Excretion in humans

1. Overview

Excretion is the process by which organisms eliminate the waste products of metabolism and substances in excess of requirements. This process is vital for maintaining a constant internal environment (homeostasis), as the accumulation of metabolic wastes like urea and carbon dioxide can become toxic and damage cells.

Key Definitions

• Excretion: The removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances in excess of requirements.
• Deamination: The removal of the nitrogen-containing part of amino acids to form urea.
• Assimilation: The movement of digested food molecules into the cells of the body where they are used, becoming part of the cells.
• Urea: A toxic nitrogenous waste product formed in the liver from the breakdown of excess amino acids.

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Core Content

Main Excretory Products and Organs

• Carbon Dioxide ($CO_2$): Produced during aerobic respiration in all living cells. It is transported in the blood to the lungs, where it is excreted during exhalation.
• Urea: Formed in the liver from excess proteins/amino acids. It is excreted by the kidneys in urine.
• Excess Water and Ions: Taken in through diet; the kidneys regulate their levels and excrete the excess in urine.

The Urinary System Structure

• Kidneys: Two bean-shaped organs that filter the blood to remove waste and produce urine.
• Ureters: Two tubes that carry urine from the kidneys to the bladder.
• Bladder: A muscular sac that stores urine until it is released.
• Urethra: The tube through which urine passes from the bladder to outside the body.

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Extended Content (Extended Curriculum Only)

The Liver’s Role in Excretion

The liver is the primary site for processing nutrients and toxins:

1. Assimilation: The liver converts absorbed amino acids into proteins (like plasma proteins, e.g., fibrinogen).
2. Urea Formation: The body cannot store excess amino acids. To prevent waste, the liver breaks them down.
3. Deamination: This is the specific process where the nitrogen-containing part of the amino acid is removed. This part is converted into urea, while the remaining carbon-based part can be used for energy.
4. Toxicity: Urea is toxic. If it builds up in the blood, it can cause cell damage and organ failure, which is why excretion is essential.

Kidney Anatomy

• Cortex: The outer layer where filtration occurs.
• Medulla: The inner section containing the loops of the nephrons and collecting ducts.

The Nephron: Structure and Function

The nephron is the microscopic functional unit of the kidney.

Step 1: Filtration (in the Glomerulus)

• Blood enters a cluster of capillaries called the glomerulus under high pressure.
• Small molecules—water, glucose, urea, and ions—are squeezed out of the blood into the nephron tubule.
• Note: Blood cells and large proteins stay in the blood because they are too big to pass through the capillary walls.

Step 2: Selective Reabsorption As the filtrate moves through the nephron tubule, the body "takes back" what it needs:

• Glucose: 100% of glucose is reabsorbed back into the blood (via active transport).
• Ions: Some ions are reabsorbed to maintain correct blood concentration.
• Water: Most of the water is reabsorbed back into the blood to prevent dehydration.

Step 3: Formation of Urine

• The substances that are not reabsorbed (all of the urea, excess water, and excess ions) continue through the tubule to the collecting duct.
• This liquid is now urine, which travels to the bladder via the ureter.

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Key Equations

There are no mathematical equations for this topic, but the conceptual "equation" for urine is: $$\text{Urine} = \text{Urea} + \text{Excess Water} + \text{Excess Ions}$$

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Common Mistakes to Avoid

• ❌ Wrong: Confusing excretion with egestion.
  • ✓ Right: Excretion is the removal of metabolic waste (made inside cells, e.g., urea). Egestion is the removal of undigested food (feces) that never entered the cells.
• ❌ Wrong: Swapping the names of the ureter and urethra.
  • ✓ Right: Remember: Ureter (two "e"s for the two tubes from the kidneys); Urethra (one tube for the urine to "run away" from the body).
• ❌ Wrong: Thinking the kidneys make urea.
  • ✓ Right: The liver makes urea via deamination; the kidneys simply filter it out of the blood.

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Exam Tips

• Command Word "State": If asked to "state the products excreted by the lungs," keep it simple: "Carbon dioxide."
• Command Word "Describe": When describing the role of the nephron, remember to mention filtration and reabsorption as two distinct steps.
• Structure-Function Links: If asked how the glomerulus is suited for filtration, mention that it consists of a knot of capillaries providing a large surface area and high pressure.
• Typical Values: In data questions, look for glucose levels. In a healthy person, there should be 3.1 or 3.2 mmol/L (or similar) in the blood, but 0 in the urine. If glucose is in the urine, it suggests a problem with reabsorption (like diabetes).
• Identify Diagrams: Be prepared to label the cortex (outer) and medulla (inner) on a kidney diagram. This is a very common "identify" task in Paper 2 and Paper 4.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [6 marks]

Question:

(a) State two substances, other than carbon dioxide, that are excreted from the human body. [2]

(b) Define the term 'excretion'. [2]

(c) Explain why excretion is essential for human survival. [2]

Worked Solution:

(a)

1. Urea Urea is a nitrogenous waste product from the breakdown of amino acids.
2. Excess ions The body needs to maintain ion balance.

How to earn full marks:

• 1 mark for each correct substance.
• Accept any two of: urea, excess water, excess ions (e.g., sodium, chloride).
• Do not accept faeces or undigested food.

(b)

1. Excretion is the removal of metabolic waste products Defining the process of excretion.
2. from the body. Specifying that the waste products are removed from the body.

How to earn full marks:

• 1 mark for mentioning waste products.
• 1 mark for specifying the removal is from the body (not just produced).
• Full marks can be earned if the answer implies that waste products are produced due to metabolism.

(c)

1. Excretion removes toxic substances like urea Identifying the toxicity of urea.
2. If urea accumulates, it can damage cells and disrupt bodily functions. Explaining the consequences of urea buildup.

How to earn full marks:

• 1 mark for mentioning the toxicity of urea or other metabolic waste.
• 1 mark for stating the consequences of toxicity, such as cell damage or disruption of bodily functions.

Common Pitfall: Many students confuse excretion with egestion. Remember that excretion is the removal of metabolic waste products, while egestion is the removal of undigested food. Also, be sure to specify that the waste products are removed from the body.

Exam-Style Question 2 — Extended Response [10 marks]

Question:

The diagram shows a simplified nephron.

(a) Identify the labelled structures in the diagram. [4]

(b) Describe the process of filtration that occurs at the glomerulus. [3]

(c) Explain how the nephron reabsorbs glucose and water back into the blood. [3]

Worked Solution:

(a)

1. Glomerulus is the network of capillaries where filtration begins. Identifying the glomerulus.
2. Bowman's capsule is the cup-shaped structure surrounding the glomerulus, collecting the filtrate. Identifying Bowman's capsule.
3. Proximal convoluted tubule is the coiled section where most reabsorption occurs. Identifying the proximal convoluted tubule.
4. Collecting duct is the tube that collects urine from multiple nephrons. Identifying the collecting duct.

How to earn full marks:

• 1 mark for each correctly identified structure.
• Spelling must be reasonably accurate.

(b)

1. Blood enters the glomerulus at high pressure through the afferent arteriole, which has a wider diameter than the efferent arteriole. Describing the high pressure in the glomerulus.
2. This high pressure forces small molecules like water, glucose, ions, and urea out of the blood through the capillary walls. Stating the substances forced out.
3. These substances pass into the Bowman's capsule, forming the glomerular filtrate. Describing where the substances go.

How to earn full marks:

• 1 mark for mentioning high pressure and the difference in diameter of the arterioles.
• 1 mark for listing at least two substances that are filtered out.
• 1 mark for stating that these substances enter Bowman's capsule.

(c)

1. Glucose is reabsorbed in the proximal convoluted tubule by active transport, against its concentration gradient. Stating where glucose is reabsorbed and by what mechanism.
2. Water is reabsorbed by osmosis along the nephron, particularly in the loop of Henle and collecting duct, following the concentration gradient created by the reabsorption of ions. Stating where water is reabsorbed and by what mechanism.
3. Reabsorbed glucose and water then enter the surrounding blood capillaries, returning to the bloodstream. Describing where the reabsorbed substances go.

How to earn full marks:

• 1 mark for stating that glucose is reabsorbed in the proximal convoluted tubule by active transport.
• 1 mark for stating that water is reabsorbed by osmosis.
• 1 mark for stating that the reabsorbed substances enter the blood capillaries.

Common Pitfall: Students often forget that the high pressure in the glomerulus is due to the difference in diameter between the afferent and efferent arterioles. Also, remember that glucose is reabsorbed by active transport, while water is reabsorbed by osmosis.

Exam-Style Question 3 — Short Answer [5 marks]

Question:

(a) State where urea is formed in the human body. [1]

(b) Outline the process of deamination. [2]

(c) Explain why the liver converts excess amino acids into urea. [2]

Worked Solution:

(a)

1. Urea is formed in the liver. Stating the location of urea formation.

How to earn full marks:

• 1 mark for stating the liver.

(b)

1. Deamination is the removal of the nitrogen-containing amino group (NH2) Identifying the removal of the amino group.
2. from excess amino acids in the liver. Specifying the source of the amino group.

How to earn full marks:

• 1 mark for mentioning the removal of the amino group.
• 1 mark for specifying that it is from amino acids.

(c)

1. Excess amino acids cannot be stored in the body. Stating that amino acids cannot be stored.
2. The amino group removed from amino acids is toxic, so it is converted to urea, which is less toxic and can be safely transported to the kidneys for excretion. Explaining the toxicity of the amino group and conversion to less toxic urea.

How to earn full marks:

• 1 mark for stating that amino acids cannot be stored.
• 1 mark for explaining the toxicity of the amino group and the need to convert it to less toxic urea.

Common Pitfall: A common mistake is thinking urea is formed in the kidneys. It's crucial to remember that deamination and urea formation occur in the liver. Also, be clear that it's the nitrogen-containing part of the amino acid that's toxic.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A patient has a kidney disorder that affects the function of their nephrons. This results in higher than normal concentrations of protein and glucose being detected in their urine.

(a) Suggest how the kidney disorder might be affecting the glomerulus. [3]

(b) Explain how the presence of glucose in the urine is related to the function of the proximal convoluted tubule. [3]

(c) Describe the roles of the kidneys, ureters, bladder, and urethra in the excretion of urine. [3]

Worked Solution:

(a)

1. The glomerulus normally filters water, glucose, ions, and urea, while preventing larger molecules like proteins from passing through. Stating the normal function of the glomerulus.
2. The kidney disorder might be causing damage to the glomerulus's filtration membrane. Suggesting damage as a cause.
3. This damage could increase the permeability of the glomerulus, allowing larger molecules like protein to pass through into the filtrate, which is then not reabsorbed. Explaining how damage leads to protein in the filtrate.

How to earn full marks:

• 1 mark for stating the normal function of the glomerulus.
• 1 mark for suggesting damage to the glomerulus's filtration membrane.
• 1 mark for explaining how damage leads to increased permeability and protein in the filtrate.

(b)

1. Glucose is normally completely reabsorbed in the proximal convoluted tubule by active transport. Stating the normal reabsorption of glucose in the PCT.
2. The kidney disorder may be affecting the active transport proteins in the proximal convoluted tubule, reducing their efficiency or number. Suggesting a problem with active transport.
3. If these active transport proteins are not functioning properly, glucose will not be fully reabsorbed and will remain in the filtrate, resulting in glucose in the urine. Explaining how impaired active transport leads to glucose in the urine.

How to earn full marks:

• 1 mark for stating the normal reabsorption of glucose in the PCT.
• 1 mark for suggesting a problem with active transport proteins.
• 1 mark for explaining how impaired active transport leads to glucose in the urine.

(c)

1. The kidneys filter the blood, removing urea, excess water, and ions to produce urine. Describing the function of the kidneys.
2. The ureters are tubes that transport urine from the kidneys to the bladder through peristaltic contractions. Describing the function of the ureters.
3. The bladder is a muscular sac that stores urine until it reaches a certain volume, triggering the urge to urinate. The urethra carries urine from the bladder to the outside of the body during urination. Describing the function of the bladder and urethra.

How to earn full marks:

• 1 mark for stating function of kidney.
• 1 mark for stating function of ureter.
• 1 mark for stating the function of bladder and urethra.

Common Pitfall: When discussing kidney disorders, be specific about which part of the nephron is affected and how that affects its function. For example, simply saying "the kidney is damaged" is not enough; you need to explain how the damage impacts filtration or reabsorption. Also, remember that the ureters use peristalsis to move urine.