14.4.1. Transport of Oxygen

Haemoglobin is a red coloured iron containing pigment present in the RBCs. O₂ can bind with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of O₂.

Haemoglobin, found in red blood cells (RBCs), is crucial for oxygen transport. It can bind oxygen molecules to form oxyhaemoglobin. This binding is reversible, meaning that haemoglobin can release the oxygen when needed by the body's tissues. Each haemoglobin molecule has the capability to bind four oxygen molecules, enabling efficient oxygen transport.

Think of haemoglobin as a delivery truck that carries oxygen to neighborhoods (tissues) in the city (the body). Just like a truck can load and unload packages, haemoglobin can pick up oxygen in the lungs and drop it off where it's needed, delivering four packages (oxygen molecules) at once.

Binding of oxygen with haemoglobin is primarily related to partial pressure of O₂. Partial pressure of CO₂, hydrogen ion concentration and temperature are the other factors which can interfere with this binding.

The amount of oxygen that binds to haemoglobin depends largely on the partial pressure of oxygen (pO₂). Higher pO₂ levels lead to increased oxygen binding. Other factors, such as the level of carbon dioxide (pCO₂), acidity (hydrogen ion concentration), and temperature, can also affect this binding. For instance, if CO₂ levels are high, it can lead to a release of oxygen from haemoglobin.

Consider a sponge that absorbs water. The sponge absorbs more water when it’s dry (high pO₂) and releases it when it’s wet (high CO₂ levels). Similarly, haemoglobin picks up oxygen when conditions are right in the lungs and releases it in the tissues where CO₂ levels are high.

A sigmoid curve is obtained when percentage saturation of haemoglobin with O₂ is plotted against the pO₂. This curve is called the Oxygen dissociation curve.

The relationship between the percentage of hemoglobin saturated with oxygen and the partial pressure of oxygen is illustrated by a sigmoid (S-shaped) curve known as the oxygen dissociation curve. This curve is important for understanding how easily hemoglobin can bind or release oxygen under varying conditions.

Imagine climbing a mountain. As you ascend, the air gets thinner (lower pO₂) and you find it harder to breathe (lower saturation of oxygen). The sigmoid curve reflects this experience – at certain heights, even with less oxygen, your body can still function well at lower saturation levels due to the curve's shape.

In the alveoli, where there is high pO₂, low pCO₂, lesser H+ concentration, and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin, whereas in the tissues, where low pO₂, high pCO₂, high H+ concentration, and higher temperature exist, the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin.

In the lungs (alveoli), the conditions are optimal for hemoglobin to pick up oxygen (high pO₂ and low pCO₂). In contrast, in body tissues, where oxygen is utilized, conditions change—pO₂ drops while pCO₂ increases and acidity rises. These changes signal hemoglobin to release oxygen so it can be used in cellular respiration.

Think of a restaurant where fresh food is delivered. In the 'kitchen' (lungs), there are plenty of ingredients (oxygen) to cook with, so the chefs (hemoglobin) are happy to take in as much as they can. Once the meal is prepared and served (in the body), the kitchen is busy cooking and uses up the ingredients, forcing the chefs to let go of the food they were holding onto.

O₂ gets bound to haemoglobin in the lung surface and gets dissociated at the tissues. Every 100 ml of oxygenated blood can deliver around 5 ml of O₂ to the tissues under normal physiological conditions.

Hemoglobin binds oxygen in the lungs (alveoli) where oxygen levels are high. As blood circulates and reaches tissues where oxygen is needed, the differing conditions encourage hemoglobin to release the oxygen. On average, 100 ml of oxygen-rich blood delivers about 5 ml of oxygen to active tissues, supporting their metabolic needs.

Imagine a delivery service where each truck carries 5 packages of fresh produce (oxygen). As the truck reaches various neighborhoods (tissues), it drops off the packages (oxygen) that the residents need, ensuring that everyone gets the nourishment they require.

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