Every moment, your cells are carrying out chemical reactions. These reactions keep you alive, but they also produce substances that must not be allowed to accumulate in the body. One important example is urea, a nitrogen-containing waste substance that is carried in the blood.
This creates an interesting problem. The kidneys must remove unwanted substances from the blood without throwing away everything that happens to be dissolved in it. Glucose, amino acids, mineral ions and water are small enough to enter the early filtrate too. The kidney therefore works in two broad stages: it first filters, then it takes back what the body needs. Understanding those two stages makes the whole topic much easier.
Try these five questions before reading the chapter. They target the ideas students most often mix up when first learning about excretion and urine formation.
- 5 diagnostic questions
- Common kidney and excretion misconceptions
- Instant explanation for every answer option
What Is Excretion?
Excretion is the removal of metabolic waste products and toxic substances from the body. The word metabolic is important. It tells us that these wastes arise from chemical reactions taking place inside the body.
One important nitrogen-containing waste is urea. Urea is formed in the liver when excess amino acids are broken down. It is carried in the blood to the kidneys, where much of it is removed and eventually leaves the body in urine.
Why must wastes be removed? If metabolic waste products accumulate, they can interfere with normal cellular processes and become harmful to the body.
Excretion is not the same as egestion
Students often use these two words as though they mean the same thing, but biologically they describe different processes.
| Process | What is removed? | Example |
|---|---|---|
| Excretion | Metabolic waste products and toxic substances | Urea removed from the blood by the kidneys |
| Egestion | Undigested material that was not absorbed from the alimentary canal | Faeces leaving the body |
The Human Urinary System
The urinary system provides a route for waste substances removed by the kidneys to leave the body. You should be able to identify the kidneys, ureters, bladder and urethra and state what each structure does.
| Structure | Function |
|---|---|
| Kidneys | Process the blood, removing urea and regulating the amounts of water and mineral ions that leave the body in urine. |
| Ureters | Tubes that carry urine from the kidneys to the bladder. |
| Bladder | A muscular organ that temporarily stores urine. |
| Urethra | Carries urine from the bladder to the outside of the body. |
Urine pathway: kidney → ureter → bladder → urethra → outside the body
Blood entering and leaving a kidney
Blood reaches each kidney through a renal artery. This blood contains useful substances as well as waste substances such as urea. After the kidney has processed the blood, blood leaves through the renal vein.
Blood in the renal vein still contains some urea. The kidney does not remove every urea molecule in a single passage, but the amount of urea in the blood is reduced as the kidneys continually process the circulation.
The Nephron: Where Urine Is Produced
Each kidney contains many microscopic units called nephrons. A nephron consists of a renal corpuscle followed by a long tubule. This is where the composition of the filtrate is changed until urine is eventually formed.
Do not try to memorise the nephron as a complicated drawing. Instead, follow the path of the filtrate and connect the structures to the two processes that matter most: ultra-filtration and selective reabsorption.
Stage 1 — Ultra-filtration: small substances are forced from the blood into Bowman's capsule to form the glomerular filtrate.
Stage 2 — Selective reabsorption: useful substances are transferred from the filtrate back into the blood as the filtrate passes along the nephron.
Stage 1: Ultra-filtration
At the beginning of the nephron is a network of capillaries called the glomerulus, surrounded by Bowman's capsule. Blood in the glomerulus is under high pressure.
This pressure forces water and small dissolved substances through the filtration barrier and into Bowman's capsule. This process is called ultra-filtration.
The fluid produced is called the glomerular filtrate. It contains substances such as water, glucose, amino acids, mineral ions and urea.
Blood cells and large plasma proteins normally remain in the blood because they are too large to pass through the filtration barrier.
| Substance | Normally enters the filtrate? | Reason |
|---|---|---|
| Water | Yes | Small enough to pass through the filtration barrier |
| Glucose | Yes | Small enough to be filtered |
| Amino acids | Yes | Small enough to be filtered |
| Mineral ions | Yes | Small enough to be filtered |
| Urea | Yes | Small enough to be filtered |
| Large plasma proteins | No | Too large to pass through the filtration barrier |
| Blood cells | No | Too large to pass through the filtration barrier |
Important: Ultra-filtration does not separate substances according to whether they are useful or waste. Glucose and amino acids are useful, but they still enter the filtrate because they are small enough to pass through the filtration barrier.
Stage 2: Selective reabsorption
If all the glomerular filtrate simply became urine, the body would lose large amounts of useful substances. The nephron therefore takes selected substances back into the blood.
This process is called selective reabsorption.
Much of this reabsorption occurs in the proximal convoluted tubule. In a healthy person, all glucose and amino acids are normally reabsorbed. Much of the water and mineral ions required by the body are also returned to the blood.
Further along the nephron, additional water and mineral ions can be reabsorbed. The fluid that remains eventually passes into the collecting duct and contributes to the formation of urine.
Urine normally contains urea, excess mineral ions and water.
| Substance | In early filtrate? | Normally in final urine? |
|---|---|---|
| Glucose | Yes | No — normally reabsorbed |
| Amino acids | Yes | No — normally reabsorbed |
| Urea | Yes | Yes |
| Water | Yes | Yes — some remains in the urine |
| Mineral ions | Yes | Yes — excess ions may be excreted |
| Large plasma proteins | No | No |
| Blood cells | No | No |
Putting the two processes together
Ultra-filtration: blood → small substances enter Bowman's capsule.
Selective reabsorption: filtrate → useful substances return to the blood.
Final result: the remaining fluid contributes to the formation of urine.
This distinction is one of the most important ideas in the chapter. Ultra-filtration puts substances into the nephron; selective reabsorption takes useful substances back out of the filtrate.
How the Kidneys Link to Water Balance
The amount of water reabsorbed from the nephron can also be adjusted, helping the body maintain a suitable water balance. This connects kidney function to osmoregulation and anti-diuretic hormone (ADH).
Link to Homeostasis: more ADH causes more water to be reabsorbed into the blood, producing a smaller volume of more concentrated urine. Less ADH causes less water to be reabsorbed, producing a larger volume of more dilute urine.
ADH is explored more fully when you study homeostasis. For now, the important connection is that the kidney can vary how much water is returned to the blood.
Kidney Failure and Dialysis
If the kidneys fail, they can no longer regulate the composition of the blood effectively. Waste products such as urea may accumulate, while the concentrations of water and mineral ions can also become abnormal.
Dialysis is a treatment that removes unwanted substances from the blood when the kidneys cannot perform this function adequately.
How dialysis works
During haemodialysis, blood from the patient is passed through a dialysis machine. Inside the machine, the blood flows alongside dialysis fluid, with the two fluids separated by a partially permeable membrane.
Small dissolved substances can cross this membrane, while blood cells and large plasma proteins remain in the blood.
| Feature of dialysis | Why it matters |
|---|---|
| Fresh dialysis fluid contains little or no urea | Maintains a concentration gradient so urea diffuses from the blood into the dialysis fluid |
| Dialysis fluid contains glucose at approximately the normal blood concentration | Reduces net loss of useful glucose from the blood |
| Dialysis fluid contains suitable concentrations of mineral ions | Allows excess ions to be removed without unnecessarily removing ions required by the body |
| Partially permeable membrane | Allows small substances to cross while retaining blood cells and large plasma proteins |
| Large membrane surface area | Provides a large area for diffusion |
| Fresh dialysis fluid is continually supplied | Helps maintain concentration gradients for diffusion |
Dialysis depends on concentration gradients
Suppose the patient's blood contains a high concentration of urea while fresh dialysis fluid contains little or no urea. A concentration gradient exists across the membrane, so there is a net diffusion of urea from the blood into the dialysis fluid.
Glucose is different. The dialysis fluid contains approximately the normal concentration of glucose found in blood. This greatly reduces the concentration gradient for glucose and therefore reduces unnecessary net loss of glucose from the patient's blood.
In an examination, avoid writing only that “the dialysis machine filters the blood”. A stronger explanation refers to the partially permeable membrane, concentration gradients and diffusion of small dissolved substances.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| “Excretion means removing faeces.” | Faeces mainly contains material that was not digested or absorbed. Its removal is egestion. | Link excretion to metabolic waste products such as urea. |
| “Useful substances such as glucose are not filtered.” | Glucose is small enough to enter the glomerular filtrate. | State that glucose is filtered first and then selectively reabsorbed. |
| “Selective reabsorption produces the filtrate.” | The filtrate is produced during ultra-filtration. | Remember: ultra-filtration first, selective reabsorption second. |
| “The ureter and urethra are the same tube.” | They connect different structures. | Ureter: kidney → bladder. Urethra: bladder → outside. |
| “Normal urine contains blood cells and large plasma proteins.” | These are normally too large to pass through the filtration barrier. | Normal urine mainly contains water, urea and excess mineral ions. |
| “Dialysis fluid should just be pure water.” | This could create concentration gradients that cause useful substances to diffuse out of the blood. | Explain how dialysis fluid is formulated to remove unwanted substances while reducing loss of useful substances. |
What you should be able to do
Define excretion
Explain that excretion removes metabolic waste products and toxic substances from the body, and distinguish it from egestion.
Identify the urinary organs
State the functions of the kidneys, ureters, bladder and urethra and trace the route taken by urine.
Recognise the main nephron structures
Identify the glomerulus, Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.
Explain ultra-filtration
Describe how high pressure at the glomerulus causes water and small dissolved substances to enter Bowman's capsule.
Explain selective reabsorption
Describe how useful substances in the filtrate are returned to the blood.
Explain dialysis
Use concentration gradients, diffusion and a partially permeable membrane to explain how unwanted substances are removed from blood.
These ten questions go beyond simple recall. Use what you know about ultra-filtration, selective reabsorption and dialysis to reason through each situation.
- 10 mastery questions
- Exam-style application and reasoning
- Instant explanation for every answer option
Frequently asked questions
Questions Students Often Ask About the Kidneys
Once you understand ultra-filtration followed by selective reabsorption, most kidney questions become much more logical. These final questions clear up some distinctions that commonly cause confusion.
What is the difference between excretion and egestion?
Excretion removes metabolic waste products and toxic substances from the body. Egestion removes undigested material that was never absorbed from the alimentary canal. Passing faeces is therefore egestion rather than excretion.
Why is glucose filtered if the body needs it?
Ultra-filtration does not distinguish molecules according to whether they are useful. Glucose molecules are small enough to pass into the glomerular filtrate. The nephron then prevents their loss by selectively reabsorbing glucose back into the blood.
What is the easiest way to distinguish ultra-filtration from selective reabsorption?
Think about the direction of movement. During ultra-filtration, small substances move from the blood into Bowman's capsule. During selective reabsorption, useful substances move from the filtrate back into the blood.
What does the glomerulus do?
The glomerulus is a network of capillaries at the start of the nephron. Blood there is under high pressure, causing water and small dissolved substances to pass through the filtration barrier into Bowman's capsule.
Why are blood cells and large plasma proteins normally absent from urine?
They are normally too large to pass through the filtration barrier during ultra-filtration, so they remain in the bloodstream.
What is the difference between the ureter and the urethra?
A ureter carries urine from a kidney to the bladder. The urethra carries urine from the bladder to the outside of the body. Remember: kidney → ureter → bladder → urethra.
Why does dialysis fluid contain no urea but contain glucose?
Fresh dialysis fluid contains little or no urea so that a concentration gradient causes urea to diffuse out of the blood. It contains glucose at approximately the normal blood concentration so there is little concentration gradient for glucose, reducing its unnecessary loss.
Why can blood cells not leave the blood during dialysis?
The dialysis membrane is partially permeable. Small dissolved molecules can cross it, but blood cells and large plasma proteins are too large and remain in the patient's blood.
Does ADH belong to the kidney topic?
ADH acts on the kidneys and changes the amount of water reabsorbed, so kidney function is closely connected to ADH. More ADH results in greater water reabsorption and a smaller volume of more concentrated urine. The control of ADH is explored more fully under homeostasis and osmoregulation.
The central idea of kidney function is straightforward: filter first, then selectively take back what the body needs. Once you can follow a substance from the blood into Bowman's capsule, along the nephron and either back into the blood or onwards into urine, questions about nephron function and dialysis become much easier to reason through.