 Calcitriol, do you know who this is? It's someone you know. Calcitriol is your friend and you know it because it's actually vitamin, vitamin D3. And you probably are thinking how can you secrete or how can calcitonin, no, that's our next one. How can parathyroid hormone affect calcitriol? And I'm going to tell you how it does this. First of all, let's figure out how calcitriol does its thing. Have you heard of the connection between the lovely sunshine and vitamin D? In fact, sunshine is necessary for producing, let's take in this green person, a calcitriol precursor in the skin, which then travels to, I'm going to draw you a little, what's it called? Pictionary, what is this? Okay, I'll give you a little hint. But of course, this is the liver. This little precursor was built in the skin and it travels to the liver and it gets turned into something else and then guess where it travels? That thing travels, okay, this one will be easier. Pictionary, that one's easy. To the kidney where now we get our functional vitamin D3. This is calcitriol and it's functional. Okay, calcitriol, what does it actually do? Well, one of the most important things that it does is it increases absorption of calcium in the gut. So there are calcium transporters called calcium binding proteins that get what? Exocytost or placed into the luminal membrane in the intestinal tract and increase calcium absorption from the digestive system. This is why the argument is, dude, you got to have your vitamin D in order to get your calcium. Let's add vitamin D to milk so we can absorb the calcium and milk even better. In addition to this, calcitriol increases absorption or reabsorption of calcium in the proximal convoluted tubule. Kidney's coming back at you. So if you have low calcium, you're going to produce calcitriol which will increase calcium reabsorption, increases blood calcium levels, increases absorption of calcium in the gut. My Wi-Fi is unsecure. Thank you very much for telling me that. I think that's everything that you need to know about this. Am I right? How do you feel? Shall we finish it up strong with the... Both calcitriol and parathyroid hormone are increasing calcium concentrations in the blood. I think we should have someone who's doing the opposite if we're going to maintain homeostatic balance. Perhaps. Let's talk about calcitonin.