Vitamin D and Calcium Levels

00:01 There are several considerations in regulating the body calcium levels.

00:05 We’ve talked about the blood but there’s several things to consider.

00:08 First of all, vitamin D in our diet.

00:11 Are we getting enough calcium in our diet? And is our body making sufficient amounts of vitamin D to support the calcium that we need.

00:19 The parathyroid hormone or the PTH that I talked about is active in the kidneys and the bone as we shall see.

00:25 And this helps to either, in the case of PTH, to release calcium so that the body has enough calcium travelling to the blood stream or in the case of a protein called calcitonin to actually absorb that calcium back into the bone and reduce the blood levels of calcium.

00:41 We’ll see that in just a second.

00:43 So the bones as I said are important reservoirs of calcium.

00:47 A place to put excess calcium when the body has excess calcium and a place to provide calcium when the calcium levels are too low.

00:55 These two hormones, calcitonin and PTH, that I have described play important roles in that process.

01:01 The bone making that is the taking of calcium out of the blood and putting it into the bone occurs in specialized bone cells called osteoblasts.

01:08 This reduces blood calcium and stimulated by calcitonin.

01:13 On the other, when the blood needs more calcium, bones are broken down in specialized cells called osteoclasts.

01:20 These cells are stimulated by the hormone known as PTH.

01:25 Now, another important variable in calcium levels in the blood is phosphate concentrations.

01:30 Calcitonin decreases phosphate levels.

01:34 Calcitonin acts on the kidneys and it stimulates the process of phosphate reabsorption that is preventing the phosphate from being excreted in the urine.

01:43 As a consequence phosphate concentration in the blood supply goes up and with phosphate concentration, the blood supplies goes up, bone making is favored.

01:53 On the other hand, PTH favors the opposite process.

01:56 It favors the excretion of phosphate by the kidneys.

02:00 So the blood levels of phosphate fall and the falling blood phosphate levels favors the release of calcium from bones.

02:09 In the addition to the stimulation of increasing the calcium levels from the bone by PTH, PTH also stimulates the absorption by stimulating the formation of active vitamin D, okay? The increased levels of vitamin D favors the taking of calcium from the diet and putting it into the blood supply, thus favoring the increase in concentration of blood calcium.

02:34 PTH synthesis, however, is negatively regulated by the amount of ionized calcium that’s in the blood serum.

02:41 Meaning that the higher the levels of calcium that’s in the blood, the less PTH is made.

02:48 High serum calcium reduces this secretion because PTH is made by the parathyroid gland.

02:55 And by negatively impacting the parathyroid gland, less PTH is made.

03:01 So conversely low calcium, however, favors PTH released by the parathyroid gland and stimulation of the processes that I’ve described here.

03:10 Well, that’s a mouthful of things.

03:11 Let’s look at the cellular level and see what’s actually happening with these individual hormones.

03:16 On the left, I show what happens in the body with respect to calcitonin.

03:20 So we can imagine we have a certain amount of free calcium ions that are present inside the cell.

03:25 In this case, I’ve shown an abundant level of calcium ions that are present in the blood supply.

03:31 When this happens, calcitonin will favor the making of bone.

03:34 The making of bone will reduce the levels of calcium that’s occurring inside of the blood supply and more bone cells will be made.

03:42 Another way that calcitonin affects this is by changing the phosphate levels.

03:47 Remember that what calcitonin does is it raises the levels of phosphate by stimulating the reabsorption of phosphate by the kidney.

03:55 When phosphate levels increase, then more of that calcium is gobbled up and converted into bone.

04:00 This is the way that calcitonin actually makes the bone.

04:04 So when calcitonin is present, blood supply of calcium will go down, the increase in bone concentration or bone density will actually go up.

04:14 PTH works in the opposite way.

04:17 Bone breaking occurs when the calcium levels fall too low.

04:21 The release of calcium from the bone increases the blood concentration of calcium as you can see here.

04:28 The other thing that happens with PTH, remember, is that PTH favors the excretion of phosphate by the kidneys.

04:34 So as phosphate is being excreted by the kidneys more calcium remains in the blood supply because less of it is being converted into bone by calcitonin.

04:44 Now, I said that PTH is affected by the levels of calcium that’s present in the blood supply.

04:50 High calcium levels will favor lower amount of PTH being released by the parathyroid gland.

04:56 This causes a lower level of vitamin D to be made and lower level of vitamin D being made reduces the amount of calcium that is taken from the intestine in our diet.

05:07 On the other hand, when there are low concentrations of calcium present in the blood, then a higher level of PTH is made.

05:14 The higher level of PTH favors a higher level of vitamin D being made and a higher level of vitamin D being made favors additional absorption of calcium from the diet.

05:23 So PTH is ultimately the molecule or the protein that is most important in controlling these overall levels because it’s actually sensing the calcium levels affecting vitamin D accordingly.