Transcription
Hey guys, it's your favorite medical channel, Medicosis Perfectionelos, where medicine makes perfect sense. After talking about all of hematology—we talked about anemia, leukemia, lymphoma, multiple myeloma—today, it's time to talk about platelets and hemostasis. Yes, it's happening. Touchdown, baby. So let's get started.
As you know, your blood consists of plasma and cells. Cells include red blood cells, white blood cells, and platelets. Red blood cells are the most numerous; they are in millions. Platelets are only in thousands or hundreds of thousands. Plasma has water and proteins. It has albumin, which is the most numerous. If it's the most numerous, it's going to contribute to the osmotic pressure, because osmosis only cares about number and not the mass. Globulin is large, less numerous, and we have three types: alpha, beta, and gamma. Beta globulins are the coagulation factors; gamma globulins are the antibodies.
First, let's get this out of the way: Hemostasis is different from homeostasis. Hemo means blood; stasis means stable. We have two types: We have primary hemostasis and secondary hemostasis. Think of them as two separate things. I know they are interconnected, but it's different. Think about anatomy. In anatomy, they told you that you have only one heart. In medicine, this is BS. You have two hearts: You have the right heart and the left heart. They are two separate things. Yes, there is a relationship between them, but to think as a doctor, think of them as two distinct entities. Same thing here. Think of primary hemostasis and secondary hemostasis as two different things. Primary hemostasis is by the platelets; secondary hemostasis is a process caused by the beta globins, the coagulation factors. If you can think about them as two different things, you can solve any question.
The normal platelet count is 150,000 to 400,000 per microliter. Theoretically speaking, less than 150,000 is thrombocytopenia, but clinically speaking, I don't care about thrombocytopenia unless the platelet count falls below 50,000. Same thing: more than 400,000 theoretically is called thrombocytosis. But clinically speaking, I don't care about thrombocytosis unless platelets are more than 750,000. Why is that? It's the bell-shaped curve. Let's say that here is the normal range from here to here. So here, as you know, you will be your normal platelet count from 50,000 to 400,000. Okay? Then here is just standard deviation. And here we start to care about thrombocytopenia. Here we start to care about thrombocytosis. It's similar to intelligence. Okay? If you have an intelligence of 100 and your friend has 103, does that mean he is smarter than you? No. It doesn't mean anything. But there is, like, a three-degree difference. I don't care, because if you look at the curve, we only care when it's significant. Same thing with platelets. So when grandma comes and tells me, "I see here the platelet counts are 500,000. And according to the reference range, they are high. They are supposed to be only 400,000," I ask her one question: "Do you have any symptoms?" No. I just send her home. Follow up with me. You'll be fine.
When you leave blood alone in a test tube, a vertical test tube, it will sediment. It's called gravity. And, by the way, in my video on ESR, erythrocyte sedimentation rate, I made a mistake. I told you to measure this distance as the ESR. No, we measure it from above. So because the blood starts as here and starts to decrease, starts to sediment, we measure this distance from here to here, and the Westergren tube is numbered from here. So zero, one, two, three. So any idiot can measure the level of the red blood cell. You measure this number, and this is your ESR. You start from the top, not from the bottom. I made a mistake. Sorry about that.
For A+ students, this buffy coat area, which is between the plasma on top and red blood cells on the bottom, sometimes can appear green. Why? Because it includes the white blood cells, and they have myeloperoxidase created or secreted by the neutrophils. Platelets are in this buffy coat. It's very thin. Sometimes you can see it. That's fine. But this tells you that the red blood cells are in millions. Platelets are nothing compared to the red blood cells. That's why they are less than 1%, a very thin buffy coat layer.
Is there a difference between plasma and serum? Yes, indeed. There is. Again, leave the blood alone, and it will start to sediment. Leave it to clot. Do not add any anticoagulant or coat the test tube, and then you will have a blood clot. Remove this blood clot and throw it in the trash. What's left is called the serum. Yeah. Just get blood, let it clot, remove the clots, throw it in the trash. What's left is the serum. Serum is defibrillated plasma. I love this word: defibrillated. So here's the blood clot. The blood clot includes red blood cells and platelets. Yes. They are called thrombocytes. They are the cells of thrombosis, and they have the plasma coagulation factors. Throw this in the trash, and you end up with the serum. So plasma minus clot equals serum. Serum is plasma without fibrinogen or the clotting factors. Serum has a high concentration of serotonin. No kidding. That's why we called it serotonin. 'In' means protein, and 'sero' because it's in the serum. By the way, serotonin is also involved in depression; it's in your brain. That's why antidepressants are SSRIs—selective serotonin reuptake inhibitors—and serotonin is also involved in carcinoid syndrome.
Platelets, also known as thrombocytes—'cyte' means cell, 'thrombo' means thrombus—so they are the cells of thrombosis. What an honor. Decreased number of platelets is thrombocytopenia. Increased number of platelets is thrombocytosis or thrombocythemia. But, again, we don't care about thrombocytosis unless it's more than 750,000. We don't care about thrombocytopenia unless it's below 50,000, because of the bell-shaped curve and the standard deviation.
Platelets are synthesized from the glorious, magnificent megakaryocytes. Megakaryocytes are made in the bone marrow. While leaving the bone marrow, they squeeze through some sinusoids. They break down into pieces—actually, thousands of pieces. Each piece is a platelet. So platelets are not cells; they are just pieces of the magnificent megakaryocyte. Platelets are pieces; they are not cells. So somebody will ask, "Do platelets have a nucleus?" This is a stupid question, like the question that asks, "Does truth exist?" Well, is that true? Again, same foolish question. If I told you platelets are not cells, then they don't have a nucleus. Are they able to divide? They are not cells, honey, so they cannot divide. Okay. How do they regenerate? They don't. Only the megakaryocyte is able to produce more, but platelets by themselves cannot regenerate or divide. Platelets are just pieces of the membrane of the megakaryocyte and a small part of its cytoplasm, but the megakaryocyte had a big, low-low beta nucleus; platelets have no nucleus whatsoever. On bone marrow biopsy, we don't see platelets; we only see megakaryocytes, because while squeezing out of the sinusoids, then they get destroyed into thousands of pieces called platelets. In order for you to see platelets, you need a peripheral blood smear—what the British call it, the blood film.
Where do you find platelets? In the peripheral blood. One-third of the platelet pool is hiding in the spleen. That's why splenomegaly decreases the platelet count, because when you have splenomegaly, now you have, let's say, half of the number of platelets in the spleen, leaving less platelets in the peripheral blood. When you order a platelet count, we are only counting the peripheral blood. So splenomegaly will decrease the platelet count; a huge spleen can eat more platelets. That's why one of the methods to treat thrombocytopenia is to remove the spleen. When you remove the spleen, you force all of the platelets into the peripheral blood. So platelets now have nowhere to hide; they are forced into the bloodstream, increasing the platelet count and back to normal.
Here's a very important point. Yeah, we sometimes blame the spleen for the thrombocytopenia. However, the spleen can only eat so many platelets, which means if the platelet count is 20,000 or below, don't blame the spleen. There is something else going on. The spleen cannot eat that many platelets. So, please go fishing and look for something else if the platelet count is less than 20,000.
What stimulates platelet production? The hormone thrombopoietin. Remember red blood cells? Erythropoietin stimulates red blood cell formation; same thing here, thrombopoietin—TPO—synthesized by the glorious liver. That's why in severe liver disease there is thrombocytopenia, because now you have less TPO. Also, interleukin-6 increases TPO, then will lead to increased platelets. Interleukin-6 is involved in inflammation or some malignancies. That's why increased interleukin-6 increases TPO, increases platelet count. We call this secondary thrombocytosis. Do you remember primary thrombocytosis? Yes. It's essential thrombocytosis, one of the myeloproliferative disorders, together with multiple myeloma. Then decreased platelet count will stimulate TPO production to increase platelet count back to normal. This is called negative feedback.
The lifespan of platelets is between eight and twelve days. Who removes platelets? The splenic macrophages. Who regenerates the platelets? Now the platelets bother their parents—the megakaryocytes found in the bone marrow.
In the next video, we're going to discuss platelet structure. So make sure to subscribe. Just want to say thank you for all the nice people who support this channel on Patreon. If you'd like to get all of my notes, go to patreon.com/medicosis. Thank you so much for watching. Until next time. Be safe, stay happy, and study hard. Smash like.