Physiology of Respiration

- Respiration is the process by which the body takes in oxygen from the air and removes carbon dioxide, a waste product of cellular metabolism.

- It involves several interconnected processes, including pulmonary ventilation, gas exchange, transport of gases in the blood, and cellular respiration.

1. Pulmonary Ventilation

Definition:   The process of inhaling oxygen-rich air into the lungs (inspiration) and exhaling carbon dioxide-rich air out of the lungs (expiration).

Mechanism of Breathing:  

  - Inspiration: Diaphragm contracts and flattens, intercostal muscles contract, causing the rib cage to expand, increasing the volume of the thoracic cavity and lowering the air pressure inside the lungs. Air rushes into the lungs to equalize the pressure.

  - Expiration: Diaphragm relaxes, returning to its dome-shaped position, intercostal muscles relax, causing the rib cage to recoil inward, decreasing the volume of the thoracic cavity and increasing the air pressure inside the lungs. Air is forced out of the lungs to equalize the pressure.

2. Gas Exchange

Definition:   The exchange of oxygen and carbon dioxide between the alveoli in the lungs and the pulmonary capillaries.

Process:  

  - Oxygen Diffusion: Oxygen moves from the alveoli, where its concentration is high, across the alveolar-capillary membrane into the bloodstream, where its concentration is lower. It binds to hemoglobin in red blood cells for transport.

  - Carbon Dioxide Diffusion: Carbon dioxide moves from the bloodstream, where its concentration is high, across the alveolar-capillary membrane into the alveoli, where its concentration is lower. It is then exhaled out of the body.

3. Transport of Gases in the Blood

Oxygen Transport:  

  - Most oxygen is transported in the blood bound to hemoglobin molecules in red blood cells, forming oxyhemoglobin.

  - Some oxygen is dissolved directly in the plasma.

  - Oxygen-hemoglobin dissociation curve illustrates the relationship between partial pressure of oxygen and hemoglobin saturation.

Carbon Dioxide Transport:  

  - Carbon dioxide is transported in the blood in three forms:

    1. As dissolved carbon dioxide in plasma.

    2. As carbaminohemoglobin, where carbon dioxide binds to amino acids in hemoglobin.

    3. As bicarbonate ions (HCO3-) formed when carbon dioxide combines with water in red blood cells, catalyzed by the enzyme carbonic anhydrase.

4. Cellular Respiration

Definition:   The process by which cells use oxygen to produce energy (ATP) from glucose and release carbon dioxide as a byproduct.

Aerobic Respiration:  

  - Involves the breakdown of glucose in the presence of oxygen to produce ATP, carbon dioxide, and water.

  - Occurs in the mitochondria of cells.

  - Generates a much larger amount of ATP compared to anaerobic respiration.

5. Regulation of Respiration

Neural Control:  

  - Medulla Oblongata and Pons: Respiratory centers in the brainstem that regulate the rate and depth of breathing.

  - Chemoreceptors: Located in the carotid bodies and aortic bodies, sense changes in blood levels of oxygen, carbon dioxide, and pH, providing feedback to the respiratory centers to adjust breathing accordingly.

Chemical Factors:  

  - Increased carbon dioxide levels (hypercapnia), decreased oxygen levels (hypoxia), or decreased blood pH (acidosis) stimulate increased ventilation to restore homeostasis.

  - Hyperventilation and hypoventilation are mechanisms to adjust carbon dioxide levels and blood pH.

Voluntary Control:  

  - Higher brain centers, including the cerebral cortex, can consciously modify breathing patterns, such as holding one's breath or breathing slowly.