Profile
Settings
Refer your friends
Sign out
Generally, the gaseous exchange occurs in the alveoli of the lungs. Oxygen and carbon dioxide play a vital role in the exchange of gases. Oxygen enters the bloodstream through capillaries by diffusion, where oxygen transport takes place to the tissues via plasma and red blood cells (RBCs). Conversely, carbon dioxide diffuses out of the bloodstream and into the alveoli, expelled out from the body.
Haemoglobin is an iron-containing molecule that becomes oxyhemoglobin when it binds to oxygen. Each molecule of haemoglobin carries about four molecules of oxygen. Typically, 100ml of oxygenated blood carries 5ml of oxygen.
Two ways for the Transport Oxygen to Tissues
Transport of oxygen to the organs or tissues occurs via blood and plasma:
Transport of Oxygen through Plasma
The transport of O2 through plasma occurs because some O2 is dissolved in plasma, which comprises about 3% of the total O2 transport.
Transport of Oxygen by RBC
A significant amount of oxygen, about 97%, is transported by binding to haemoglobin in RBC. Haemoglobin is an iron-containing pigment present in red blood cells, and this is responsible for the red colour of RBC. Haemoglobin consists of four polypeptides, and each polypeptide is attached to one heme group. One O2 can bind to the ferrous ion (Fe) at the centre of the haem group. Each haemoglobin molecule carries four molecules.
In the lungs, where the oxygen tension is high, oxygen transport will occur by binding to haemoglobin (dark red). Oxygen binds with the haemoglobin to form the oxyhemoglobin, bright red-coloured. This process is termed oxygenation of haemoglobin.
And at tissues (or other organs) where the oxygen tension is low, oxyhemoglobin splits into oxygen and haemoglobin, thereby transporting oxygen from the above-dissociated oxygen.
The Relationship between haemoglobin and Oxygen Partial Pressure
The attachment of oxygen to haemoglobin is predominantly determined by O2 partial pressure. A sigmoid curve is formed when the % saturation of haemoglobin with O2 is plotted (on the y-axis) against the pO2 (on the x-axis). This is known as the oxygen dissociation curve.
Haemoglobin is 100% saturated when each haemoglobin molecule carries four oxygen molecules. It is said to be 50% saturated when each haemoglobin, on average, carries two oxygen molecules.
About 97% of haemoglobin is saturated in systemic arteries, where pO₂ is about 95mmHg. Haemoglobin is 75% saturated when pO₂ is 40mmHg, the pO₂ of tissue cells at rest.
It means that oxygen transport by oxyhemoglobin to resting tissues is about 22%. The remaining is in the form of ‘reserve’ in the blood itself. Hence, oxyhemoglobin ensures oxygen transport for survival for 4-5 minutes after stopping the heart or when breathing is interrupted. Inactive tissues such as skeletal muscles, pO₂ is much below 40 mmHg. Under these conditions, there is more “unloading tension”, and a large percentage of O₂ is released from haemoglobin.
For example, at a pO₂ of 20mmHg, the haemoglobin saturation percentage is only 35%. 50% haemoglobin is saturated at P50 (oxygen tension). The normal P50 is 26.7 mmHg.
Factors affecting the Transport of Oxygen (Affinity of haemoglobin for Oxygen)
The partial pressure of CO₂, H+ ion concentration, and temperature are the other factors that can interfere with the transport of oxygen and binding with haemoglobin.
In alveoli
The conditions in the alveoli that are favourable for the synthesis of oxyhemoglobin include high pO2, low pCO2, reduced H+ ions, and lower temperatures.
In tissues
Conditions favourable for dissociating oxygen from oxyhemoglobin exist in tissues with low pO2, high pCO2, higher H+ ion concentration, and higher temperatures. Under these conditions, the oxygen-haemoglobin dissociation curve shifts to the right.
Bohr Effect – Determining the O2 Affinity
The effect of CO₂ and the resultant H+ on the oxygen affinity of haemoglobin is termed the Bohr effect. The oxygen dissociation curve is used to study the effect of factors like pCO₂, H+ concentration, etc., on the binding of O₂, with haemoglobin. Under normal physiological circumstances, every 100 ml of oxygenated blood may provide approximately 5 ml of O2 to the other tissues.
Within limits, as temperature increases, the amount of O₂ released from haemoglobin also increases. Heat is released as a by-product of the metabolic reactions, and the released heat contracts muscle fibres, leading to a rise in body temperature. Metabolically active cells require more O₂ and liberate more heat.
The heat, in turn, promotes the release of O₂ from oxyhemoglobin. Consequently, the saturation curve shifts to the right. A substance found in red blood cells called 2,3-bisphosphoglycerate (BPG), formerly called 2,3 diphosphoglycerate (DPG), decreases the affinity of haemoglobin for O₂ and thus helps unload O₂ from haemoglobin. 2,3-bisphosphoglycerate is formed in red blood cells during glycolysis.
Conclusion
Oxygen is vital for all living beings. And it is taken by different organisms in different ways. In this article, we have discussed all the possible information about oxygen transport within the human body, including the factors affecting its transportation and the types of oxygen transport.
We hope you can grasp the information that you are searching for. If you have any doubts, you can ask them in the comment box. If you need a similar article, don’t hesitate to bookmark our page.
