The gas exchange that occurs in the alveoli is a vital process in human respiration. Oxygen and carbon dioxide must be efficiently exchanged to ensure proper cell function and metabolic balance. This involves the principles of diffusion, the role of haemoglobin, and the transport of carbon dioxide.
Principles of Diffusion in Alveoli
Simple Diffusion
- Concentration Gradient: Oxygen and carbon dioxide move along their respective concentration gradients. Oxygen flows from a high concentration in the alveoli to a lower concentration in the blood. Conversely, carbon dioxide moves from a higher concentration in the blood to a lower concentration in the alveoli.
- Surface Area and Thickness: The large surface area provided by millions of alveoli and the thin walls (about one cell thick) ensure a fast diffusion rate.
- Permeability: Alveolar walls are permeable to oxygen and carbon dioxide but not to larger molecules like proteins, allowing these gases to diffuse freely.
Partial Pressure Gradients
- Oxygen: The partial pressure of oxygen (P_O2) is around 100 mm Hg in the alveoli and 40 mm Hg in the capillaries, encouraging oxygen to flow into the blood.
- Carbon Dioxide: The partial pressure of carbon dioxide (P_CO2) is approximately 45 mm Hg in the capillaries and 40 mm Hg in the alveoli, leading to diffusion into the alveoli.
Haemoglobin and Oxygen Transport
Oxygen Binding
- Oxyhemoglobin Formation: Hemoglobin molecules in red blood cells can carry up to four oxygen molecules, forming oxyhemoglobin (HbO2).
- Cooperative Binding: The binding of one oxygen molecule to haemoglobin enhances the affinity for further oxygen molecules, a phenomenon known as cooperative binding.
Oxygen Release
- Oxygen Dissociation: In the tissues where P_O2 is low, oxygen is released from haemoglobin.
- Bohr Effect: If the carbon dioxide level or temperature is high or pH is low, more oxygen is released, known as the Bohr Effect.
Carbon Dioxide Transport in the Blood
Carbon Dioxide Dissolved in Plasma
- Approximately 7% of carbon dioxide is transported as a dissolved gas in the plasma.
Carbon Dioxide as Bicarbonate Ions
- Around 70% of carbon dioxide reacts with water in red blood cells, forming carbonic acid (H2CO3), which then dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+).
- Carbonic Anhydrase: This enzyme catalyses the reaction, speeding it up significantly.
- Chloride Shift: The bicarbonate ions exit the red blood cells, and chloride ions enter to maintain electrical neutrality.
Carbon Dioxide Bound to Hemoglobin
- Roughly 23% of carbon dioxide binds to haemoglobin at different sites from where oxygen binds.
- Carbaminohemoglobin: This complex is formed when carbon dioxide binds to haemoglobin.
- Haldane Effect: The ability of haemoglobin to carry carbon dioxide is increased when oxygen is released (deoxygenation).
Interaction Between Oxygen and Carbon Dioxide Transport
- Chloride Shift Mechanism: This ensures charge balance within red blood cells during the transportation of bicarbonate ions.
- Bohr and Haldane Effects: These effects ensure that oxygen and carbon dioxide are delivered and removed efficiently at the tissue and lung levels.
Alveolar Structure and Function in Gas Exchange
Structure
- Type I Alveolar Cells: Thin cells that allow rapid diffusion of gases.
- Type II Alveolar Cells: Secrete surfactant, reducing surface tension and preventing alveolar collapse.
- Alveolar Macrophages: These cells ingest foreign particles, keeping the alveolar surface clean.
Function
- Ventilation-Perfusion Matching: The blood flow and airflow must be appropriately matched for efficient gas exchange.
- Surfactant Role: By reducing surface tension, surfactant ensures that alveoli remain open, particularly during exhalation.
FAQ
Emphysema damages the alveolar walls, reducing the surface area available for gas exchange. This leads to less efficient diffusion of oxygen and carbon dioxide. The destruction of alveolar walls also compromises the elasticity of the lung tissue, making it more difficult for the lungs to expand and contract. As a result, individuals with emphysema may struggle to breathe and have reduced oxygenation of the blood.
Surfactant is a lipid-rich substance that reduces surface tension within the alveoli. This prevents the alveoli from collapsing upon expiration, ensuring they remain open and capable of gas exchange. By reducing surface tension, surfactant allows the alveoli to expand more easily during inspiration, promoting efficient oxygenation of the blood.
At higher altitudes, the atmospheric pressure is lower, which reduces the partial pressure of oxygen in the air. Consequently, the concentration gradient for oxygen between the alveoli and the blood is decreased. This leads to a reduced rate of oxygen diffusion into the blood, which may cause symptoms of altitude sickness, such as shortness of breath.
The chloride shift is a process where chloride ions move into red blood cells as bicarbonate ions move out, maintaining electrical neutrality. Carbon dioxide is converted into bicarbonate in red blood cells, and as bicarbonate ions leave the cells, chloride ions enter to balance the charge. This shift facilitates the transport of carbon dioxide in the blood.
The alveolar walls are very thin, usually just one cell layer thick, to facilitate efficient gas exchange. A thinner barrier decreases the distance that gases must travel between the alveoli and the blood, allowing rapid diffusion. If the walls were thicker, the rate of diffusion would decrease, hindering the exchange of oxygen and carbon dioxide, and thereby affecting the overall function of the respiratory system.
Practice Questions
Oxygen and carbon dioxide exchange in the alveoli is facilitated by simple diffusion across concentration gradients. Oxygen diffuses from the alveoli, where its partial pressure (P_O2) is higher, into the blood, while carbon dioxide diffuses from the blood into the alveoli, where its partial pressure (P_CO2) is lower. The thin alveolar walls and large surface area allow this diffusion to occur efficiently. Partial pressure gradients act as driving forces for the diffusion process, allowing oxygen and carbon dioxide to move down their respective concentration gradients.
Haemoglobin plays a crucial role in oxygen transport by binding oxygen molecules to form oxyhemoglobin. Each haemoglobin can carry up to four oxygen molecules, and the binding of one oxygen molecule enhances the affinity for additional oxygen molecules (cooperative binding). In tissues where P_O2 is low, oxygen is released. The Bohr Effect further influences oxygen release when the carbon dioxide level is high or the pH is low. Under these conditions, more oxygen is released from haemoglobin, allowing more oxygen to be delivered to metabolically active tissues. This ensures that oxygen supply is matched to tissue demand.
