How does Carbon dioxide get form a respiring cells to the lumen of a alveolus in the lung
How does Carbon dioxide get form a respiring cells to the lumen of a alveolus in the lung?
Carbon Dioxide, or CO2 is a key product of respiration. It helps the body to identify where more oxygen is needed for respiration, however it is also, eventually needs to be removed from the body. Respiration is constantly occurring in the body as it produces ATP which is needed for many processes such as Active transport. This means that there is a constant production of Carbon dioxide which needs to be removed from the body in a constant fashion in order too maintain respiration. In this essay I will explain how carbon dioxide moves from a respiring cell to the the alveoli in the lungs.
Once a cell respires and produces carbon dioxide as a product of this reaction there are three different ways in which the carbon dioxide can be carried out of the body. Firstly the carbon dioxide can be dissolved in solution, secondly it can be buffered with water as carbonic acid and lastly and perhaps the most common form of transport would be the binding of carbon dioxide or proteins such as haemoglobin. 75% of all carbon dioxide is transported by the blood and the remaining 25% is carried by the plasma of the blood.
Carbon dioxide is incredibly soluble so can easily diffuse into the blood, in fact it is around 20 times more soluble than oxygen. It obeys a law named Henrys law which states that the number of molecules in a solution is proportional to the partial pressure of the liquid surface. Arterial blood consists of around 2.5ml/100ml of dissolved carbon dioxide and venous blood, around 3ml/100ml of dissolved carbon dioxide. A cardiac output (the rate of blood the heart pumps/min) of around 5litres/min will carry 150ml of dissolved carbon dioxide of which 25ml or so of carbon dioxide will be exhaled.
The Bohr shift is the key reason that the haemoglobin actually binds with the carbon dioxide. To understand this, it has to be noted that haemoglobin picks up oxygen in the lounge and released it to areas of low oxygen partial pressure. This effectively means that in areas where there is little oxygen, or it has all been used up are the ares where the haemoglobin will release the oxygen bound to it. This dissociation of oxygen from the molecule (as can be shown by the oxygen dissociation graph) is also affected by carbon dioxide. This carbon dioxide diffuses from the cells in the blood plasma (as described above) and from this blood plasma some of the carbon dioxide diffuses into the red blood cells. In the cytoplasm of these red blood cells there as a enzyme called carbonic anhydrase, that enzyme catalysed the following reaction:
Haemoglobin then combines with the hydrogen ions, this forms haemoglobinic acid (HHb). This then releases the oxygen from the molecule. The presence of a high partial pressure of carbon dioxide causes the haemoglobin to release oxygen, this is what is known as the Bohr shift. This Bohr shift is one description of how Carbon dioxide is carried in the blood. One of the products of the dissociation of the dissolved carbon dioxide is hydrogen-carbonate ions. These are formed in the cytoplasm of the red blood cell as this is where the enzyme carbonic anhydrase is found. Most of the hydrogen-carbonate ions then diffuse out of the red blood cell into the blood plasma. 85% of the carbon dioxide is transported in this way. Some of the carbon dioxide does not dissociate in this way and remains as carbon dioxide molecules. Some of these molecules dissolve in the blood plasma (5%) of the total amount. The remainder of there molecules diffuse into the red blood cells and combine directly with the amine groups. About 10% of the total carbon dioxide is carried in this way.
Once in the blood stream the carbon dioxide makes it’s way to the heart. This deoxygenated blood is moving towards the heart in order to get oxygenated by the lungs and then pumped around the body by the heart. Because the pressure in the veins is low the blood moves mainly at the contraction of the heart (systole) and as it has to flow against gravity mostly it requires this higher pressure. The blood flows from the capillaries where into venues. These collect the blood form the capillary beds and join up to the larger veins of the body. The walls of the vessels are relatively thin as they are taking the blood back at a low pressure (to the heart). They have a thing tunica media and a thin tunica externa. They also have a larger lumen in comparison to the thickness of there walls. The veins also have valves, these are semilunar valves and they close between each systole to prevent the back flow of blood. Once the blood reaches the heart by flowing through the veins It enters the vena cava and then the right ventricle flowing down into the right ventricle which has thinner muscle walls as the blood does not need to be pumped at a pressure so high as it is just going to the lungs through the pulmonary artery.
Once the blood reaches the lungs, the reaction described above which cause the blood to bond with carbon dioxide effectively reverses. This causes the carbon dioxide to diffuse into the alveoli and take oxygen place in the alveoli. This stimulates the carbon dioxide of the carbaminohaemoglobin to leave the red blood cell (situated in the capillary at this time) and hydrogen-carbonate and hydrogen ions to recombine to form carbon dioxide molecules once more. This leaves the haemoglobin molecules free to form with oxygen once more.