Test: Exchange & Transport of Gases (NCERT) - NEET MCQ

Alveoli are tiny air sacs in the lungs where the exchange of gases, such as oxygen and carbon dioxide, occurs. This exchange is vital for respiration, and alveoli serve as the primary sites for this process.

Hence, Option A is correct.

The Hamburger phenomenon, also known as the chloride shift, is a process that occurs during the transport of carbon dioxide (CO₂) in the blood.

• CO₂ Transport: When CO₂ is produced as a waste product in tissues, it diffuses into the bloodstream. Within red blood cells, CO₂ reacts with water to form carbonic acid (H₂CO₃), which rapidly dissociates to form bicarbonate ions (HCO₃^-) and hydrogen ions (H⁺).
• Bicarbonate Diffusion: The bicarbonate ions then diffuse from the red blood cells into the plasma. This movement of bicarbonate ions from the cells to the plasma creates a temporary imbalance in the ionic balance.
• Chloride Shift: To maintain the ionic balance, Cl^- ions present in the plasma diffuse into the erythrocytes. This exchange of bicarbonate ions with chloride ions is what is referred to as the Hamburger phenomenon or the chloride shift.
• Restoration of Ionic Balance: The movement of Cl^- ions into the erythrocytes from plasma helps to maintain the electrical neutrality of the blood cells and plasma. This allows the efficient transport of CO₂ from tissues to the lungs, where it is expelled from the body.

High altitudes have an effect on the human body, particularly on the oxygen-carrying capacity of the blood. This is due to the change in atmospheric pressure and oxygen concentration. Here are the details:

• In response to lower oxygen availability, the body compensates by producing more red blood cells (RBCs). This is to ensure that there is enough hemoglobin, the molecule in RBCs which binds to oxygen, to absorb the required amount of oxygen for the body to function properly.

• The term "Partial Pressure" refers to the pressure that an individual gas would exert if it occupied the total volume of the mixture on its own.
• In a mixture of gases, each gas contributes to the total pressure of the mixture. This contribution by an individual gas is known as its partial pressure.
• This concept is based on Dalton's law of partial pressures, which states that the total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture.

For example, if air is made up of nitrogen, oxygen, and other trace gases, then the atmospheric pressure (also known as barometric pressure) is made up of the partial pressures of nitrogen, oxygen, and the other trace gases.

Solubility refers to the maximum amount of a substance (solute) that can dissolve in a given amount of solvent at a particular temperature. In physiology, the solvent is usually water (H₂O) inside the body. CO₂ is about 20 times more soluble in water than O₂. Consequently, for any given partial pressure, more CO₂ molecules will be dissolved in blood and other body fluids than O₂ molecules.

In conclusion, the higher solubility of CO₂ in water compared to O₂ makes its diffusion more efficient in the body.

The iron atom at the center of the haem group of hemoglobin is able to bind O₂, enabling the Hb molecule to carry O₂, from lungs, where PO₂, is high to tissue where PO₂, is less. Carbonic anhydrase, an enzyme present in RBCs, catalyses the reaction between CO₂, and water to form carbonic acid which subsequently dissociates.

This reaction facilitates the transfer of CO₂, from the tissues to the blood and from the blood to the alveoli of the lungs.

The diffusion membrane of the lungs is comprised of several layers that facilitate the exchange of gases. These layers include:

Squamous Epithelium

• This is the thin layer of cells that lines the interior surface of the alveoli (tiny air sacs in the lungs). Squamous epithelium cells are very flat, which allows for efficient gas exchange between the alveoli and the blood in the capillaries.

Basement Membrane

• The basement membrane is a layer of extracellular matrix that anchors the epithelial tissue to its underlying connective tissue. It provides support and also acts as a barrier, regulating the movement of molecules and cells between the tissue above and the tissue below.

In the lungs, the squamous epithelium and the basement membrane together form a thin barrier. This barrier is semi-permeable, allowing oxygen and carbon dioxide to pass through during respiration. The oxygen moves from the air in the alveoli into the blood in the capillaries (oxygenation), and carbon dioxide moves from the blood into the alveoli (decarbonization), from where it is expelled from the body during exhalation. This is why options A, C, and D are incorrect, as they do not accurately describe the layers of the lung's diffusion membrane.

Concentration or partial pressure of CO₂(PCO₂ ) is 40 mm Hg in alveolar air and 32 mm Hg in expired air.

• The alveoli are tiny air sacs in the lungs where the exchange of oxygen and carbon dioxide takes place. When we breathe in, oxygen from the air we inhale passes into the bloodstream at the alveoli and carbon dioxide, a waste product of metabolism, transfers from the blood to the alveoli, from where it is expelled from the body when we exhale. This makes the concentration of CO₂ in alveolar air high.
• Expired air, also known as exhaled air, is the air we breathe out. It contains less oxygen and more carbon dioxide than the air we inhale, but not as much as alveolar air. This is because some of the carbon dioxide in the alveolar air is retained in the body to maintain a certain level of carbon dioxide in the blood and tissues, which is necessary for the body's pH balance.
• Inspired air is the air we breathe in. It contains more oxygen and less carbon dioxide than both alveolar air and expired air. This is because the atmosphere has a much lower concentration of carbon dioxide than the body.

Therefore, there is more CO₂ in alveolar air than in expired air, making option A correct. The CO₂ concentration in inspired air is the lowest, making options C and D incorrect.

Nearly 20-25% of CO₂ is transported by RBCs whereas 70% of it is carried as bicarbonate in plasma. About 7% of CO₂ is carried in a dissolved state through plasma. About 97% of O₂ is carried by RBCs in the blood. About 3% of O₂ is transported in a dissolved state through the plasma.

Excess CO₂ mainly stimulates the respiratory centre of the brain and increase the inspiratory and expiratory signal to the respiratory muscles. O₂ does not have a significant direct effect on the respiratory centre of the brain in controlling respiration.

Response To Rising PCO₂

• When PCO₂ levels in the blood rise, it means there is an accumulation of CO₂, the waste product of cellular respiration.
• The chemoreceptors detect this increase and send signals to the respiratory center in the medulla oblongata.
• The medulla oblongata then stimulates the muscles involved in breathing to increase the rate and depth of respiration. This is done to expel the excess CO₂ and restore the balance of gases in the blood.

Why Not Rising PO₂ or Falling PCO₂/PO₂?

• Falling levels of PO₂ (partial pressure of oxygen) can also stimulate breathing but this usually happens only when oxygen levels drop significantly, such as in high altitude conditions. At normal conditions, the primary drive for breathing is rising PCO₂, not falling PO₂.
• Falling PCO₂ or PO₂ would generally reduce the urge to inhale, not increase it. This is because a drop in these gases would indicate that the body is not in urgent need of oxygen or removal of CO₂.

The concentration gradient for oxygen in the human body exists from the alveoli to the tissues. This is due to the process of respiration, where oxygen is taken in and carbon dioxide is expelled.

• Alveoli to Blood: Oxygen enters the body through the respiratory system, specifically the lungs. It is in the alveoli, tiny air sacs within the lungs, where oxygen is transferred from the inhaled air into the bloodstream. This is due to the concentration gradient where oxygen is higher in the alveoli and lower in the deoxygenated blood arriving via the pulmonary arteries. This process is known as diffusion.
• Blood to Tissues: Oxygenated blood is then transported from the lungs to the heart, which pumps it around the body through the circulatory system. When the oxygen-rich blood reaches the body’s tissues and cells, another gradient is present. The oxygen concentration is higher in the blood than in the body's tissues, so oxygen diffuses from the blood into the cells where it is needed for metabolic processes.

As a result, the overall concentration gradient for oxygen in the body goes from the alveoli (where oxygen concentration is high) to the tissues (where oxygen concentration is lower).

The alveolar diffusion rate is influenced by several factors, but the reactivity of gases is not one of them. Let's discuss each option in detail:

A: Solubility of Gases
The solubility of gases plays a crucial role in the rate of alveolar diffusion. Gases that dissolve easily in liquid diffuse faster through the alveoli into the bloodstream. For instance, carbon dioxide is approximately 20 times more soluble in blood than oxygen, meaning it diffuses more rapidly.

B: Thickness of the Membranes
The thickness of the alveolar membrane also impacts the diffusion rate. The thinner the membrane, the faster the diffusion rate. This is because gases have to travel a shorter distance across a thinner membrane. Diseases like pneumonia and pulmonary edema can thicken the membrane, slowing down diffusion.

C: Pressure Gradient
The pressure gradient is another significant factor. Gases move from areas of high pressure to areas of low pressure. In the lungs, oxygen moves from the alveoli (high pressure) to the blood (low pressure), while carbon dioxide moves in the opposite direction. A steeper pressure gradient results in faster diffusion.

D: Reactivity of the Gases
The reactivity of gases does not affect the alveolar diffusion rate. Diffusion is a physical process, not a chemical one. It relies on concentration gradients and physical properties of gases like solubility and size rather than their chemical properties or reactivity. So, whether a gas is reactive or inert has no impact on its rate of diffusion across the alveolar membrane.

So, the correct answer is D: reactivity of the gases.

Respiration is a metabolic process that all living organisms perform to obtain energy by combining oxygen and food molecules, resulting in the release of waste products, heat, and ATP (adenosine triphosphate), the energy currency of the cell. However, utilization of CO₂ by cells for catabolic reactions is not a part of the respiration process in humans. Here's why:

It's important to remember that while CO₂ plays a significant role in the respiratory and circulatory systems, its main function is not to be used in catabolic reactions, but rather to be expelled from the body as a waste product.

• CO₂ is a Byproduct of Respiration: Carbon dioxide (CO₂) is a waste product produced during the process of cellular respiration.
• CO₂ Transport: Carbon dioxide does not play a direct role in the transport of gas.
• Photosynthesis vs. Respiration: The utilization of CO₂ is a characteristic of plants, not humans. Plants use carbon dioxide in the process of photosynthesis to produce glucose and release oxygen.

The part ′B′ in the graph represents partial pressure of oxygen.

The Oxygen dissociation curve represents the relationship between the partial pressure of oxygen (usually on the X-axis) and the percentage of saturation of Hemoglobin (usually on the Y-axis).

About 20−25% of CO₂ is carried by Hb as carbamino-haemoglobin. CO₂ reacts directly with amine radicals (NH₂) of Hb of form an unstable compound called carbaminohaemoglobin which releases CO₂ in the lungs.

 About 70% of CO₂ is transported by plasma in the form of bicarbonate ions. When CO₂ enters into the RBCs combines with water, forming carbonic acid (H₂CO₃). H₂CO₃​ is unstable and quickly dissociates into hydrogen ions and  bicarbonate ions. All these reactions are catalyse by the enzym carbonic anhydrase present in RBCs.

The relationship between the partial pressure of O₂ (PO₂) and percentage saturation of the Hb with O₂ is graphically illustrated by a curve called oxygen dissociation curve. The haemoglobin is most saturated with oxygen in the pulmonary vein, as this vein is carrying oxygenated blood from the Iungs towards the left auricle of the heart. From the left auricle the blood moves to the left ventricle where saturation of Hb with O₂ slightly reduces. Vena cava carries deoxygenated blood from all the organs of the body towards right auricle, thus, Hb is less saturated with O₂.

Expiration is normally brought about by the relaxation of inspiratory muscles. Oxyhaemoglobin can hold much less CO₂ in the form of carbaminohemoglobin than what oxyhaemoglobin can. Aperson cannot expel all the air from the lungs even after forceful expiration. The collie of air which remains in the lungs after the most forceful expiration is called residual volume. It is about 1100 L to 1200 mL. A rise in pCO₂ decrease the O₂ affinity of hemoglobin.

Vital capacity is the maximum volume of air a person can breathe in after a forced expiration or the maximum  of air a person can breathe out after a forced expiration. piston. During gaseous exchange the gasses diffuse from high partial pressure to low partial pressure. About 20-25% CO2 is carried by hemoglobin as carbaminohemoglobin. Earthworms respire through the body wall.

Carbon monoxide comobines with Hb far more readily than O₂ (CO has about 200 times greater affinity for Hb as compared to O₂), forming a relativelly stable compound carboxyhaemoglobin. This cause low supply of O₂ to the body cells leading to headache, nausea, dizziness, paralysis and even death.

Hb contains a protein portion called globin and a pigment portion called heam. The heam portion consists of four atoms of iron, each capable of combining with a molecule of O₂, Thus, one Hb carries 4 molecules of O₂

Fetal haemoglobin is the main oxygen transport carrier in human fetus during the last seven months of development in the uterus and persists in the newborn until it is about 6 months old. Functionally, fetal haemoglobin has a higher affinity to bind with oxygen molecules than the adult (or maternal) haemoglobin, giving the developing fetus better access to oxygen from the mother's blood stream.

The exchange of gases in the alveoli of the lungs takes place by simple diffusion. The exchange of gases between the alveoli and blood in the lung is the result of difference in partial pressure of respiratory gases.

The binding of carbon dioxide (CO₂) with haemoglobin is influenced by several factors. However, the primary factor is the partial pressure of carbon dioxide (pCO₂).

Partial Pressure of Carbon Dioxide (pCO₂)

• The partial pressure of carbon dioxide (pCO₂) in the blood is the primary factor influencing the binding of carbon dioxide with haemoglobin.
• The higher the pCO₂, the greater the amount of carbon dioxide that will bind with haemoglobin. Conversely, the lower the pCO₂, the less carbon dioxide will bind to haemoglobin.
• This is due to the fact that haemoglobin has a higher affinity for oxygen than it does for carbon dioxide. Thus, when the pCO₂ is high, more carbon dioxide molecules will be available to bind with haemoglobin.
• On the other hand, when the pCO₂ is low, fewer carbon dioxide molecules will be available, and haemoglobin will preferentially bind with oxygen instead.

Carbonic anhydrase is an enzyme found in red blood cells and other tissues in the body. It plays a crucial role in the rapid interconversion of carbon dioxide and water into carbonic acid, protons, and bicarbonate ions. This process is vital for the transportation of carbon dioxide from the tissues to the lungs.

When temperature decreases, oxy-Hb curve will become more steep. The steep rise of the curve indicates high affinity of Hb for O₂.

When the temperature decreases, the oxy-Hb curve becomes more steep. Here's why:

• Shift to the left
• Increase in Oxygen Affinity
• Steeper Curve

It's important to note that while this increased oxygen affinity might seem beneficial, it can also lead to problems. If the hemoglobin holds onto the oxygen too tightly and doesn't release it to the tissues that need it, it can lead to hypoxia, or a deficiency in the amount of oxygen reaching the tissues.

Charcoal on burning produces carbon monoxide (CO). CO has about 200 times more affinity for Hb than O₂. On combining with Hb, it forms a stable compound carboxyhaemoglobin. Because of this compound, Hb cannot carry sufficient O₂ to the tissues ultimately leading to death.

The particle pressure of O₂ in the expired air is 116mm Hg. In alveolar air it is 104mm Hg, in arterial blood it is 95mm Hg and in venous blood it is 40mm Hg.

• After taking a deep breath, the concentration of CO₂ in our blood decreases because we have expelled a large amount of it from our bodies.
• The urge to breathe is not primarily triggered by the need for oxygen, but rather by the presence of carbon dioxide in our bloodstream.
• Therefore, when the level of CO₂ in our blood is low, the urge to breathe is also reduced, allowing us to hold our breath for a few seconds without discomfort.

This is why the correct answer is C: Less CO₂ in blood.
However, it's important to note that this is a temporary state. Our cells continuously produce CO₂ as a byproduct of metabolism, so the CO₂ levels in our blood will soon rise again, prompting us to resume normal breathing.

The concentration gradient for carbon dioxide (CO₂) in the body exists from tissues to alveoli. This is due to the fact that:

• Cellular Respiration: During cellular respiration in the body's tissues, CO₂ is produced as a waste product. This process results in higher levels of CO₂ in the tissues compared to the blood.
• Blood Transport: The blood picks up the CO₂ from the tissues and transports it towards the lungs. This leads to a concentration gradient where the CO₂ concentration is higher in the blood near the tissues, and lower in the blood near the lungs.
• Gas Exchange: In the alveoli of the lungs, gas exchange occurs. The blood releases the CO₂, and it moves across the concentration gradient into the alveoli. This is because the concentration of CO₂ is higher in the blood than in the alveoli, causing CO₂ to diffuse from an area of high concentration (blood) to an area of low concentration (alveoli).
• Exhalation: The CO₂ in the alveoli is then exhaled out of the body, maintaining the concentration gradient that drives this process of gas exchange.