Option D6 Gas Transport Flashcards
Type 1 and Type 2 Pneumocytes
- Type I pneumocytes are very thin in order to mediate gas exchange with the bloodstream (via diffusion)
- Type II pneumocytes secrete a pulmonary surfactant in order to reduce the surface tension within the alveoli
Capillaries
- The capillaries are located close to the pneumocytes and are composed of a very thin, single-layer endothelium
- The capillaries transport oxygen within red blood cells, while white blood cells may extravasate into the lung tissue
Haemoglobin
composed of four polypeptide chains, each with an iron-containing heme group that reversibly binds oxygen
O2 molecule binds to haemoglobin
- This means haemoglobin will have a higher affinity for O2 in oxygen-rich areas (like the lung), promoting oxygen loading
- Conversely, haemoglobin will have a lower affinity for O2 in oxygen-starved areas (like muscles), promoting oxygen unloading
Oxygen dissociation curve
show the relationship between oxygen levels (as partial pressure) and haemoglobin saturation
Adult haemoglobin
- The oxygen dissociation curve for adult haemoglobin is sigmoidal (i.e. S-shaped) due to cooperative binding
- There is a low saturation of haemoglobin when oxygen levels are low (haemoglobin releases O2 in hypoxic tissues)
- There is a high saturation of haemoglobin when oxygen levels are high (haemoglobin binds O2 in oxygen-rich tissues)
Fetal haemoglobin
- The haemoglobin of the fetus has a slightly different molecular composition to adult haemoglobin
- Consequently, it has a higher affinity for oxygen (dissociation curve is shifted to the left)
- This is important as it means fetal haemoglobin will load oxygen when adult haemoglobin is unloading it (i.e. in the placenta)
- Following birth, fetal haemoglobin is almost completely replaced by adult haemoglobin (~ 6 months post-natally)
- Fetal haemoglobin production can be pharmacologically induced in adults to treat diseases such as sickle cell anaemia
Myoglobin
- Myoglobin is an oxygen-binding molecule that is found in skeletal muscle tissue
- It is made of a single polypeptide with only one heme group and hence is not capable of cooperative binding
- Consequently, the oxygen dissociation curve for myoglobin is not sigmoidal (it is logarithmic)
- Myoglobin has a higher affinity for oxygen than adult haemoglobin and becomes saturated at lower oxygen levels
- Myoglobin will hold onto its oxygen supply until levels in the muscles are very low (e.g. during intense physical activity)
- The delayed release of oxygen helps to slow the onset of anaerobic respiration and lactic acid formation during exercise
Three mechanisms where carbon dioxide is transported between lungs and tissues
- Some is bound to haemoglobin to form HbCO2 (carbon dioxide binds to the globin and so doesn’t compete with O2 binding)
- A very small fraction gets dissolved in water and is carried in solution (~5% – carbon dioxide dissolves poorly in water)
- The majority (~75%) diffuses into the erythrocyte and gets converted into carbonic acid
Transport as Carbonic Acid
- When CO2 enters the erythrocyte, it combines with water to form carbonic acid (reaction catalysed by carbonic anhydrase)
- The carbonic acid (H2CO3) then dissociates to form hydrogen ions (H+) and bicarbonate (HCO3–)
- Bicarbonate is pumped out of the cell in exchange with chloride ions (exchange ensures the erythrocyte remains uncharged)
- The bicarbonate in the blood plasma combines with sodium to form sodium bicarbonate (NaHCO3), which travels to the lungs
- The hydrogen ions within the erythrocyte make the environment less alkaline, causing haemoglobin to release its oxygen
- The haemoglobin absorbs the H+ ions and acts as a buffer to maintain the intracellular pH
- When the red blood cell reaches the lungs, bicarbonate is pumped back into the cell and the entire process is reversed
Carbonic acid
Aqueous carbon dioxide can combine with water in the blood plasma to form carbonic acid.
- Carbonic acid may then lose protons (H+) to form bicarbonate (HCO3–) or carbonate (CO32–)
- The released hydrogen ions will function to lower the pH of the solution, making the blood plasma less alkaline
Chemoreceptors in balancing blood pH
Chemoreceptors are sensitive to changes in blood pH, and can trigger body responses in order to maintain a balance:
- Lungs regulate the amount of carbon dioxide in the bloodstream by changing the rate of ventilation
- Kidneys control the reabsorption of bicarbonate ions from the filtrate and clear any excess in the urine
Blood pH
A narrow tolerance range of 7.35 to 7.45 to avoid the onset of disease.
- Maintained by plasma proteins that act as buffers
Buffering solution
Resists changes to pH by removing excess H+ ions (increasing acidity) or OH- ions (increasing alkalinity).
- Amino acids are zwitterions, meaning they have positive and negative charges, and are able to buffer pH changes
- Amine groups in amino acids can take on H+ ions
- Carboxyl group in amino acids can release H+ ions (forms water with OH- ions)
Oxyhemoglobin dissociation curve
Demonstrates the saturation of hemoglobin by oxygen under normal conditions.
- pH changes alter the affinity of hemoglobin for oxygen, consequently altering the uptake and release of oxygen of hemoglobin
Bohr effect
A decrease in pH shifts the oxygen dissociation curve to the right.
- Carbon dioxide lowers blood pH by forming carbonic acid, causing hemoglobin to release oxygen
- Cells with increased metabolism (ex. respiring tissues) release larger amounts of carbon dioxide, resulting in the promotion of hemoglobin to release oxygen
Respiratory control center
Located in the medulla oblongata, it responds to stimuli from chemoreceptors in order to control ventilation.
- Central chemoreceptors (medulla oblongata): detect changes in carbon dioxide levels
- Peripheral chemoreceptors (carotid and aortic bodies): detect carbon dioxide and oxygen levels as well as blood pH
Respiratory control center during exercise
Metabolism increases during exercise, resulting in a buildup of carbon dioxide and a reduction in the supply of oxygen.
- Changes are detected by chemoreceptors and impulses are sent to the respiratory control center
- Signals are sent to the diaphragm and intercostal muscles to increase the rate of ventilation (involuntary)
- As ventilation rate increases, carbon dioxide levels in the blood drop and oxygen levels to rise, restoring blood pH
- Long term effects: improved vital capacity
Partial pressure
The pressure exerted by a single type of gas when found within a mixture of gases.
- Determined by the concentration of gases within the mixture and total pressure of the mixture
Respiration at high altitudes
Lower air pressure and lower partial pressure of oxygen causes difficulty for hemoglobin to uptake and transport oxygen.
- Respiring tissues consequently receive less oxygen, leading to symptoms such as fatigue, headaches, and a rapid pulse
Acclimatization to high altitudes
- RBC production will increase in order to maximize oxygen uptake and transport
- RBCs will have a higher hemoglobin count with a higher affinity for oxygen
- Vital capacity will increase to improve rate of gas exchange
- Muscles will produce more myoglobin and have increased vascularization to improve overall oxygen supply
- Kidneys will begin to secrete alkaline urine (improves buffering of blood pH)
- People living permanently at high altitudes will have a greater lung surface area and larger chest sizes
- Athletes can incorporate high altitude training to adopt benefits prior to competition
Emphysema
A lung condition whereby the walls of the alveoli lose their elasticity due to damage to the alveolar walls.
- Loss of elasticity results in the abnormal enlargement of alveoli, leading to lower surface areas for gas exchange
- Degradation of alveolar walls can cause holes to develop, merging alveoli into big air sacs (pulmonary bullae)
Causes of emphysema
- Smoking (chemical irritants damage lung tissue and produce the enzyme elastase, which breaks down elastic fibers in the alveolar wall)
- Hereditary deficiency in the elastase inhibitor (small proportion)
Treatments for emphysema
Currently no cure, but treatments can relieve symptoms and delay progression.
- Changes in lifestyle (stop smoking!)
- Bronchodilators: relax bronchiolar muscles and improve airflow
- Corticosteroids: reduce inflammatory responses
- Elastase activity can be blocked by an enzyme inhibitor if concentrations are not overly high
- Oxygen supplementation: ensures adequate oxygen intake
- Surgery and alternative medicines
