Chemical mechanism of regulation of respiration

CHEMICAL MECHANISM

Chemical mechanism of regulation of respiration is operated through the chemoreceptors. Chemoreceptors are the sensory nerve endings, which give response to changes in chemical constituents of blood.

Changes in Chemical Constituents of Blood which Stimulate Chemoreceptors

1. Hypoxia (decreased pO2)

2. Hypercapnea (increased pCO2)

3. Increased hydrogen ion concentration.

Types of Chemoreceptors

Chemoreceptors are classified into two groups:

1. Central chemoreceptors

2. Peripheral chemoreceptors.

CENTRAL CHEMORECEPTORS

Central chemoreceptors are the chemoreceptors present in the brain.

Situation

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

Chemo receptors are in close contact with blood and cerebrospinal fluid. Mechanism of Action

Central chemoreceptors are connected with respiratory centers, particularly the dorsal respiratory group of neurons through synapses. These chemoreceptors act slowly but effectively. Central chemoreceptors are responsible for 70% to 80% of increased ventilation through chemical regulatory mechanism.

Main stimulant for central chemoreceptors is the increased hydrogen ion concentration. However, if

hydrogen ion concentration increases in the blood, it cannot stimulate the central chemoreceptors because, the hydrogen ions from blood cannot cross the bloodbrain barrier and blood-cerebrospinal fluid barrier.

On the other hand, if carbon dioxide increases in the blood, it can easily cross the blood-brain barrier and

Bloodcerebrospinal fluid barrier and enter the interstitial fluid of brain or the cerebrospinal fluid. There, the carbon dioxide combines with water to form carbonic acid. Since carbonic acid is unstable, it immediately dissociates into hydrogen ion and bicarbonate ion.

CO2 + H2O H2CO3 H+ + HCO3–

Hydrogen ions stimulate the central chemoreceptors. From chemoreceptors, the excitatory impulses are

sent to dorsal respiratory group of neurons, resulting in increased ventilation (increased rate and force of

breathing). Because of this, excess carbon dioxide is washed out and respiration is brought back to normal. Lack of oxygen does not have significant effect on the central chemoreceptors, except that it generally depresses the overall function of brain.

PERIPHERAL CHEMORECEPTORS

Peripheral chemoreceptors are the chemoreceptors present in carotid and aortic region. Mechanism of Action

Hypoxia is the most potent stimulant for peripheral chemoreceptors. It is because of the presence of oxygen sensitive potassium channels in the glomus cells of peripheral chemoreceptors. Hypoxia causes closure of oxygen sensitive potassium channels and prevents potassium efflux. This leads to depolarization of glomus cells (receptor potential) and generation of action potentials in nerve

ending. These impulses pass through aortic and Hering nerves and excite the dorsal group of neurons. Dorsal group of neurons in turn, send excitatory impulses to respiratory muscles, resulting in increased ventilation. This provides enough oxygen and rectifies the lack of oxygen.

In addition to hypoxia, peripheral chemoreceptors are also stimulated by hypercapnea and increased

hydrogen ion concentration. However, the sensitivity of peripheral chemoreceptors to hypercapnea and

increased hydrogen ion concentration is mild. 

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