Question

​​​​​​Human Physiology

1. Explain the concept of transport maximum. Under what scenario would glucose appear in the urine?
2. Explain in detail the process that establishes and maintains the medullary osmotic gradient in the kidneys. Then describe the effect this osmotic gradient has on the water in the renal tubules.

Answer 1

Glucose is reabsorbed by secondary active transport. All the glucose is reabsorbed in proximal tubules by sodium depemdent glucose cotransporter (SGLT)

The TmG (Transport maximum for glucose i.e. the maximum rate of absorption of glucose by the tubule) is 375 mg/min in males and 300 mg/min in females. The actual value of renal threshold is much less than this; it is 200 mg/dL in arterial and 180 mg/dL in venous blood. This deviation in the renal threshold (from the calculated predicted value) in called SPLAY .The reason for splay is heterogeneity of nephrons (i.e. not all nephrons have TmG of 375 mg/min); further, not all nephrons are maximally active simultaneously .

If plasma glucose is more than 180 mg/dl then it reaches the transport maximum saturation for glucose and carriers are already saturated so cant be able to reabsorb extra glucose and will result in renal glycosuria ( glucose in urine ).

At the filtrate Normal serum osmolality is 285-295 mosm/kg of water .

Filtrate is not changed even after the process of reabsorption from proximal convoluted tubule ( PCT) .This phenomenon is "Isoosmotic Reabsorption"

Counter Current system - Two tubes closed to each other but the direction of flow of fluids in two tubes is in opposite direction .

At the tip of Loop of Henle ( serum osmolality is 1200 mosm) After PCT through the thin descending Limb water is being reabsorbed to reach maximum serum osmolaity as It is completely permeable to water and impermeable to solutes like Nacl . This process is known called as "COUNTER CURRENT MULTIPLICATION."

Lowest serum osmolality 50 mosm is there at Distal convoluted tubule because thick descending limb of Loop of Henle is permeable to solutes Nacl and impermeable to water .Now as solutes are present in medullary interstitum medullary osmotic gradient is increased .

Now from 50 mosm to 800-900 mosm /kg of water is achieved by using ADH hormone Antidiuretic hormone

Along with Loop of Henle Vasa recta are present around peritubular capillaries and responsible for "COUNTER CURRENT EXCHANGE"

For ADH to act Following factors are required
a).Medullary Interstitial osmolality
b) Urea
C) Vasa recta

Thus "COUNTER CURRENT MULTIPLICATION " establishes the medullary osmotic gradient and is maintained by COUNTER CURRENT EXCHANGE".