IMAT Biology · Human Anatomy and Physiology

The Kidney, the Nephron and Water Balance

The kidney filters the blood under pressure, takes back everything useful, and adjusts how much water it keeps under the control of ADH.

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Why this matters

The kidney appears in 2012, 2019, 2020 and twice in 2022: which parts of a nephron contain urea, which parts reabsorb water, what forces water out of the glomerulus, and how a change in ADH changes the urine.

Concept 1 of 3: The nephron: filtration, reabsorption and secretion

The kidney works by throwing almost everything out and then taking back what it wants. High pressure squeezes a large volume of fluid out of the blood; the tubule then reclaims the glucose, salts and most of the water, and what is left becomes urine.

Definition

Each kidney has about a million nephrons. The outer cortex holds the glomeruli and the convoluted tubules; the inner medulla holds the loops of Henle and the collecting ducts. Urine flows to the renal pelvis, ureter, bladder and urethra.

Urea is made in the liver from excess amino acids (deamination) and carried to the kidney in the blood.
Ultrafiltration: the arteriole entering the glomerulus (afferent) is wider than the one leaving it (efferent), so the pressure is high. Small molecules pass into Bowman's capsule; blood cells and large proteins stay in the blood.
About 180 L of filtrate is made per day but only about 1.5 L of urine, so about 99% of the water is reabsorbed.
Urea is filtered freely, so it is present in every part of the nephron. Its concentration rises along the tubule as water is removed.
Glucose in the urine means more glucose was filtered than the tubule could reabsorb, as in untreated diabetes.

Region	What happens	Water reabsorbed here?
Glomerulus and Bowman's capsule	Ultrafiltration: water, glucose, amino acids, ions and urea leave the blood	No: this is filtration, not reabsorption
Proximal convoluted tubule	Reabsorbs all glucose and amino acids and most $\mathrm{Na^+}$ ; cells have microvilli and many mitochondria	Yes, about two thirds of it
Descending limb of the loop of Henle	Permeable to water, which leaves into the salty medulla	Yes
Ascending limb of the loop of Henle	Pumps out $\mathrm{Na^+}$ and $\mathrm{Cl^-}$ , making the medulla salty; impermeable to water	No
Distal convoluted tubule	Adjusts $\mathrm{Na^+}$ , $\mathrm{K^+}$ and $\mathrm{H^+}$ (blood pH)	Some
Collecting duct	Water leaves by osmosis into the salty medulla, controlled by ADH	Yes, a variable amount

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Urea is in every part of the nephron

Urea is filtered in the glomerulus and is never fully reabsorbed, so the fluid in every region, from the capsule to the collecting duct, contains it. An option naming a region with no urea is wrong.

The glomerulus filters; it does not reabsorb

Water leaves the blood in the glomerulus. Reabsorption of water happens in the tubules and the collecting duct, so the glomerulus is the one structure not involved in water reabsorption.

Concept 2 of 3: Net filtration pressure in the glomerulus

Three pressures act across the glomerulus. Blood pressure pushes fluid out. The proteins left in the blood pull water back by osmosis, and the fluid already in the capsule pushes back. Filtration happens only while the push out is bigger than the two pushes back.

Definition

Glomerular blood pressure comes from the contraction of the left ventricle, kept high because the efferent arteriole is narrow. It drives filtration.
Oncotic pressure: plasma proteins cannot be filtered, so they stay in the blood and draw water back by osmosis. It opposes filtration.
Capsule pressure: the fluid already in Bowman's capsule pushes back. It opposes filtration.
If blood pressure falls sharply (for example after heavy bleeding), filtration can stop.

Net filtration pressure

P_{\text{net}} = P_G - (P_C + \pi_G)

$P_G$ blood pressure in the glomerular capillaries, in mmHg
$P_C$ fluid pressure in Bowman's capsule, in mmHg
$\pi_G$ oncotic pressure of the plasma proteins, in mmHg

Worked example

In a glomerulus the capillary blood pressure is 55 mmHg, the capsule pressure is 15 mmHg and the oncotic pressure is 30 mmHg. Find the net filtration pressure. What happens if heavy bleeding lowers the capillary pressure to 45 mmHg?

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Blood proteins oppose filtration

Plasma proteins stay in the blood and hold water there by osmosis. They pull water back into the capillaries, so they never help water move into the tubule. The only force pushing water out is the blood pressure from the heart.

Concept 3 of 3: Osmoregulation: ADH, aldosterone and the concentration of urine

When the blood gets too concentrated, the brain releases a hormone that makes the collecting ducts leaky to water. Water then flows out of the duct into the salty medulla and back into the blood, leaving a small volume of concentrated urine.

Definition

Osmoreceptors in the hypothalamus detect the water content (water potential) of the blood.
ADH (antidiuretic hormone, also called vasopressin) is made in the hypothalamus and released into the blood from the posterior pituitary.
ADH makes the cells of the collecting duct (and the distal tubule) insert aquaporins, water channels, into their membranes, so more water is reabsorbed.
Antidiuretic means less urine. More ADH: small volume of concentrated urine. Less ADH: large volume of dilute urine. The same amount of urea is then dissolved in more water, so its concentration falls.
Aldosterone, from the adrenal cortex, makes the distal tubule and collecting duct reabsorb more $\mathrm{Na^+}$ when blood volume or pressure is low; water follows.
Alcohol reduces ADH release. A lack of ADH causes diabetes insipidus: large volumes of dilute urine.

Situation	ADH release	Collecting duct	Urine
Dehydration, sweating or a salty meal	Increases	More aquaporins, more water reabsorbed	Small volume, concentrated
Drinking a lot of water	Decreases	Fewer aquaporins, less water reabsorbed	Large volume, dilute
Alcohol, or a drug that blocks ADH release	Decreases	Less water reabsorbed	Large volume; lower urea concentration
Damage to the posterior pituitary	Little or none	Almost no water reabsorbed there	Very large volumes of dilute urine

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Less ADH means more urine but lower urea concentration

If urea production is unchanged, less ADH leaves more water in the urine, so the same urea is more diluted and its concentration falls. The volume rises; the concentration does not.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Net filtration pressure in the glomerulus
Net filtration pressure
$P_{\text{net}} = P_G - (P_C + \pi_G)$

Reference tables (2)

The nephron: filtration, reabsorption and secretion6 rows

Region	What happens	Water reabsorbed here?
Glomerulus and Bowman's capsule	Ultrafiltration: water, glucose, amino acids, ions and urea leave the blood	No: this is filtration, not reabsorption
Proximal convoluted tubule	Reabsorbs all glucose and amino acids and most $\mathrm{Na^+}$ ; cells have microvilli and many mitochondria	Yes, about two thirds of it
Descending limb of the loop of Henle	Permeable to water, which leaves into the salty medulla	Yes
Ascending limb of the loop of Henle	Pumps out $\mathrm{Na^+}$ and $\mathrm{Cl^-}$ , making the medulla salty; impermeable to water	No
Distal convoluted tubule	Adjusts $\mathrm{Na^+}$ , $\mathrm{K^+}$ and $\mathrm{H^+}$ (blood pH)	Some
Collecting duct	Water leaves by osmosis into the salty medulla, controlled by ADH	Yes, a variable amount

Osmoregulation: ADH, aldosterone and the concentration of urine4 rows

Situation	ADH release	Collecting duct	Urine
Dehydration, sweating or a salty meal	Increases	More aquaporins, more water reabsorbed	Small volume, concentrated
Drinking a lot of water	Decreases	Fewer aquaporins, less water reabsorbed	Large volume, dilute
Alcohol, or a drug that blocks ADH release	Decreases	Less water reabsorbed	Large volume; lower urea concentration
Damage to the posterior pituitary	Little or none	Almost no water reabsorbed there	Very large volumes of dilute urine

Watch out for (4)

Urea is in every part of the nephron
→ The nephron: filtration, reabsorption and secretion
The glomerulus filters; it does not reabsorb
→ The nephron: filtration, reabsorption and secretion
Blood proteins oppose filtration
→ Net filtration pressure in the glomerulus
Less ADH means more urine but lower urea concentration
→ Osmoregulation: ADH, aldosterone and the concentration of urine

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