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Lecture 18

BIOC34H3 Lecture Notes - Lecture 18: Nephron, Cholecalciferol, Homeostasis

Biological Sciences
Course Code
Stephen Reid

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Lecture 18: Calcium Regulation, Glucose Regulation and Clearance
1. Calcium Regulation
Calcium levels in blood are regulated by hormones. Calcium exchange occurs between the blood and is stored in
bone, gut and kidney.
Absorb calcium from digested food in gut or reabsorb it from kidney. Rates of absorption from gut or reabsorption
from kidneys can be regulated depending on whether plasma calcium levels are high or low. Calcium from bones can
be added to blood or deposited back into bone. Calcium removal from bone is resorption whereas calcium deposit
into bones is calcification or deposition.
3 primary hormones: parathyroid hormone, calcitrol (both involved in raising
plasma calcium), and calcitonin (involved in lowering plasma calcium). Parathyroid
hormone and calcitonin made in parathyroid and thyroid glands.
Calcitonin is produced by thyroid gland in response to high Ca2+ in blood
plasma. Calcitonin causes deposition of calcium into bone and reduces
reabsorption of calcium by kidneys.
Parathyroid hormone (PTH) produced by parathyroid glands raises plasma
calcium by stimulating reabsorption of calcium in kidneys and the release of
calcium from bones to blood. PTH also stimulates production of calcitrol which
causes calcium uptake from gut to increase.
Calcitrol produced when plasma calcium levels are low. Convert cholesterol
into 7-dehydrocholesterol which is converted to vitamin D3 by sun (as blood
moves in vessels near the surface of the skin). Vitamin D3 is converted in liver to
25-hydroxyvitamin D3. If plasma calcium levels are low, kidneys are
stimulated by parathyroid hormone to turn 25-hydroxyvitamin D3 into calcitrol.
2. Glucose Handling by the Kidney
Normally, 100% of filtered glucose is reabsorbed by kidneys, passing through
tubular epithelium via active glucose/Na+ co-transport, and into peritubular fluid
via facilitated diffusion (Na+ is reabsorbed via Na+/K+ exchange).
Filtered Load = GFR x [solute]plasma
Transport maximum of substance refers to maximum operating capacity of
transport and carrier proteins that handle that particular substance.
Renal threshold of substance is the plasma concentration of a substance at
which that substance begins to appear in the urine.
When filtered load is equal to transport maximum, then renal threshold for that
substance has been met.
Calculate renal threshold for glucose (the point at which glucose levels in plasma are so high that it appears in urine) by
determining when filtered load equals transport maximum. Transport maximum for glucose is 375 mg/ml.
The filtered load is equal to GFR (125 ml/min) multiplied by the concentration of glucose in the plasma (1 mg/ml).
The filtered load is therefore 125 mg of glucose filtered every minute (mg/min).
Renal threshold is achieved when filtered load = transport maximum. Therefore:
Transport Maximum = GFR x Renal Threshold
Transport maximum is 375 mg/ml and GFR is 125 ml/min. Renal threshold for glucose = 3 mg/ml (300 mg/dL).
When concentration of glucose in plasma reaches 3 mg/ml, transport processes are saturated and glucose will begin to
appear in urine. In reality, glucose appears in urine when plasma concentration of glucose is 1.8 mg/ml (180 mg/dL).
Graph illustrates glucose handing by kidney. When plasma glucose levels are low,
amount of glucose filtered at glomerulus = amount that is reabsorbed by kidney. In this
case, no glucose is excreted. Reabsorption = filtration until point when plasma glucose
levels reach 300 mg/dL. At this point, we have reached theoretical renal threshold.
However, reabsorption fails to keep up with filtration and glucose excretion begins
when plasma glucose levels are much lower (about 180 mg/dL). The reason for this is
that not all of the glucose in the kidney tubules has access to the glucose transport
systems. Glucose that remains within the centre of the kidney tubule lumen is not
available to be transported (reabsorbed). Therefore the actual renal threshold is less
than the theoretical value.
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