Oxygen transportation

Oxygen is essential for humans to maintain life-sustaining aerobic respiration, and it is arguably the most commonly administered drug in the fields of anaesthesia and critical care medicine. During oxidative phosphorylation, which occurs within the mitochondrial inner membrane, oxygen serves as a terminal electron acceptor at the end of the electron transport chain, resulting in the synthesis of adenosine triphosphate (ATP), the coenzyme that provides energy to all active metabolic processes.

In terms of human physiology, oxygen transport can be divided into two categories: that which occurs through convection and that which occurs through diffusion. Convection is a term used to describe the movement of oxygen within the circulation that occurs as a result of bulk transport in this context. This is an active process that necessitates the expenditure of energy, which in this case comes from the pumping of the heart. However, diffusion describes the passive movement of oxygen down a concentration gradient, such as from the microcirculation into the tissues, whereas absorption describes the active movement of oxygen (and ultimately the mitochondria).

Haemoglobin is a protein which is found in red blood cells that consists of four subunits: two alpha subunits and two beta subunits.  Each subunit has a haem group in the centre that contains iron and binds one oxygen molecule.  This means that each haemoglobin molecule can be able to bind with four oxygen molecules, forming oxyhaemoglobin.  Haemoglobin molecules with a greater number of oxygen molecules bound are brighter red, hence why oxygenated arterial blood is brighter red and deoxygenated venous blood is darker red.

Upon entering the bloodstream from the lungs, oxygen is taken up by haemoglobin (Hb), which can be found in the red blood cells.

Oxygen binding to haemoglobin:

The shape of haemoglobin is determined by the number of oxygen molecules that are bound to it. The change in shape also results in a change in the affinity of the compound for oxygen. In proportion to the increase in the number of oxygen molecules bound to haemoglobin, the affinity of haemoglobin for oxygen increases. 

It is known as the Tense State (T-state) of haemoglobin when no oxygen is bound to it and the haemoglobin has a low affinity for oxygen. The haemoglobin changes its shape at the point where oxygen first binds to it, resulting in the Relaxed State (R-state), which has a higher affinity for oxygen. This change can be represented graphically on a graph of oxygen saturation versus partial pressure of oxygen.

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Oxygen delivery at tissues:

It is shown in the diagram above that the percentage of oxygen bound to haemoglobin is proportional to the partial pressure of oxygen (po2) at a particular site. The dissociation of oxyhaemoglobin into oxygen and haemoglobin occurs when it reaches a tissue that has a low partial pressure of oxygen (e.g. skeletal muscle), resulting in an increase in local partial pressure of oxygen. In contrast, when it reaches a tissue with a high partial pressure of oxygen (for example, the pulmonary circulation), haemoglobin will continue to take up more oxygen, resulting in a lower partial pressure of oxygen.

Factors affecting oxygen affinity:

The affinity of haemoglobin for oxygen can be affected by a variety of factors, including:

• pH/pCO2: When the concentration of H+/pCO2 rises and the pH falls, haemoglobin enters the T state and its affinity for oxygen decreases. The Bohr effect is the name given to this phenomenon. However, when H+/pCO2 decreases and pH increases, the affinity of haemoglobin for oxygen increases in the opposite direction.
• 2,3-diphosphoglycerate (2,3-DPG): 2,3-DPG, also known as 2,3-BPG, is a chemical found in red blood cells that is produced as a byproduct of the glucose metabolic pathway. Because 2,3-DPG binds to the beta chains of haemoglobin, increased 2,3-DPG levels result in increased binding of 2,3-DPG to haemoglobin, resulting in a decrease in the affinity of haemoglobin for oxygen, which is harmful. Inversely, when 2,3-DPG levels are decreased, as is the case when tissue metabolism is reduced, there are fewer 2,3-DPG molecules binding to haemoglobin, resulting in a higher affinity for oxygen due to the increased number of opportunities for it to bind.
• Temperature:  When temperatures are raised, as they are in active muscles, there is an increase in heat production, which reduces the affinity of haemoglobin for oxygen. The production of heat is reduced at lower temperatures, for example, when tissue metabolism is reduced, and the affinity of haemoglobin for oxygen is increased when temperatures are lowered.

When haemoglobin has a high affinity for oxygen, the oxyhaemoglobin dissociation curve shifts as a result. When the oxygen affinity increases, the curve shifts to the left, whereas when the oxygen affinity decreases, the curve shifts to the right.

Conclusion:

The transport of oxygen is important for aerobic respiration to occur. The transport of oxygen throughout the human body occurs through both convection and diffusion mechanisms. In terms of human physiology, oxygen transport can be divided into two categories: that which occurs through convection and that which occurs through diffusion.

Haemoglobin is a protein found in red blood cells that is composed of four subunits: two alpha subunits and two beta subunits.The shape of haemoglobin is determined by the number of oxygen molecules that are bound to it.