiration (breathing out). During inhalation, the diaphragm moves downwards, the external intercostal muscles between the ribs contract, lifting the ribs and the sternum. This increases the volume of the thoracic cavity due to which the air pressure inside the pulmonary cavity decreases. Since the atmospheric pressure is higher than the pulmonary air pressure, air is forced to move into the lungs, i.e. inspiration takes place. After that, the diaphragm moves upward, the intercostal muscles relax and the ribs, along with the sternum, move inward. This decreases the volume of the thoracic cavity. As a result, the pulmonary air pressure increases slightly more than the atmospheric pressure and the air is exhaled out, i.e., expiration takes place. 

Terms Related to the Mechanism of Breathing

• Tidal volume (TV): The amount of air breathed in or out of the lungs during one respiratory cycle. It is 500 ml for healthy individual

• Inspiratory reserve volume (IRV): The  air volume that can be inhaled forcefully after a normal inspiration. It is about 3000ml

• Expiratory reserve volume (ERV): The air volume that can be exhaled forcefully after a normal expiration. It is 1000-1100 mL

• Residual volume (RV): The volume of air present in the lungs even after maximum forceful expiration. It is 1200mL

• Inspiratory capacity (IC): The amount of air that can be inhaled after a normal expiration. [TV + IRV]. It is 3500mL

• Expiratory capacity (EC): The amount of air that can be exhaled after a normal inspiration. [TV + ERV]. It is 1500mL

• Functional residual capacity (FRC): The amount of air left in the lungs after a normal expiration. [ERV + RV]. It is 2200-2500mL

• Vital capacity (VC): The maximum volume of air that a person can breathe in or breathe out after a forceful inhalation or exhalation.  [TV + IRV + ERV]. It is 4500mL

• Total lung capacity (TLC): The total amount of air in the lungs after a forceful inhalation. [VC + RV]. It is 6000mL

Exchange of Gases

The exchange of gases takes place at two places.

1. Between the alveoli and the blood capillaries

2. Between the blood and the tissues

The exchange of gases at these sites occurs by simple diffusion due to differences in the pressure or concentration gradient of O2 and CO2. The walls of the alveoli and blood capillaries are made of very thin, single layer membranes through which diffusion can easily take place. 

In the inhaled air, the partial pressure of oxygen is higher, and the partial pressure of carbon dioxide is much lower than in the alveolar blood. That’s why, in the alveoli, the O2 diffuses into the blood and the CO2 diffuses out of the blood in the alveoli. From here, the CO2 is expelled out with the exhaled air and the O2 is carried by the blood to the tissues.

In the tissues, the concentration of CO2 is higher while the concentration of O2 is lower due to continuous catabolic processes. Hence, the CO2 diffuses out of the tissues into the blood and the O2 diffuses out of the blood and into the tissues. Then, this oxygen is used by the tissues for respiration while the carbon dioxide is transported by the blood, back to the alveoli where it is removed from the blood. 

Conclusion

The respiratory rhythm is maintained by the respiratory centre present in the medulla, to suit the demands of the body. The chemosensitive areas of the brain recognise the changes in the concentration of carbon dioxide(CO2) and send signals to the respiratory centre to adjust the respiratory rhythm accordingly. The concentration of oxygen (O2) plays an insignificant role in maintaining this rhythm.