Ca and Oxygen in Blood VIA Urinary System

99% of filtered calcium

is reabsorbed by kidneys

65% of filtered calcium

is reabsorbed by PCT

25% of filtered calcium

is reabsorbed in ascending loop of Henle

reabsorption of calcium in the DCT (8%)

is most tightly regulated and influenced by hormones

PTH

increases calcium reabsorption in DCT; stimulates kidneys to convert vitamin D to calcitriol (active form) increasing calcium absorption in gut

calcitonin

decreases calcium reabsorption, leading to more calcium being excreted in urine

PTH effect on phsophate

decreases phosphate reabsorption in PCT, causing more phosphate to be excreted

calcium-phosphate precipitation in tissues

prevents calcium from performing other functions

proximal tubule

calcium reabsorption here is mostly passive and follows sodium and water movement

ascending loop of Henle

reabsorption is passive and driven by private voltage in tubular lumen created by sodium and potassium transport

distal convoluted tubule

main site of hormonal regulation; calcium reabsorption is active and occurs via calcium channels and transporters in cells

when blood calcium drops

PTH is released to restore normal levels

PTH stimulates bone reabsorption

osteoclasts break down bone, releasing calcium into blood

PTH increases calcium reabsorption in kidneys

occurs at DCT, prevents calcium from being excreted in urine

PTH inhibits phosphate reabsorption in PCT

leads to increased phosphate excretion in urine

when blood calcium returns to normal

PTH secretion decreases in negative feedback loop

calcitrol

active form of vitamin D, crucial for calcium and phosphate balance

precursor formation in skin

UV light from sunlight converts cholesterol derivative in skin into Vitamin D3 (cholecalciferol)

liver conversion

vitamin D3 is converted to vitamin D2

kidney activation

PTH converts vitamin D2 to calcitriol

calcitriol increases

calcium and phospahte absorption from intestines

calcitriol acts on bone and kidneys

to reabsorb calcium and phosphate from bone into blood; increased osteoclast activity

calcitriol increases reabsorption of calcium

in distal tubules, helping to conserve calcium and reduce its loss in urine

calcitriol influences phosphate absorption

by increasing intestinal absorption of phosphate rather than direct action on kidney

oxygen sensing cells

peritubular interstitial cells in renal cortex are responsible for detecting oxygen tension in blood

hypoxia inducible factor

when oxygen levels drop, proteins accumulate in cells and act as transcription factor; turning on EPO production

EPO is synthesized and secreted

into bloodstream, especially during hypoxic conditions

EPO bone marrow response

stimulates erythroid progenitor cells in bone marrow to proliferate and mature into RBCs

EPO restoration of oxygen carrying capacity

as RBC cell numbers increase, blood’s ability to carry oxygen improves, correcting hypoxia

negative feedback of EPO

once oxygen delivery to kidney returns to normal, HIF is degraded, EPO production decreases, and RBC production slows down