Mechanism And Regulation Of Respiration

 This article deals about the 5 stages of respiratory mechanism and 2 types of respiratory regulation.

Mechanism Of Respiration

The mechanism of respiration involves the following steps:

Breathing or pulmonary ventilation.

External respiration.

Transport of O2 to tissue.

Internal respiration.

Transport of CO2 from tissue.

1. Breathing Or Pulmonary Ventilation

This is movement of air into and out of the lungs.

Breathing supplies oxygen to the alveoli, and eliminates carbon dioxide.

The main muscles involved in breathing are the intercostal muscles and the diaphragm.

There are 11 pairs of intercostal muscles occupying the spaces between the 12 pairs of ribs. They are arranged in two layers, the external and internal intercostal muscles.

The diaphragm is a dome-shaped muscular structure separating the thoracic and abdominal cavities.

Breathing depends upon changes in pressure and volume in the thoracic cavity. Since air flows from an area of high pressure to an area of low pressure, changing the pressure inside the lungs determines the direction of airflow.

Breathing involves two process, they are:

i) Inspiration

It takes place when the volume of the thoracic cavity Is increased and the air pressure is decreased.

Simultaneous contraction of the external intercostal muscles and the diaphragm expands the thorax.

As the diaphragm + external intercostals contracts (moves downward) lung volume increases.

It involves following events:

First of all, external intercostal muscle contracts and internal intercostal muscles relax.

Due to contraction of external intercostal muscles, ribs are pulled upward, resulting in increase in thoracic cavity size.

The thoracic cavity further enlarges due to contraction of diaphragm, lowering the diaphragm and increases the size of thoracic cavity.

With increase in size of thorax, lungs expand simultaneously.

As lungs expand, the air pressure is reduced inside, so to equalize the pressure, atmospheric air rushes inside the lungs.

ii) Expiration

It takes place when the size of the thoracic cavity is reduced and air pressure is increased.

It involves following events:

The internal intercostal muscle contracts and external intercostal muscles relax.

Due to contraction of internal Intercostal muscle, ribs are pulled inward, resulting in decrease in size of thoracic cavity.

Furthermore the diaphragm is pushed upward due to Its relaxation.

With the decrease in size of thoracic cavity, lungs are compressed.

As lungs are compressed, pressure increases, so the air is forced outside.

2. External Respiration

This is the exchange of gases by diffusion between alveoli and blood in the alveolar capillaries, across the respiratory membrane.

Diffusion of oxygen and carbon dioxide depends on pressure differences, e.g. between atmospheric air and the blood, or blood and the tissues.

Gas exchange during the respiration process takes place in the alveolus at its surface that separates the alveolus with the capillary.

The exchange of O2 and CO2 occurs through diffusion which is the net movement of gas molecules from a region that has a higher partial pressure to another region that has a lower partial pressure.

The venous blood in alveolar capillaries contains high levels of CO2 and low level of O2.

CO2 then diffuses from higher level (venous blood) to lower level (alveoli) until equilibrium is maintained. By the same process O2 diffuses from alveoli to venous blood until equilibrium.

3. Transport Of Oxygen To Tissue

Oxygen is carried in the blood to the tissue in two from:

i) Oxyhaemoglobin (98.5%)

It is a chemical combination of O2 with haemoglobin.

Hb4 + 4O2 = Hb4O8 (oxyhaemoglobin).

ii) Solution in Plasma Water (1.5%)

O2 dissolves in plasma of blood and carries to tissues.

When the level of O2 is high in blood, it combines with haemoglobin to form oxyhaemoglobin.

Oxyhaemoglobin is unstable, and under certain conditions readily dissociates releasing oxygen. Factors that increase dissociation include low O2 levels, low pH and raised temperature.

4. Internal Respiration

Internal respiration is exchange of gases which takes place in tissue, so also known as cellular respiration.

In tissue, oxygen carried in the form of Oxyhaemoglobin gets dissociated to liberate free O2.

Hb4O8 = Hb + O2

The free O2 then oxidized the glucose in the presence of respiratory enzymes to liberate CO2, water and energy.

C6H12O6 + 6O2 = 6CO2 + 6H2O + Energy

Energy is utilized by the tissue for its vital activities, while the CO2 is diffused from the tissue.

5. Transport Of Carbon Dioxide From Tissue To Lungs

Carbon dioxide is one of the waste products of metabolism.

It is excreted by the lungs and is transported by three mechanisms:

i) As Carbonic Acid (H2CO3)

Some CO2 dissolved in the plasma to form carbonic acid (7%).

Carbon dioxide mixed with water of blood plasma to form carbonic acid.

CO2 + H2O =H2CO3

ii) Bicarbonate Ions (HCO3­-)

In the plasma (70%).

Carbonic acid formed in blood plasma quickly ionizes to form bicarbonates and hydrogen ions in the presence of enzyme carbonic anhydrase.

CO2 + H2O = H+ + HCO3­-

Bicarbonate ions combined with sodium or potassium present in blood to form sodium bicarbonate (NAHCO3) or Potassium bicarbonate (KHCO3) and transported in this form.

iii) As Carbamino Haemoglobin

Some CO2 combines with Haemoglobin to form carbamino haemoglobin in RBCs (23%).

CO2 + NHbNH2 = HbNH.COOH (carbamino haemoglobin).

Finally, CO2 is carried to lungs and expelled out by the expiration process of breathing.

mechanism of respiration
Process Of Respiration By Caroline Tokarshi | SlideShare. 

Regulation\Control Of Respiration

Respiration is regulated by two mechanisms:

Nervous or neural mechanism.

Chemical mechanism.

1. Neural Mechanism

Respiration is under the control of two distinct neural mechanism: 

Autonomic control.

Voluntary control.

Usually respiration is under autonomic control (involuntary) and doing activities such as speaking, singing etc. Respiration is under voluntary control.

The voluntary system of respiration is situated in the cerebral cortex.

The autonomic system of respiration is located in the pons and medulla oblongata and is collectively referred to as respiratory centre (collects sensory information about the level of oxygen and carbon dioxide in the blood and determines the signals to be sent to the respiratory muscles.)

Stimulation of the respiratory muscles provide respiratory movements which leads to alveolar ventilation.

Respiratory centers are situated in the reticular formation of the brainstem and depending upon the their position in brainstem, the respiratory centers are classified into two groups:

Medullary centers.

Pontine centers.

Medullary Centers

Divided into two:

Inspiratory Center

Inspiratory center is situated in the upper part of medulla oblongata.

This center is also called the dorsal group of respiratory neurons.

It is formed by the nucleus of tractus solitarius.

Function: It is concerned with inspiration.

Expiratory Center

It is situated in medulla oblongata anterior and lateral to the inspiratory center.

It is also called the ventral group of respiratory neurons.

It is formed by neurons of nucleus ambiguus and nucleus retro ambiguous.

Function: This center is inactive during quiet breathing and the inspiratory center is the active center, but during forced  breathing or when the inspiratory center is inhibited it becomes active.

Pontine Centers

Divided into two:

Pneumotaxic Center

It is situated in upper Pons.

It is formed by the nucleus parabrachialis.

Function: It controls medullary respiratory centers, particularly the inspiratory center through apneustic center. It always controls the activity of the inspiratory center so that duration of inspiration is controlled.

Apneustic Center

It is situated in lower Pons.

Function: this center increases depth of inspiration by acting directly on the inspiratory center.

2. Chemical Mechanism

The chemical mechanism of the respiration is operated through the chemoreceptors.

They are the receptors which give response to change in the chemical constituents of blood like:



Increased hydrogen ions concentration (decreased blood pH).

Chemoreceptors are classified into two groups:

Central chemoreceptors.

Peripheral chemoreceptors.

Central Chemoreceptors 

The chemoreceptors present in the brain are called central chemoreceptors 

They are situated in the deeper part of medulla oblongata, close to the dorsal group of neurons. This area is known as chemosensitive area and neurons are called 

as chemoreceptors.

Action: They are very sensitive to an increase in hydrogen ion concentration. Hydrogen ions cannot cross the blood brain barrier and blood cerebrospinal fluid barrier.

On the other hand if carbon dioxide increases in the blood as it is a gas it can cross both the barrier easily and after entering the brain it combines with water to form carbonic acid.

As carbonic acid is unstable, it immediately dissociates into hydrogen and bicarbonate ions.

The hydrogen ion now stimulates the central chemoreceptors which stimulates the dorsal group of the respiratory center (inspiratory group) and increases rate and force of breathing.

Peripheral Chemoreceptors 

The receptors which are present in peripheral portions of the body are called peripheral chemoreceptors. 

Action: They are very sensitive to reduction in partial pressure of oxygen. Whenever, the partial pressure of oxygen decreases these chemoreceptors become activated and send

impulses to inspiratory center and stimulate them. Thereby increases rate and force of respiration and rectifies the lack of oxygen.