Transcription
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In this video, we're going to look at the red blood cell life cycle, or erythrocyte life cycle. So here I'm drawing a blood vessel. A blood vessel contains red blood cells. Here are mature red blood cells. Mature red blood cells are responsible for transporting oxygen around our body.
But where do the red blood cells come from? Well, they all arise from the bone marrow. So here I'm drawing a bone. The red blood cells come from stem cells within the bone marrow. Erythropoiesis is the term used to describe erythrocyte synthesis, or the production of red blood cells. Hematopoietic stem cells can differentiate into proerythroblast and then early erythroblast. In the early erythroblast stage, we see ribosome synthesis, and this is important because once the early erythroblast becomes a late erythroblast, the ribosomes are synthesizing a lot of hemoglobin. And so we have hemoglobin accumulation. Hemoglobin is the molecule within red blood cells that actually carries the oxygen.
So this late erythroblast can then develop into a normoblast. A normoblast already contains all this hemoglobin. A normoblast will then become a reticulocyte. A reticulocyte loses its nucleus. However, it is not a red blood cell just yet. A reticulocyte actually stays in the bone marrow for several days before entering circulation. And once entered into circulation, the reticulocyte, after about one to two days, or 24 to 48 hours, will mature and become an erythrocyte. Erythrocytes are the red blood cells that we know that circulate and carry oxygen in our body.
But erythrocytes don't circulate in our body forever. It has a lifespan of about 128 days. When it's aged or when it's damaged, and so when it's aged or damaged, it has to be removed. Red blood cell removal occurs in three main organs: the spleen, the liver, and the bone marrow. So here I'm drawing a few old red blood cells that need to be cleared out. And here I'm drawing the spleen. Within a spleen, we have macrophages, and it is these macrophages that will actually engulf these old erythrocytes and break them down.
And so when the old red blood cells are broken down, you end up with a lot of hemoglobin. The hemoglobin can then be broken down further into globin and its heme component. Globin is then broken down to amino acids, and the heme is broken down to two main products: bilirubin, essentially, and iron. These macrophages are not only found in the spleen, as I mentioned, but they're also found in the liver and bone. And they do exactly the same thing.
Most of the breakdown product of red blood cells are recycled. So, for example, here the amino acids will enter back into circulation and then will travel to the bone marrow to be used in erythropoiesis again, the production of red blood cells. The iron and bilirubin also have similar fates. So the iron, it can't actually travel in the blood just like that. It has to be attached to a transporter. And so iron obtains an iron transporter from the liver known as transferrin, and it's this way how iron makes its way back to the bone marrow for erythropoiesis.
Bilirubin goes back to the liver either through albumin transporter or whatnot, but essentially bilirubin ends up in the liver. Now, bilirubin has a few fates. Most of the time, bilirubin is actually secreted or excreted through the bile system. So bilirubin will enter the bile and then will be secreted into the duodenum of the small intestine, and this is through the bile duct. So bilirubin is excreted in feces, or it is reabsorbed and is excreted in urine.
Now, that we're in this picture of the stomach and small intestine, it's important to note that when we eat nutrients, some of these nutrients are important in red blood cell production. They are absorbed in the small intestine. So nutrients absorbed for erythropoiesis include amino acids, monosaccharides, lipids, vitamin B12, folic acid, and iron. And all these are used in erythropoiesis because we need these to make red blood cells. And as I mentioned, the main function of red blood cells is to transport oxygen to tissues.
So now let's look at one important hormone or factor that influences erythropoiesis in times of hypoxia, which is where when we have a decrease in oxygen levels. This will stimulate the kidneys to produce an important hormone called erythropoietin. And erythropoietin is such an important hormone in erythropoiesis because it stimulates erythropoiesis. And so when you have kidney failure, for example, your kidneys are not able to produce erythropoietin, so you usually develop anemia.
I hope you enjoyed this brief video on red blood cell lifecycle. Thank you.