Gas Exchange in the Lungs and Tissues

The rate of gas exchange is influenced by the surface area available for diffusion, the concentration gradient, the permeability of the barrier, and the distance between the gases.

Oxygen levels (PO2), carbon dioxide levels (PCO2), and pH (acidity) of arterial blood directly affect ventilation. Changes in these parameters trigger the respiratory system to adjust breathing rate and depth.

Partial pressure of a gas in solution is a measure of the gas's contribution to the total pressure of the solution. It's different from the gas's concentration, which describes the actual amount of gas dissolved in the solution.

The solubility of a gas in liquid affects its diffusion. Gases that are highly soluble in liquids, like CO2, diffuse more readily than gases that are less soluble, like O2.

Gas exchange in the lungs occurs between alveoli and pulmonary capillaries. Oxygen diffuses from alveoli into the blood, and carbon dioxide diffuses from the blood into alveoli.

Carbon dioxide (CO2) transported in the blood through three main mechanisms: dissolved in plasma (7%), bound to hemoglobin as carbaminohemoglobin (23%), and as bicarbonate ions (HCO3-) in plasma (70%).

In red blood cells, CO2 combines with water, catalyzed by carbonic anhydrase, forming carbonic acid (H2CO3). Carbonic acid then dissociates into bicarbonate ion (HCO3-) and hydrogen ions (H+). This reaction is reversible.

Central chemoreceptors, located in the CNS, primarily respond to changes in pCO2. Peripheral chemoreceptors, found in carotid bodies, are most sensitive to changes in pO2 but also respond to pH and pCO2.

Central chemoreceptors monitor the pH of cerebrospinal fluid. When pCO2 rises, it crosses into the cerebrospinal fluid and is converted into bicarbonate and H+. The increase in H+ stimulates the central chemoreceptors to increase ventilation, bringing pCO2 back down.

Peripheral chemoreceptors sense low pO2, increased pCO2, and decreased pH. They signal the brainstem respiratory centers to increase ventilation, thereby increasing oxygen intake and removing excess CO2.

Located in the medulla, the DRG is responsible for initiating inspiration. It receives sensory input from the pons and sends signals to the inspiratory muscles (diaphragm and intercostal muscles).

A group of neurons in the medulla responsible for active expiration and strong inspiration. It acts as a pacemaker, generating the basic rhythm of breathing.

Located in the pons, these groups influence the rhythm and depth of breathing. They coordinate with other respiratory centers to ensure smooth transitions between inspiration and expiration.

Oxygen requires a significantly higher concentration gradient than CO2 for dissolving in the blood. This is because CO2 is 20 times more soluble in blood than oxygen.

Gas movement in the blood depends on three factors: pressure gradient of the gas, solubility of the gas in liquid, and temperature (which is relatively constant in the human body).

Hemoglobin, found in red blood cells, is responsible for transporting 98% of oxygen in the blood. When oxygen binds to hemoglobin (Hb), it forms oxyhemoglobin (HbO2). This binding releases oxygen at tissues with low oxygen levels.

Partial pressure of oxygen in plasma (pO2) directly affects the amount of oxygen bound to hemoglobin. Higher pO2 means more oxygen carrying capacity.

The oxyhemoglobin saturation curve depicts the relationship between oxygen partial pressure (pO2) and the percentage of hemoglobin saturated with oxygen. This curve's sigmoid shape reflects oxygen binding cooperatively to hemoglobin.

The oxyhemoglobin curve shifts due to changes in pH, temperature, and 2,3-BPG. A shift to the right indicates a lower affinity for oxygen, while a shift to the left means a higher affinity for oxygen.

CO2 converts to bicarbonate (HCO3-) in red blood cells with the help of the enzyme carbonic anhydrase. This reaction is reversible, and its equilibrium shifts depending on the CO2 concentration.

CO2 is transported in the blood in three main ways: as dissolved CO2 in plasma, bound to hemoglobin, and as bicarbonate (HCO3-) ions. Bicarbonate is the most significant form of CO2 transport.