Human Respiratory System - functions, parts and parameters

The respiratory system provides a means for acquiring oxygen (O2) and eliminating carbon dioxide (CO2). Humans have a well developed respiratory system and respiration involves inspiration (breathing in) and expiration (breathing out) exchange of gases in the lungs and its transport to the tissues.

Types of Respiration:

Respiration is the interchange of gases between an organism and the medium in which it lives. Respiration is divided into two types.
Internal Respiration: It is the exchange of gases between blood stream and nearby cells.
External Respiration: External or lung respiration is the exchange of gases between lungs and blood stream. The respiratory rate, respiratory volume, respiratory air flow etc are important variables of respiration.
The mechanism of breathing involves the action of muscles that change the volume of the thoracic cavity (chest) to generate inspiration and expiration. Inspiration results from the contraction of diaphragm and intercostals muscles whereas the expiration results from their relaxation.

Functions of Respiratory System:

1. It helps to avoid sudden changes in blood pH and body fluids.
2. The respiratory organs provide maximum surface area for diffusion of oxygen and CO2.
3. With the help of respiratory system, gases are constantly renewed.
4. Respiratory system protects the surface membranes from harsh environments such as temperature.


Parts of Respiration:

The main parts of respiration are given in the block below.


The function of each block is discussed below.

1. Nose and nasal cavities: The air from outside to inside and vice-versa is guided through the nose and nasal cavities. Actually nose provides the entry of air during inspiration and exit of air during expiration.

2. Pharynx (Throat): The throat is subdivided into three parts – Nasopharynx, Oropharynx, Hypopharynx.

3. Larynx (Voice Box): The vocal cords are located in the larynx. It is called voice box because it is due to the vibration of vocal cords when air is forced upwards, that the sound is produced.

4. Trachea (Windpipe): It is a vertical tube that allows the passage of air to and from the lungs.

5. Bronchi: Trachea is divided into two branches which divide into each lung. Each branch is called bronchus.

6. Bronchioles: The bronchi is also divided into many smaller branches. Bronchioles are the smallest bronchial branches. Air inspired through nose passed through the trachea and which divided into bronchi and terminates at the bronchioles.

7. Alveoli (air sacs): The air sacs trap the air and allow the exchange of gases to blood capillaries. Alveoli have a maximum capacity of around 9L in adult men and 7L in adult women.

8. Lung capillaries: The alveoli is surrounded with thin tubes carrying blood. These are called lung capillaries and they allow the exchange of gases.

9. Lungs: The lungs consists of two cone shaped spongy organs that contain the alveoli (air sacs) that trap the air for gas exchange with blood. Blood enters the lungs through pulmonary arteries and after oxygenation it leaves through pulmonary veins.

Parameters of Respiration:

The parameters of respiration indicate the state of the respiratory function and the lung volumes and capacities under specific conditions. The various parameters of respiration are discussed below. Several factors can affect the lung volumes. A person who is born and lives at sea level will develop a slightly smaller lung capacity than a person who lives at high altitude levels. Also taller people, non-smokers and athletes are found to have more lung capacity when compared to shorter people, smokers and non – athletes respectively. Also the various values vary with the age of the person. In conditions such as asthma the volumes are normally lower but the flow rates are normally obstructed.

1. Dead air (About 150 mL): We know that the air enters the lungs through nose and the nasal cavities. Only a certain portion of air entering the respiratory system reaches the alveoli. The volume of air that is not available for gas exchange with the blood is called dead air.

2. Tidal Volume (TV – Male 500 Ml/ Female 390 mL): It is called the depth of breathing and it is defined as the volume of gases inspired or expired during each respiratory cycle. In other words it is the volume of air an individual is normally breathing in and out.

3. Inspiratory Reserve Volume (IRV Male 3L/ Female 2.3 L): It is the maximum amount of gas that can be inspired with effort from end inspiratory position. Or it is the extra inspiration from low peak tidal volume. It is also called ‘complemental air’.

4. Expiratory Reserve Volume (ERV – Male 1.2L/ Female 0.93 L): It is the maximum amount of gas that can be expired from end expiratory level. Or it is the extra expiration from low peak tidal volume.

5. Residual Volume (RV – Male 1.2L / Female 0.93L): Even if we expire with maximum effort some amount of gas remains in lungs. Residual volume is the amount of gas remaining in lungs after maximal expiration.

6. Total Lung Capacity (TLC – Male 6L/ Female 4.7L): It is the amount of gas contained in the lungs at the end of maximal inspiration. It is the sum of Inspiratory Capacity (IC) and Functional Residual Capacity (FRC). Inspiratory capacity is defined as the maximal amount of gas that can be inspired from resting expiratory level. It is about 3.6 L. Functional Residual Capacity (FRC) is the amount of gas containing in lungs at resting expiratory level. TLC = FRC + IC

7. Minute volume (MV): It is the volume of air breathed for one minute.

8. Vital Capacity (VC – Male 4.6L/ Female 3.6 L): Vital Capacity is the maximum amount of air that a person can expel from the lungs after first filling the lungs to their maximum extent. It is equal to the sum of Inspiratory Reserve Volume (IRV), Expiratory Reserve Volume (ERV) and Tidal Volume (TV). So, VC = IRV+ERV+TV.

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