Oxygen is transported in the blood in two ways: Show
Bound to HaemoglobinOnce oxygen has entered the blood from the lungs, it can be bound by haemoglobin (Hb) in the red blood cells. Haemoglobin is a protein comprised 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 each haemoglobin molecule can bind four oxygen molecules, forming oxyhaemoglobin. Haemoglobin molecules with a greater number of oxygen molecules bound are brighter red. This is why oxygenated arterial blood is brighter red and deoxygenated venous blood is darker red. Zephyris at the English language Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons Fig 1 – Haemoglobin molecule. Note the four haem groups in green Oxygen Binding to HaemoglobinHaemoglobin changes shape based on the number of oxygen molecules bound to it. The change in shape also alters its affinity to oxygen. As the number of oxygen molecules bound to haemoglobin increases, the affinity of haemoglobin for oxygen increases. This is known as cooperativity. When no oxygen is bound, the haemoglobin is said to be in the Tense State (T-state), with a low affinity for oxygen. At the point where oxygen first binds, the haemoglobin alters its shape into the Relaxed State (R-state), which has a higher affinity for oxygen. We can plot this change on a graph of oxygen saturation over partial pressure of oxygen. By Hazmat2 [Public domain], via Wikimedia Commons Fig 2 – Saturation of Hb with oxygen against partial pressure of oxygen Oxygen Delivery at TissuesAs shown on the diagram above, the percentage of oxygen bound to haemoglobin is related to the partial pressure of oxygen (pO2) at a given site. When oxyhaemoglobin reaches a tissue with a low pO2 (e.g. skeletal muscle), it will dissociate into oxygen and haemoglobin, resulting in an increase in local pO2. When it reaches a tissue that has a high pO2 (e.g. in the pulmonary circulation), haemoglobin will continue to take up more oxygen, resulting in a lowered pO2. Factors Affecting Oxygen AffinityVarious factors can affect the affinity of haemoglobin for oxygen:
The affinity of haemoglobin for oxygen also results in a shift in the oxygen-haemoglobin dissociation curve. An increase in oxygen affinity results in the curve shifting to the left, whereas a decrease in oxygen affinity results in the curve shifting to the right. Clinical Relevance – Carbon Monoxide PoisoningCarbon Monoxide (CO) is a colourless, odourless gas. It can be released from faulty boilers or combustion engines. Carbon Monoxide poisoning occurs when CO reacts with haemoglobin at the site of oxygen binding. Haemoglobin has an affinity for CO that is 210x greater than its affinity for oxygen. This means that once carbon monoxide binds to haemoglobin, it is irreversible. Symptoms of CO poisoning are headache, nausea and tiredness. Interestingly, respiration rate is usually spared as the partial pressure of oxygen dissolved in the blood is maintained at normal levels. Haemoglobin bound to CO has a cherry-red colour and this may be visible in nails beds and mucous membranes of patients with CO poisoning. Treatment is with 100% oxygen and referral for hyperbaric oxygen treatment. CO poisoning is considered fatal when 70-80% of haemoglobin is bound with carbon monoxide. |