Haemoglobin, the red blood protein responsible for oxygen transport, binds about \(1.34 \mathrm{~cm}^{3}\) of oxygen per gram. Normal blood has a haemoglobin concentration of \(150 \mathrm{~g} \mathrm{dm}^{-3}\). Haemoglobin in the lungs is about 97 per cent saturated with oxygen, but in the capillary is only about 75 per cent saturated. What volume of oxygen is given up by \(100 \mathrm{~cm}^{3}\) of blood flowing from the lungs in the capillary?

Understanding haemoglobin saturation is critical for comprehending how our blood carries oxygen from the lungs to other parts of the body. Haemoglobin is a complex protein in red blood cells that has the unique capability to bind to oxygen. The term 'saturation' refers to the percentage of haemoglobin molecules in the blood that are bound to oxygen at any given time.

When blood flows through the lungs, haemoglobin picks up oxygen and becomes saturated — ideally to 100%. However, because of physiological factors and other variables, haemoglobin is often less than fully saturated. For example, normal saturation levels in the lungs are approximately 97%, which is considered sufficient for healthy oxygen delivery. Meanwhile, in the capillary, where oxygen is released to the tissues, saturation can drop to 75%. These saturation levels are key indicators of the body's oxygenation status and are used to calculate the oxygen content that blood can carry and release as it travels through the body.

The haemoglobin saturation curve, a graphical representation showing the relationship between oxygen tension (partial pressure) and haemoglobin saturation, is essential for understanding how haemoglobin's affinity for oxygen changes with varying physiological conditions, such as pH levels, carbon dioxide concentrations, and temperature.

The oxygen binding capacity of haemoglobin is an essential concept in understanding how oxygen is transported in the blood. Each haemoglobin molecule can carry up to four oxygen molecules, one on each of its iron-containing heme groups. The oxygen binding capacity is determined by the amount of haemoglobin in the blood and its ability to bind oxygen.

In exercise terms, haemoglobin can bind approximately 1.34 ml of oxygen per gram. Given that normal blood has a concentration of haemoglobin at 150 g/dm³, we can use this information to calculate the total volume of oxygen that can be carried by the blood when fully saturated. A higher haemoglobin concentration means a greater capacity for carrying oxygen, which is critical during periods of increased demand, such as during exercise or at high altitudes.

Various factors can impact oxygen binding capacity, including the partial pressure of oxygen, levels of carbon dioxide, blood pH, and temperature. For example, a decrease in pH (acidosis) or an increase in temperature usually results in haemoglobin releasing more oxygen to the tissues, a property known as the Bohr effect.

Calculating blood oxygen content involves understanding how much oxygen is actually being carried by the blood at any given moment. This calculation is vital for assessing the efficiency of oxygen transport and detecting potential issues such as hypoxia, where tissues do not receive enough oxygen.

To calculate the oxygen content in a volume of blood, we need to know the haemoglobin concentration, the oxygen binding capacity of haemoglobin, and the current saturation level. Using the example from the exercise, by multiplying the haemoglobin concentration (after converting it to the appropriate units) with the oxygen binding capacity and the saturation level, we can determine the volume of oxygen carried in a specific amount of blood, be it 100 cm³ or any other volume.

For instance, if 100 cm³ of blood transitions from 97% saturation in the lungs to 75% saturation in peripheral capillaries, the oxygen content can be calculated at each saturation level. The difference between these two values will give the volume of oxygen delivered to the tissues. This calculation is significant in medical settings to ensure that a patient is receiving adequate oxygen and to monitor how well their body is utilizing it.