HUMAN RESPIRATORY SYSTEM

All human entities require energy for carrying out their cellular and metabolic functions. For fulfilling their energy requirements they take nutrients in the form of food. These food molecules are then broken down to smaller molecules or insoluble substances and are digested by enzymes to their simple forms. These molecules are then absorbed and subsequently assimilated by body cells. The assimilated molecules holds energy in the form of chemical bonds. Their bond energy is then released by oxidation process occurring in the living cells.

All organisms use respiration as a beneficial process for release of energy. It is basically an oxidation process of food substances within the tissue to form CO₂ and H₂O with consequent release of energy. The energy produced is stored in the form of ATP(Adenosine Tri Phosphate, energy currency of cell) and CO₂ and H₂O are considered as the by-product of this respiration process.

In other simple words, respiration is the process that involves five basic steps:

1- Intake of oxygen from the environment.

2- Stepwise oxidation of food with the help of incoming oxygen.

3- Elimination of CO₂ produced as a by-product in the process of oxidation.

4- Release of small amount of energy during oxidation.

5- Storing of energy so that it can be released in useful forms (such as ATP).

TYPES AND PHASES OF RESPIRATION

There are mainly two types of respiration

AEROBIC RESPIRATION

It is the process in which intake of molecular oxygen and release of CO₂ occurs simultaneously. The organisms that follow aerobic respiration are termed as aerobes.

Aerobic respiration occur in two ways:

• DIRECT RESPIRATION– It involves the exchange of environmental oxygen with the CO₂ in the body cells without the use of respiratory organs or blood. For example; it occurs in aerobic bacteria, protists, plants, sponges, coelenterates, flatworms, roundworms and most arthropods.
• INDIRECT RESPIRATION– It occurs by the aid of special organs like skin, gills, lungs and blood. For example; it occurs in crustaceans, amphibians, many annelids, molluscs, reptiles, birds, mammals, etc.

PHASES OF INDIRECT AEROBIC RESPIRATION

Indirect respiration includes following two phases, i.e., external respiration and internal respiration. These two phases are preceded by a preliminary phase called breathing.

Breathing: A preliminary phase

Breathing is not the synonym of respiration but it is the first step of respiration. It is a physical process which refers to the movements of air in and out of the body.

1- External Respiration: It refers to the intake of oxygen by the blood from water or air into the respiratory organs and removal of CO₂ from it.

2- Internal Respiration(Cellular Respiration): It includes following four processes, i.e.,

• Uptake of oxygen by tissue cells from the blood via tissue fluid.
• Oxidation of food to provide energy, water and CO₂ in tissue cells.
• Storage of energy produced by oxidation process in the form of phosphate bonds of ATP.
• Release of CO₂ into the blood by tissue cells via tissue fluid.

ANAEROBIC RESPIRATION

It is the process in which energy is produced without the usage of oxygen. In this, the organisms that carry out anaerobic respiration (called anaerobes), may or may not produce CO₂.

For example: Yeast produce ethanol or ethyl alcohol and CO₂ (i.e., without using oxygen) by oxidising glucose.

C₆H₁₂O_{6(glucose)——>}2C₂H₅OH_(ethanol) + 2CO₂ + Energy

Also some bacteria metabolise glucose to form lactic acid without using oxygen and without producing CO₂.

 C₆H₁₂O_{6(glucose)——->} 2CH₃CHOHCOOH_{(lactic acid)} + Energy

Anaerobic Respiration is also known as fermentation.

VARIOUS ORGANS OF HUMAN RESPIRATORY SYSTEM

The human respiratory system consists of various organs. The lungs are the main organs where exchange of respiratory gases, i.e., CO₂ and O₂ takes place. Various organs of the human respiratory system are as follows:

NOSTRILS These are the holes of the nose through which air enters inside the body. These are paired openings that open into nasal cavities.

NASAL CAVITIES The two nasal cavities are separated from each other by a thin cartilaginous median vertical partition called nasal septum.

PHARYNX  It comprises of nasopharynx, oropharynx and laryngopharynx. The pharynx provides passage to both air and food.

LARYNX It is also called voice-box. Until puberty there is little difference in the size of the larynx in man and woman. Thereafter, it grows larger and becomes prominent in man, and is called Adam’s apple(in man). It is basically the upper part of trachea and allow the air to pass through it to reach trachea.

TRACHEA(WIND PIPE) The larynx leads into a long tube, the trachea or wind pipe, bearing rings of hyaline cartilage which are incomplete posteriorly. At its lower end, the trachea bifurcates into a pair of primary bronchi, which enter the right and left lungs. Bronchi are reinforced to prevent their collapse and are lined with ciliated epithelium and mucous producing cells. These bronchi are further branched into small tubes and bronchioles. They are terminated into grape like sac clusters called alveoli.

ALVEOLI This is surrounded by a network of thin walled capillaries.

LUNGS These are paired structures located in the thoracic cavity, (i.e., one on either side of the heart.) The thoracic cavity is closed below by the diaphragm. Each lung is enclosed in two membranes called the pleurae. The lungs are soft, spongy and elastic organs, which are pinkish in colour. The upper most portion of each lung is called the apex and the inferior most portion is called the base.

MOVEMENT OF AIR THROUGH RESPIRATORY SYSTEM

MECHANISM OF RESPIRATION

Respiration occurs in three major steps namely breathing, exchange of gases and transport of gases.

Breathing involves the inward(inspiration) movement of oxygen and outward(expiration) movement of carbon dioxide.

Gaseous exchange is the process of exchange of gases between the alveoli(primary site of exchange) and the blood due to the difference in partial pressure of gases at different sites.

After gaseous exchange, O₂ and CO₂ are transported to their target sites in various forms. Oxygen is transported in dissolved state(30%), as oxy-haemoglobin(97%) to various body tissues. Likewise, CO₂ is transported in dissolved state(7%), as bicarbonate ions(70%) and as carbaminohaemoglobin(23%) to the lungs.

RESPIRATORY QUOTIENT (RQ)

It is the ratio of volume of CO₂ released to volume of O₂ absorbed. Its value can be 0, 1, <1, >1.

RQ=Volume of CO₂ formed/Volume of O₂ utilised

RQ of carbohydrate= 1

RQ of protein= 0.90

RQ of fat= 0.70

DISORDERS OF RESPIRATORY SYSTEM

1- ASTHMA : Difficulty in breathing.

2- PNEUMONIA : Acute infection or inflammation of the alveoli.

3- EMPHYSEMA: Inflation or distension of bronchioles or alveolar  sac.

4- OCCUPATIONAL RESPIRATORY DISORDERS: It is due to the occupation of individuals especially those in grinding or stone breaking industry.

TERMS TO REMEMBER

• Hypoxia is a condition of oxygen shortage in the tissues.
• Apneus is prolonging of respiration during inspiration.
• Atelectasis means absence of air in a part or whole lungs due to collapse.
• Hay fever is an acute irritative inflammation of the mucous membrane of upper respiratory passage.
• Anoxia is the absence of oxygen in inspired gases, aterial blood or tissues.
• At rest breathing occurs about 14-18 times per minute in a normal man, being more in children.
• In human female, thoracic breathing is more predominant.
• Mammals have negative pressure breathing as it allows them to eat and breathe at the same time.
• A person can live without one lung but it limits the physical ability of the person.
• Nacrosis is a depression of the central nervous system(CNS), including the respiratory centres.
• Pharyngitis is an inflammation of the pharynx, often called sore throat.