Tissue Hypoxia – Causes – Hypoxemia and Hypercapnia

00:00 Now, we also need to think about what is happening at the cellular level.

00:06 So each individual tissue in the body could experience low O2s and we call this hypoxia.

00:14 So tissue hypoxia can occur through many different ways.

00:19 Let’s talk through what the most common are.

00:22 One is it’s not getting enough blood.

00:25 So if cardiac output is low or if tissue perfusion is low, the tissues could become hypoxic.

00:33 Another way that tissues can become hypoxic is if the blood cells themselves aren’t carrying enough O2.

00:40 Such as if a person has anemia or if they have a carbon monoxide poisoning.

00:46 Because remember carbon monoxide binds to the O2 binding sites on hemoglobin with a greater affinity than O2 and thus replaces them.

00:56 So either of those will mean that the blood that gets to the tissues is not bringing as much oxygen with it.

01:03 The final way is a hypoxemia.

01:06 So hypoxemia is a low blood PO2.

01:13 That’s different than hypoxia, which is occurring at the level of the tissue.

01:18 So now let’s talk through the different hypoxia reasons and how the body tries to fix a hypoxia.

01:24 So we have hypoxia and in the kidney, this is sensed and a hypoxia-induced factor is produced.

01:33 What this does is then stimulate erythropoietin known as EPO, synthesize its proteins and it starts to form red blood cells.

01:44 So a hypoxia overtime is sensed by the kidney and it produces a hormone called erythropoietin, which then produces more red blood cells or erythrocytes.

01:56 You may have heard of EPO or erythropoietin before, because this is sometimes a drug that athletes might use to increase the amount of red blood cells such as what happened in some Tour de Frances a number of years back.

02:14 Hypoxemias.

02:16 You need to go through the different types of hypoxemias and there is a way in which you can talk through these to understand which particular one might be prevalent at a certain time.

02:27 So low arterial partial pressures of O2.

02:31 Low PaO2 can be induced by either hypoventilation or a low PIO2 or a low partial pressure of inspired oxygen.

02:44 It could be caused by a diffusional impairment by a right to left shunt or ventilation to perfusion inequality.

02:53 These are all four main mechanisms by which a hypoxemia can occur.

02:59 If we look at our diagram, here, you can see that the blood is going to be oxygenated as it goes through the lungs.

03:07 So where is the low PaO2? That’s in the red portion of the diagram.

03:12 So if you don’t have enough O2 being delivered to peripheral tissues, they will become hypoxic.

03:21 So let’s talk through first hypoventilation or low PIO2.

03:27 So you can decrease PIO2, one of two ways.

03:32 One is you could be in a low barometric pressure environment and that's usually at altitude.

03:37 Another way is if the fraction of O2 is lower in ambient air then you would expect.

03:44 We would expect it to be at 21%.

03:47 But let’s say you were in an enclosed room without very good circulation.

03:51 Eventually the O2 levels could decrease in that area.

03:56 Hypoventilation is another way in which you can create a hypoxemia and this is that you’re not ventilating or bringing enough air into the lungs and therefore, you’re not being able to fully oxygenate your blood.