Kidney Function and Urine Concentration

The ability of the kidneys to adjust the concentration of urine depending on the body's water balance.

When the body has excess water, the kidneys excrete dilute urine with low osmolarity (around 50 mOsm/L).

When the body is dehydrated, the kidneys excrete concentrated urine with high osmolarity (up to 1400 mOsm/L).

A hormone produced by the posterior pituitary gland that regulates water reabsorption in the kidneys.

The process of removing excess water from the blood and excreting it as urine.

The ability of the kidneys to adjust water excretion without significantly altering the excretion of solutes.

The process of reabsorbing solutes without reabsorbing large amounts of water in the distal parts of the nephron.

The filtrate is diluted as it passes through the tubule due to the reabsorption of solutes at a higher rate than water in specific segments.

The minimum volume of urine that must be excreted, calculated using the maximum urine concentrating ability.

A hormone that increases the permeability of the distal tubules and collecting ducts to water, allowing for greater water reabsorption in the kidneys.

The difference in osmolarity between the renal medullary interstitial fluid and the tubular fluid, which drives water absorption.

The process by which the loop of Henle actively transports sodium, chloride, and potassium ions from the thick ascending limb into the interstitium, creating a hyperosmolar environment in the renal medulla.

The structure that carries blood through the renal medulla, absorbing water that moves out of the tubules.

The highest concentration urine the kidneys can produce.

The process of moving solutes from the tubular fluid into the interstitial fluid, contributing to the high osmolarity of the renal medulla.

The process by which urea moves from the collecting ducts into the interstitial fluid, contributing to the high osmolarity of the renal medulla.

The descending limb of the loop of Henle is highly permeable to water, allowing water to move out from the tubule into the surrounding interstitium.

The thick ascending limb actively pumps out solutes, making the interstitial fluid hyperosmotic.

The active pump in the thick ascending limb can create a maximum concentration gradient of 200 mOsmoles per liter.

The active ion pump in the thick ascending limb creates a hyperosmotic interstitium, pulling water out of the descending limb and concentrating urine.

The high osmolarity in the interstitium is achieved through a repeated cycle of solute pumping and water movement.

The descending limb becomes more concentrated as water moves out, reaching up to 500 mOsmoles per liter.

The overall process of solute pumping and water movement results in a higher osmolarity in the renal medulla, enhancing urine concentration.

The osmolarity of the interstitium, created by the countercurrent multiplier system, can reach 1200 to 1400 mOsmoles per liter.

The fluid leaving the loop of Henle is dilute because it actively transports sodium out while being impermeable to water.

The amount of water reabsorbed in the cortical collecting tubule is controlled by the concentration of antidiuretic hormone (ADH).

When ADH levels are low, the cortical collecting tubule becomes almost impermeable to water, allowing for continued solute reabsorption and further dilution.

With high ADH levels, the cortical collecting tubule becomes permeable to water, leading to rapid water reabsorption by the peritubular capillaries.

The osmolarity of the medullary interstitium is maintained because water reabsorption occurs in the cortex, not the medulla.

When the kidney forms concentrated urine, urea is passively reabsorbed from the tubule into the renal interstitial fluid.

UT-A1 and UT-B transporters, activated by ADH, facilitate urea diffusion out of the tubule and into the renal interstitial fluid, contributing to the concentration gradient.

Even though urea is reabsorbed and concentrated in the renal medullary interstitium, its concentration in urine remains high due to the active transport of water out of the tubule.

The concentration of dissolved particles (osmolarity) in the blood increases as it descends into the renal medulla, due to both solutes entering the blood and water leaving into the interstitium. This concentration reaches 1200 milliosmoles at the bottom of the vasa recta.

The ascending limb of the loop of Henle is impermeable to water but actively reabsorbs solutes like sodium, chloride, and potassium. This results in further dilution of the tubular fluid, reaching a low osmolarity around 140 milliosmoles.

The late distal tubule and cortical collecting tubules are highly permeable to water under the influence of antidiuretic hormone (ADH). With high levels of ADH, water is reabsorbed, leading to concentrated urine.

The proximal tubule, despite its high permeability to water, maintains a comparatively consistent osmolarity due to the simultaneous movement of water and solutes. This keeps the osmolarity similar to plasma infiltrate.

The ascending loop of Henle reabsorbs water into the medulla, creating a hyperosmolar environment. This is facilitated by aquaporin-1 channels that are present in this limb.

The thick ascending loop of Henle is impermeable to water but actively pumps sodium, chloride, potassium, and other ions out of the tubule into the interstitial fluid. This pumping action contributes to the hyperosmolarity of the renal medulla.

Osmo-receptors in the hypothalamus detect changes in blood osmolarity (concentration of solutes) and trigger the release of ADH, which then regulates water reabsorption in the kidneys.

The thirst mechanism is triggered when blood osmolarity increases (meaning it becomes more concentrated), prompting the individual to drink water and restore fluid balance.