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The urinary system

Trace how a nephron turns plasma into urine, and explain how the kidneys adjust blood volume, pressure and pH.

Blood delivers and removes materials everywhere else in the body. Now look at how the body cleans that blood and decides how much water to keep.

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Filtration

Filtration uses blood pressure to push water and small solutes out of the glomerular capillaries into the capsule of a nephron. It is selective by size and charge: water, glucose, amino acids, ions and urea pass, while most plasma proteins, blood cells and platelets are held back.

For example

A damaged filtration membrane lets albumin escape, and protein appears in the urine even though the kidney has no tear in its drainage route.

Read all notes for this section

Filtration: Filtration uses blood pressure to push water and small solutes out of the glomerular capillaries into the capsule of a nephron. It is selective by size and charge: water, glucose, amino acids, ions and urea pass, while most plasma proteins, blood cells and platelets are held back. A damaged filtration membrane lets albumin escape, and protein appears in the urine even though the kidney has no tear in its drainage route.
Reabsorption: Reabsorption moves useful material from the tubular fluid back into the blood through the peritubular capillaries. It is the reason urine volume is small compared with the volume filtered: more than 99% of the filtrate returns. The proximal convoluted tubule does most of this work. Glucose is normally reabsorbed completely. If blood glucose is very high, the carriers can be saturated and glucose appears in the urine.
Secretion: Secretion moves substances from the blood into the tubular fluid. It removes hydrogen and potassium ions, ammonium and some drugs or wastes that were not filtered, so it complements filtration rather than repeating it. A weak acid drug can be removed faster when the urine is made more alkaline, because more of the drug stays in the tubular fluid instead of diffusing back.
ADH: Antidiuretic hormone (ADH), also called vasopressin, is released when body fluids become too concentrated or blood volume falls. It inserts water channels called aquaporins into collecting-duct cells, so more water returns to the blood and less is lost. When ADH is low, the duct stays impermeable to water and dilute urine is produced. After sweating heavily without drinking, ADH rises, urine volume falls and the urine is darker and more concentrated.
Renin–angiotensin–aldosterone: When blood volume or pressure falls, the kidney releases renin. Renin leads to angiotensin II, which raises sodium reabsorption and stimulates the adrenal cortex to release aldosterone. Aldosterone increases sodium reabsorption and potassium secretion in the collecting duct; water follows sodium, so blood volume and pressure rise. Blood loss triggers this chain. The kidneys hold on to sodium and water instead of letting them leave in urine.
Micturition: Micturition is the discharge of urine. When bladder volume exceeds about 200–400 mL, stretch receptors signal the sacral spinal cord. The micturition reflex contracts the detrusor muscle and relaxes the internal urethral sphincter. The external urethral sphincter is skeletal muscle, so the cortex can delay or start urination within limits. A full bladder gives a conscious sense of fullness before the reflex empties it, which is why a person can usually wait for a toilet.

Organs and the nephron

Two kidneys, two ureters, one bladder and one urethra make up the urinary system. Each kidney contains about one million nephrons, and each nephron begins with a renal corpuscle. Blood arrives under pressure, and water plus small solutes are filtered into a capsule. Everything filtered then flows along a tubule. What remains at the end of the collecting duct is urine, which drains through the renal pelvis and the ureter to the bladder.

Filtration, reabsorption and secretion

Three processes make urine. Filtration in the corpuscle is driven by blood pressure; it lets water and small solutes through but holds back most proteins and all blood cells. Reabsorption moves useful material from the tubule back into the blood — more than 99% of the filtrate returns. Secretion moves substances from the blood into the tubule for removal. The kidneys filter 150–180 litres a day but excrete only 1–2 litres of urine.

Concentrating urine and ADH

The loop of Henle builds a concentration gradient in the medulla. Its descending limb loses water, and its ascending limb reabsorbs salt without water, so the surrounding tissue becomes saltier with depth. That gradient lets the collecting duct reclaim water when the body needs it. Antidiuretic hormone (ADH) controls this step: with little ADH the duct stays impermeable to water and urine is dilute, and with more ADH water channels are inserted and urine becomes concentrated.

Blood pressure, pH and micturition

Hormones adjust sodium and water to protect blood volume and pressure. When blood volume or pressure falls, renin leads to angiotensin II, which increases sodium reabsorption and stimulates aldosterone; aldosterone increases sodium reabsorption and potassium secretion, and water follows sodium. Atrial natriuretic peptide works the other way when the atria are stretched, increasing sodium and water excretion. The kidneys also excrete hydrogen ions and reabsorb bicarbonate, so they adjust blood pH together with the lungs.

Explore the illustration

Look at the whole view first. Then select one label and connect it to the idea you just read.

From blood to urine: nephron and tract

A schematic teaching view, not to scale: a small figure places the kidneys in the body, a frontal kidney section shows cortex, medulla and pelvis, an enlarged nephron shows where filtration, reabsorption and secretion happen, and the lower part of the figure shows the tract from ureter to urethra.

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01 / 22 Body position reference

Blood arrives and is filtered under pressure

Guided anatomy tour: each step identifies one structure and explains its relationship to the next. The highlight moves; the anatomy stays still. Reduced motion is enabled. Use the arrows to study one step at a time, or choose Play.

Body position reference

The kidneys lie high on the posterior abdominal wall, one on each side of the vertebral column. The right kidney usually sits slightly lower because the liver occupies space above it. They are retroperitoneal, meaning they lie behind the peritoneum rather than inside it.

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In-depth reading

Follow this example step by step, then try explaining it yourself. The extra reading is optional: you can return to it after exploring the illustration.

Worked example: why a hot day changes your urine

You work outside on a hot day and sweat heavily. Water leaves the body through the skin, so the water content of your blood falls and the concentration of solutes in your body fluids rises.
Receptors detect the higher osmolarity, and the posterior pituitary releases more antidiuretic hormone (ADH) into the blood.
ADH reaches the collecting ducts of the nephrons and inserts aquaporin-2 water channels into the cells lining them. Because the medulla around the ducts is salty, water moves out of the duct by osmosis and returns to the blood.
Less water stays in the tubular fluid, so a smaller volume of more concentrated urine is produced. Filtration itself has barely changed; what changed is how much of the filtrate was reabsorbed.

Explain it yourself

The next day you drink two litres of water while resting. What happens to ADH, to water reabsorption in the collecting duct and to urine volume, and why?

Show the explanation

ADH falls. Fewer aquaporin-2 channels are inserted, so the collecting duct is less permeable to water and less water is reabsorbed. More water therefore stays in the tubular fluid and a larger volume of dilute urine is produced. The loop of Henle still builds the medullary gradient; without ADH the duct simply cannot use it to reclaim water.

More detail — when you are ready

These notes expand the mechanisms and vocabulary. Read one topic at a time and find the structures in the illustration.

From kidney to nephron: the parts and their work

The kidneys sit high on the posterior abdominal wall, behind the peritoneum — a position called retroperitoneal. The right kidney usually lies slightly lower than the left because the liver occupies the space above it. Each kidney is held by a renal capsule, an adipose capsule and renal fascia.

A frontal section shows two regions. The outer renal cortex is pale and contains the renal corpuscles. The inner renal medulla is darker and contains cone-shaped renal pyramids; the loop of Henle and the collecting ducts run through it. Each pyramid ends at a papilla, where urine drains into a minor calyx. Minor calyces join into major calyces, which join into the renal pelvis; the pelvis narrows into the ureter.

The kidneys are small — less than 0.5% of body mass — but receive 20–25% of the resting cardiac output, about 1200 mL per minute in adults. This large blood supply is what allows them to keep adjusting plasma composition continuously.

About one million nephrons are packed into each kidney. A nephron has two parts: a renal corpuscle, where plasma is filtered, and a renal tubule, through which the filtered fluid passes. The corpuscle contains the glomerulus, a knot of capillaries, inside the glomerular (Bowman's) capsule. The tubule then runs as the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule. Several nephrons drain into one collecting duct. Collecting ducts and papillary ducts are not part of a nephron, but they complete the route to the calyces.

About 80–85% of nephrons are cortical: their corpuscles lie in the outer cortex and their short loops reach only a little way into the medulla. The rest are juxtamedullary; their long loops reach deep into the medulla, and those long loops are the ones that build the salt gradient used to concentrate urine.

Filtration: pressure, barriers and what gets through

The fluid entering the capsular space is the glomerular filtrate. In adults the kidneys filter about 150 litres a day in females and 180 litres in males. More than 99% of that returns to the blood by reabsorption, so only 1–2 litres leave as urine. The rate at which all the corpuscles together form filtrate is the glomerular filtration rate (GFR): about 125 mL per minute in males and 105 mL per minute in females.

Filtration is selective because three layers form the filtration membrane. Glomerular endothelial cells have large fenestrations that let plasma solutes out but hold back blood cells and platelets. The basal lamina, a layer of collagen fibres and proteoglycans, blocks larger plasma proteins. Outside it, podocytes wrap the capillaries with foot-like processes called pedicels; the narrow filtration slits between them are covered by a slit membrane that admits molecules below about 0.006–0.007 µm. Water, glucose, vitamins, amino acids, ammonia, urea and ions pass. Albumin is just too large, so less than 1% of it crosses.

Three pressures decide net filtration. Glomerular blood hydrostatic pressure (GBHP, about 55 mmHg) pushes fluid out and promotes filtration. Capsular hydrostatic pressure (CHP, about 15 mmHg) and blood colloid osmotic pressure (BCOP, about 30 mmHg, from plasma proteins) both oppose filtration. Net filtration pressure is 55 − 15 − 30 = 10 mmHg. That small outward pressure is enough because the membrane is thin and porous and the glomerular surface area is large.

GFR stays remarkably steady while mean arterial pressure is between about 80 and 180 mmHg. Two mechanisms achieve this: adjusting blood flow into and out of the glomerulus, and changing the surface area available for filtration. Sympathetic nerves can override this when the whole body needs to defend its blood pressure.

Reabsorption, secretion and concentrating the urine

Reabsorption returns substances from the tubular fluid to the blood. The proximal convoluted tubule does most of it: glucose, amino acids, sodium, water and bicarbonate return there. Reabsorption can be saturated. Glucose carriers normally keep glucose out of the urine entirely, but when blood glucose is very high the carriers work at their maximum and the extra glucose stays in the tubule — which is why glucose in urine is a warning sign rather than a normal finding.

Secretion moves substances the other way, from the blood into the tubular fluid. Hydrogen ions, potassium ions, ammonium and some drugs and wastes are added here. Secretion adjusts blood pH and potassium level, and it removes substances that filtration did not remove.

The loop of Henle makes it possible to produce concentrated urine. Its descending limb is permeable to water and impermeable to most solutes, so water leaves and the fluid becomes more concentrated as it descends. Its thick ascending limb reabsorbs sodium, potassium and chloride but is impermeable to water, so the fluid becomes more dilute — down to about 150 mOsm/litre — while salt accumulates in the medullary interstitial fluid. The result is a gradient that rises from about 300 mOsm/litre in the outer medulla to about 1200 mOsm/litre deep inside. Blood flow in the vasa recta runs in the opposite direction (countercurrent) so it carries water away without washing out the salt.

Collecting-duct cells then decide how much water is reclaimed. In the absence of ADH they are impermeable to water and urine is dilute, with an osmolarity that can fall as low as 65 mOsm/litre. When ADH is present, aquaporin-2 water channels are inserted into the apical membrane, water moves out into the salty medulla and the urine becomes concentrated.

Hormones, pH and micturition

Blood volume and pressure are adjusted by hormones acting on the tubule, not only by the heart and vessels.

When blood volume or pressure falls, the kidney releases renin. Renin leads to angiotensin II, which raises sodium reabsorption in the proximal tubule and stimulates the adrenal cortex to release aldosterone. Aldosterone increases sodium reabsorption and potassium secretion in the collecting duct; because water follows sodium, blood volume and pressure rise.

Antidiuretic hormone (ADH) is released when extracellular fluid osmolarity rises or blood volume falls. It increases facultative water reabsorption, which dilutes body fluids back towards normal. Atrial natriuretic peptide (ANP) is released when the atria are stretched by too much blood. It suppresses sodium and water reabsorption in the proximal tubule and collecting duct and inhibits aldosterone and ADH, so sodium and water leave in the urine and blood volume falls. Parathyroid hormone increases calcium reabsorption in the early distal tubule when plasma calcium is low.

The kidneys also regulate pH. They excrete hydrogen ions and reabsorb filtered bicarbonate, and they can secrete ammonium. Together with the lungs, which adjust carbon dioxide, they keep blood pH within its narrow range. Because the lungs handle carbon dioxide quickly and the kidneys adjust bicarbonate and hydrogen ions more slowly, the two systems complement each other.

Micturition is the discharge of urine. When bladder volume exceeds about 200–400 mL, stretch receptors signal the micturition centre in sacral spinal cord segments S2 and S3. Parasympathetic impulses contract the detrusor muscle and relax the internal urethral sphincter, while somatic motor neurons to the external urethral sphincter are inhibited. The external sphincter is skeletal muscle, so the cerebral cortex can start or delay urination within limits — which is why bladder emptying is a reflex that we learn to control.

Normal urine is mostly water with urea, creatinine, ions and other dissolved wastes. Its volume, concentration, colour and odour vary with fluid intake, diet and activity. Persistently unusual findings, such as a large volume of very dilute urine, blood or protein, are clinical findings that need proper assessment rather than self-diagnosis.