Welcome to nonstopneuron.com where learning medical concepts is as easy as watching cartoons. In this video, we will learn about oxygen-hemoglobin dissociation curve. A topic from respiratory physiology. This video is divided into three parts. First we will see how changing partial pressure of O2 affects it's binding with hemoglobin. Then we will see the curve itself. Finally we will see what this curve means in terms of transport of oxygen from lungs to tissues. Let's get started. This is blood. This is hemoglobin molecule. One hemoglobin molecule has four heme groups. One heme can bind with one oxygen molecule. So one hemoglobin molecule can bind with total of four oxygen molecules. In blood we have lots of hemoglobin molecules. These are oxygen molecules in surrounding. At low partial pressure of oxygen, affinity of hemoglobin for oxygen is low. So less oxygen molecules bind with hemoglobin. As the pressure increases, more O2 binds. At lower range of oxygen pressure increasing pressure cause relatively small increase in oxygen binding. However this bound oxygen molecules increase affinity of Hb for oxygen. So in middle range of oxygen pressure, even small increase in pressure causes marked increase in oxygen binding. This phenomenon of increasing affinity by bound oxygen molecules is called positive cooperativity. Now the hemoglobin starts saturating with oxygen. See very few sites are left for more oxygen. So at higher range, increasing pressure causes only small increase in binding. When all the hemoglobin molecules are bound, there cannot be any further binding. Now let's see the same thing in form of graph. Here we have partial pressure of oxygen on x axis and percentage of hemoglobin saturation on y axis. At the lower range of partial pressure of oxygen, there is only small increase in oxygen binding. So graph is relatively flat here. Due to positive cooperativity, further increase in oxygen pressure, causes marked increase in oxygen binding. So the graph is steep in this range. Now the hemoglobin starts saturating. So further increase in oxygen pressure causes small increase in binding. So graph is relatively flat again. This is oxygen-hemoglobin dissociation curve. It is S shaped or sigmoidal due to cooperativity. Now let's see how this concept applies to transport of oxygen. These are lungs, this is artery carrying blood from lung to peripheral tissues. This muscle represents all peripheral tissues. This is vain bringing blood back to the lungs. And these are hemoglobin molecules circulating in these vessels. At lungs partial pressure of oxygen is high, about 100 mmHg. Due to this, oxygen diffuses from lungs into the blood and binds with hemoglobin. In oxygen-hemoglobin dissociation curve it corresponds to this point. At 100 mmHg partial pressure of oxygen, hemoglobin saturation is about 97%. So arterial blood has 97% oxygen. Now this hemoglobin travels to the peripheral tissues. Here partial pressure of oxygen is about 40 mmHg. Due to low partial pressure, hemoglobin releases oxygen molecules. In curve, it corresponds to this point. Partial pressure is 40 mmHg. And due to release of oxygen, saturation decreases to 75%. Yes, only 22% of total oxygen is unloaded at peripheral sites under normal restful condition. So venous blood still has 75% oxygen. Now this hemoglobin goes back to lungs. Here, oxygen is again loaded and cycle keeps repeating. In terms of oxygen hemoglobin dissociation curve, oxygenation at lungs moves the set point from 75% to 97% saturation. Then blood goes to peripheral tissues where release of oxygen moves the set point from 97% to 75%. In this way, equilibrium keeps moving between these two points. I hope you are getting everything. This is simplest understanding of the oxygen hemoglobin dissociation curve. So many factors affect affinity of hemoglobin for oxygen and thereby affect the curve. But that's a topic for separate video. Just one more topic before closing. Till now we were talking in terms of percentage of saturation. Now let's see absolute quantity of oxygen carried by hemoglobin. At 97% saturation, 100 ml of blood contains about 20 ml oxygen in arteries. At peripheral sites, hemoglobin releases oxygen and saturation falls to 75%. At this point, oxygen content is about 15 ml oxygen per 100 ml of blood. Thus, 5 ml oxygen is release at peripheral sites, by each 100 ml blood and the same amount is reloaded from lungs. That's it for now. Also check out other videos on respiratory physiology. If you liked it, please share it with all your colleagues. If you are new here, subscribe to get notified for upcoming videos. Thanks for watching. See you in next video.