How Your Kidneys Filter About 180 Litres of Fluid a Day
Compare ammonotelic, ureotelic and uricotelic animals, learn the human excretory system and nephron, the steps of urine formation and the counter-current mechanism, hormonal control of the kidney, other excretory organs, and kidney disorders with dialysis and transplantation.
Why must the body get rid of nitrogen?
Every time your cells break down proteins, they produce nitrogen-containing waste. Left to build up, it would poison the body, so the kidneys filter the blood without rest and flush the waste out as urine.
This lesson covers nitrogenous wastes and the nephron, urine formation, hormonal control and other excretory organs, and kidney disorders with dialysis.
This lesson covers nitrogenous wastes and the nephron, urine formation, hormonal control and other excretory organs, and kidney disorders with dialysis.
How do ammonotelic, ureotelic and uricotelic animals differ, and what is the structure of the human excretory system and nephron?
Animals excrete nitrogen mainly as ammonia, urea or uric acid depending on how much water they can spare; humans are ureotelic, with two kidneys, ureters, a urinary bladder and a urethra, and each kidney holds nearly a million nephrons.
Nitrogenous wastes:
- Ammonotelic — ammonia; most toxic, needs much water; bony fish, aquatic amphibians and aquatic insects
- Ureotelic — urea, made in the liver; less toxic; mammals and many terrestrial amphibians
- Uricotelic — uric acid as a paste or pellet; least toxic, needs little water; reptiles, birds, land snails and insects
Human excretory system. Bean-shaped kidneys, each with an outer cortex and an inner medulla, send urine through the ureters to the urinary bladder, which empties through the urethra.
Structure of a nephron:
- Glomerulus — a tuft of capillaries between the afferent arteriole and the narrower efferent arteriole
- Bowman's capsule — a cup around the glomerulus; together they form the Malpighian body
- Proximal convoluted tubule (PCT), Henle's loop, distal convoluted tubule (DCT) and collecting duct, with the vasa recta running beside the loop
An everyday example. Bird droppings on a balcony railing are white and pasty because birds excrete uric acid, saving water.
The substance. Urea is made in the liver, not the kidney — the kidney only removes it from the blood.
Nitrogenous wastes:
- Ammonotelic — ammonia; most toxic, needs much water; bony fish, aquatic amphibians and aquatic insects
- Ureotelic — urea, made in the liver; less toxic; mammals and many terrestrial amphibians
- Uricotelic — uric acid as a paste or pellet; least toxic, needs little water; reptiles, birds, land snails and insects
Human excretory system. Bean-shaped kidneys, each with an outer cortex and an inner medulla, send urine through the ureters to the urinary bladder, which empties through the urethra.
Structure of a nephron:
- Glomerulus — a tuft of capillaries between the afferent arteriole and the narrower efferent arteriole
- Bowman's capsule — a cup around the glomerulus; together they form the Malpighian body
- Proximal convoluted tubule (PCT), Henle's loop, distal convoluted tubule (DCT) and collecting duct, with the vasa recta running beside the loop
An everyday example. Bird droppings on a balcony railing are white and pasty because birds excrete uric acid, saving water.
The substance. Urea is made in the liver, not the kidney — the kidney only removes it from the blood.
How is urine formed, and how does the counter-current mechanism concentrate it?
Urine is formed by glomerular filtration, reabsorption and tubular secretion, and the counter-current mechanism of Henle's loop and the vasa recta makes the medulla salty, so the collecting duct can withdraw water and concentrate the urine.
1. Glomerular filtration. Blood pressure forces plasma, without its proteins, into Bowman's capsule. The glomerular filtration rate (GFR) is about 125 mL per minute, or about 180 litres per day.
2. Reabsorption. Nearly 99 per cent of the filtrate is reabsorbed, leaving about 1.5 litres of urine a day.
- PCT — takes back nearly all glucose and amino acids and much of the sodium and water
- Descending limb — permeable to water, almost impermeable to salts
- Ascending limb — impermeable to water but lets salts out
- DCT and collecting duct — take back water and sodium under hormonal control
3. Tubular secretion. Hydrogen ions, potassium ions and ammonia are secreted into the filtrate, keeping body fluids balanced.
Counter-current mechanism:
- Filtrate flows in opposite directions in the two limbs of Henle's loop, and blood does the same in the vasa recta
- Salt from the ascending limb and urea from the collecting duct raise the concentration of the medulla from about 300 to about 1200 mOsmol per litre
- Water leaves the collecting duct into this salty medulla, making urine up to about four times as concentrated as the filtrate
An everyday example. After a long afternoon of cricket in the heat, urine becomes darker and smaller in volume because the kidneys are saving water.
The substance. Glucose in urine is a warning sign — the PCT normally takes all of it back, so glucose in urine suggests diabetes.
1. Glomerular filtration. Blood pressure forces plasma, without its proteins, into Bowman's capsule. The glomerular filtration rate (GFR) is about 125 mL per minute, or about 180 litres per day.
2. Reabsorption. Nearly 99 per cent of the filtrate is reabsorbed, leaving about 1.5 litres of urine a day.
- PCT — takes back nearly all glucose and amino acids and much of the sodium and water
- Descending limb — permeable to water, almost impermeable to salts
- Ascending limb — impermeable to water but lets salts out
- DCT and collecting duct — take back water and sodium under hormonal control
3. Tubular secretion. Hydrogen ions, potassium ions and ammonia are secreted into the filtrate, keeping body fluids balanced.
Counter-current mechanism:
- Filtrate flows in opposite directions in the two limbs of Henle's loop, and blood does the same in the vasa recta
- Salt from the ascending limb and urea from the collecting duct raise the concentration of the medulla from about 300 to about 1200 mOsmol per litre
- Water leaves the collecting duct into this salty medulla, making urine up to about four times as concentrated as the filtrate
An everyday example. After a long afternoon of cricket in the heat, urine becomes darker and smaller in volume because the kidneys are saving water.
The substance. Glucose in urine is a warning sign — the PCT normally takes all of it back, so glucose in urine suggests diabetes.
How do hormones regulate the kidneys, and how do the skin, liver and lungs help in excretion?
Kidney function is regulated by ADH, the renin-angiotensin mechanism, aldosterone and atrial natriuretic factor, while the lungs, liver and skin also remove wastes.
Hormonal regulation:
- ADH (vasopressin) — released from the posterior pituitary when body fluid is too concentrated; increases water reabsorption in the DCT and collecting duct
- Renin-angiotensin mechanism — when blood flow or pressure falls, the juxtaglomerular apparatus releases renin, which turns angiotensinogen into angiotensin I and then angiotensin II; angiotensin II narrows blood vessels and raises blood pressure
- Aldosterone — released from the adrenal cortex when angiotensin II acts on it; increases sodium and water reabsorption
- Atrial natriuretic factor (ANF) — released from the heart atria when blood pressure rises; widens blood vessels and lowers pressure
Other excretory organs:
- Lungs — remove carbon dioxide and some water
- Liver — passes pigments, cholesterol and broken-down steroid hormones into bile
- Skin — sweat removes salt, a little urea and lactic acid; sebum removes sterols and waxes
An everyday example. Feeling very thirsty and passing little urine after a day out in the sun is ADH at work, saving water.
The substance. Alcohol increases urine output — it suppresses ADH release, which is one reason it causes dehydration.
Hormonal regulation:
- ADH (vasopressin) — released from the posterior pituitary when body fluid is too concentrated; increases water reabsorption in the DCT and collecting duct
- Renin-angiotensin mechanism — when blood flow or pressure falls, the juxtaglomerular apparatus releases renin, which turns angiotensinogen into angiotensin I and then angiotensin II; angiotensin II narrows blood vessels and raises blood pressure
- Aldosterone — released from the adrenal cortex when angiotensin II acts on it; increases sodium and water reabsorption
- Atrial natriuretic factor (ANF) — released from the heart atria when blood pressure rises; widens blood vessels and lowers pressure
Other excretory organs:
- Lungs — remove carbon dioxide and some water
- Liver — passes pigments, cholesterol and broken-down steroid hormones into bile
- Skin — sweat removes salt, a little urea and lactic acid; sebum removes sterols and waxes
An everyday example. Feeling very thirsty and passing little urine after a day out in the sun is ADH at work, saving water.
The substance. Alcohol increases urine output — it suppresses ADH release, which is one reason it causes dehydration.
What are the disorders of the excretory system, and how do dialysis and kidney transplants work?
Kidney disorders such as uraemia, renal failure, kidney stones and glomerulonephritis stop the kidneys from clearing wastes, and they are treated by haemodialysis or a kidney transplant.
Disorders:
- Uraemia — urea builds up in the blood when the kidneys fail
- Renal calculi — kidney stones of crystallised salts such as oxalates
- Glomerulonephritis — inflammation of the glomeruli
Haemodialysis:
- Blood from an artery is mixed with the anticoagulant heparin and pumped into a dialysing unit
- It flows through a coiled cellophane tube bathed in dialysing fluid, which matches plasma but has no nitrogenous wastes
- Wastes diffuse out into the fluid, and the cleaned blood receives anti-heparin and returns through a vein
Kidney transplantation. A working kidney from a donor, preferably a close relative, replaces the failed one, and immune-suppressing drugs help prevent rejection.
An everyday example. Patients visiting a dialysis centre at a district hospital every week have their blood cleaned artificially because their own kidneys have failed.
The substance. Dialysis removes wastes but cannot do every job of the kidney — it does not make hormones such as erythropoietin, which is why patients may still become anaemic.
Disorders:
- Uraemia — urea builds up in the blood when the kidneys fail
- Renal calculi — kidney stones of crystallised salts such as oxalates
- Glomerulonephritis — inflammation of the glomeruli
Haemodialysis:
- Blood from an artery is mixed with the anticoagulant heparin and pumped into a dialysing unit
- It flows through a coiled cellophane tube bathed in dialysing fluid, which matches plasma but has no nitrogenous wastes
- Wastes diffuse out into the fluid, and the cleaned blood receives anti-heparin and returns through a vein
Kidney transplantation. A working kidney from a donor, preferably a close relative, replaces the failed one, and immune-suppressing drugs help prevent rejection.
An everyday example. Patients visiting a dialysis centre at a district hospital every week have their blood cleaned artificially because their own kidneys have failed.
The substance. Dialysis removes wastes but cannot do every job of the kidney — it does not make hormones such as erythropoietin, which is why patients may still become anaemic.
Exam tip
What earns full marks on excretion and the nephron?
Label a nephron diagram in the order filtrate flows — Bowman's capsule, PCT, descending limb, ascending limb, DCT, collecting duct — and mark which parts let water through.
- Ammonotelic: ammonia; ureotelic: urea; uricotelic: uric acid
- ADH saves water; aldosterone saves sodium; ANF lowers blood pressure
The trap. Writing that the ascending limb reabsorbs water. The ascending limb is impermeable to water; the descending limb lets water out.
- Ammonotelic: ammonia; ureotelic: urea; uricotelic: uric acid
- ADH saves water; aldosterone saves sodium; ANF lowers blood pressure
The trap. Writing that the ascending limb reabsorbs water. The ascending limb is impermeable to water; the descending limb lets water out.
Did you know
How do desert rats survive without drinking water?
Desert mammals such as the kangaroo rat have unusually long loops of Henle, which build a very steep salt gradient in the kidney medulla.
That gradient lets their kidneys pull back almost all the water from the filtrate, producing extremely concentrated urine. Much of the water they need comes from breaking down their food rather than from drinking.
That gradient lets their kidneys pull back almost all the water from the filtrate, producing extremely concentrated urine. Much of the water they need comes from breaking down their food rather than from drinking.
Exam relevance
Why does NEET keep returning to the nephron and urine formation?
Excretory Products and their Elimination is a recurring NEET chapter, and the nephron sits at its centre.
What gets asked. Examples of ammonotelic, ureotelic and uricotelic animals, the job of each part of the nephron, GFR and the counter-current mechanism, and the roles of ADH, renin, aldosterone and ANF.
Question types. Mostly statement-based and match-the-column questions, with diagram-based questions on a labelled nephron.
Why it matters later. ADH and aldosterone return in Chemical Coordination and Integration, and blood pressure control links back to Body Fluids and Circulation.
The trap that costs marks. Mixing up ANF and the renin-angiotensin mechanism — renin and angiotensin raise blood pressure, while ANF lowers it.
What gets asked. Examples of ammonotelic, ureotelic and uricotelic animals, the job of each part of the nephron, GFR and the counter-current mechanism, and the roles of ADH, renin, aldosterone and ANF.
Question types. Mostly statement-based and match-the-column questions, with diagram-based questions on a labelled nephron.
Why it matters later. ADH and aldosterone return in Chemical Coordination and Integration, and blood pressure control links back to Body Fluids and Circulation.
The trap that costs marks. Mixing up ANF and the renin-angiotensin mechanism — renin and angiotensin raise blood pressure, while ANF lowers it.
Key takeaways
What must you be able to do from this lesson?
- Wastes and structure: ammonia, urea and uric acid; kidneys, ureters, bladder and urethra; the nephron from glomerulus to collecting duct
- Urine formation: filtration, reabsorption and secretion, with a counter-current mechanism that concentrates urine
- Regulation and other organs: ADH, renin-angiotensin, aldosterone and ANF; lungs, liver and skin
- Disorders and treatment: uraemia, kidney stones and glomerulonephritis, with haemodialysis and transplantation
If GFR is 125 mL per minute, how many litres of filtrate do the kidneys form in a day — and why is so little of it lost as urine?
- Urine formation: filtration, reabsorption and secretion, with a counter-current mechanism that concentrates urine
- Regulation and other organs: ADH, renin-angiotensin, aldosterone and ANF; lungs, liver and skin
- Disorders and treatment: uraemia, kidney stones and glomerulonephritis, with haemodialysis and transplantation
If GFR is 125 mL per minute, how many litres of filtrate do the kidneys form in a day — and why is so little of it lost as urine?