Human Respiration: Transport of Gases, Mechanism, and Examples

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What is Respiration?

o Respiration is the exchange of gases i.e., oxygen and carbon dioxide and transported from lungs to bloodstream and to the cells of body.

o Alveoli is site for the exchange and capillaries of the blood vessels.

o These exchange and transport of gases are conducted under various factors and also external environment.

Respiration Process

1. Alveoli is the site for gas exchange of Oxygen and carbon dioxide in the lungs. Blood and tissue are also site responsible for the exchange by simple diffusion with the factors based on concentration and pressure gradient. Solubility of gases and thickness of membranes involved in gas exchange are the factors for the rate of diffusion.

2. Each gas from the mixture has its own partial pressure. Partial pressure of oxygen is different from partial pressure of carbon dioxide and changes with different sites.

3. Amount of carbon dioxide can diffuse through membrane is observed to be higher than oxygen as solubility of carbon dioxide is also higher than oxygen.

4. Diffusion of membrane has to be across three layers i.e., endothelium (alveolar capillaries, thin squamous epithelium of alveoli and substance between endothelium and epithelium. The membrane is of thickness less than a millimeter. These all factor helps in diffusion of oxygen from alveoli to tissue and carbon dioxide of tissue to alveoli.

5. Transport of gases – blood is responsible of the transport of oxygen and carbon dioxide. 97% of oxygen is carried by the red blood cells and 3% by the plasma in dissolved state. while carbon dioxide is carried by the red blood cells about 20 – 25% and 7% by the plasma in dissolved state whereas, 70% of carbon dioxide is carried as bicarbonate.

6. Transport of oxygen – oxygen binds with hemoglobin reversibly to form oxyhemoglobin. Four molecules of oxygen binds with one molecule of hemoglobin. This binding of oxygen and hemoglobin is due to the partial pressure and other factors are partial pressure of carbon dioxide, temperature and hydrogen ion concentration.

7. Oxyhaemoglobin formation can be by the high partial pressure of oxygen and low partial pressure of carbon dioxide along with low temperature and low H+ concentration in alveoli. And when in tissue there is low partial pressure of oxygen with high partial pressure of the carbon dioxide and high temperature along with the high H+ concentration results in dissociation of oxygen from the oxyhaemoglobin. Oxygen binds to the haemoglobin at the lungs surfaces and dissociate in the tissue. For every 100 mL of oxygenated blood, it delivers 5mL of oxygen to the tissue

8. Transportation of carbon dioxide – carbamino – haemoglobin is formed as haemoglobin carries 20-25% of carbon dioxide apart from oxygen. Partial pressure of oxygen plays a major role in binding of carbon dioxide with haemoglobin. This binding is opposite of oxygen. When in tissue, high partial pressure of carbon dioxide and low partial pressure of oxygen results in binding of carbon dioxide more. In alveoli when the partial pressure of carbon dioxide is low and partial pressure of oxygen high, it results in the dissociation of carbon dioxide from carbamino – haemoglobin and release in the tissue.

9. Carbonic anhydrase enzyme is present in high concentration in red blood cells and in low concentration in plasma. This enzyme catalyses the reaction to form bicarbonates.

CO2 + H2O ⇔ H2CO3 ⇔ HCO3+ H+

Catabolism results in high carbon dioxide in the tissue and diffuses in the blood and gives HCO3- and H+. The opposite reaction is observed in alveoli where partial pressure of carbon dioxide is low. Bicarbonate that is trapped in the tissue is transported to the alveoli and released in the form of carbon dioxide. With every 100 mL of deoxygenated blood 4 mL of carbon dioxide is delivered to the alveoli.

Regulation of Respiration

o The regulation of respiration is controlled by the brain. The signals transmitted are between brain and chest muscles along with the diaphragm. Medulla and pons region is responsible for the regulation of the respiration with contraction of muscles and diaphragm.

o Medulla: Respiration rhythm i.e., inspiration then follows expiration and such cycle is followed. This rhythm is under the medulla region of the brain and called as respiratory rhythmic centre. Medulla produces rhythmic nerve impulse which contract the muscles that help in inspiration. And when muscles relax it results in expiration.

o Pons: Pneumotaxic centre – this helps in controlling the rate of breathing with its pattern. This centre regulates the duration of inspiration by sending neural signals. Also, helps in the prevention of over inflation. Pons also keep in check about the amount of air to take in one breath.

o Apneustic Centre: With constant stimulation in the medulla region through the neurons helps to promote inspiration. This helps in controlling the intensity of breathing and send the signals that oppose signals of pneumotaxic centre.

o Chemo-sensitive area that are located adjacent to the medulla and are activated when there is increase of CO2 and H+ ions, then signals the rhythmic centre for the respiratory process and makes changes for the same and for elimination of the substances. There are certain chemo-receptors present in the walls of bronchi and bronchioles, medulla, aortic arch and artery. These receptors can keep in check the concentration of CO2 and H+ ions and signals to regulate the breathing.

o Factors that affect rate of respiration are CO2 concentration, temperature, water etc.

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