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Biology: Structure and function

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Biology: Structure and function
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In this chapter

  1. 1Introduction to Human Excretion
  2. 2Structure of the Urinary System
  3. 3Inside the Kidney: The Nephron
  4. 4Step 1: Ultrafiltration in the Bowman's Capsule
  5. 5Step 2: Selective Reabsorption
  6. 6Water Balance and Osmoregulation

1. Introduction to Human Excretion

Excretion is the process by which living organisms remove waste products produced by metabolic reactions inside their cells. These waste products, such as urea and carbon dioxide, are toxic if they accumulate in the body. It is vital not to confuse excretion with egestion, which is the removal of undigested food (faeces) from the gut. The main excretory organs in humans are the kidneys (excreting urea and excess water and salts as urine), the lungs (excreting carbon dioxide and water vapour), and the skin (excreting sweat, which contains water, salts, and a small amount of urea).

Key term

Excretion: The removal from organisms of toxic materials, the waste products of metabolism, and substances in excess of requirements.

Common pitfall

Confusing excretion (removal of metabolic waste like urea) with egestion (removal of undigested food as faeces).

Worked example 14 marks

Distinguish between excretion and egestion, providing a specific example of a substance removed by each process.

  1. 1

    Step 1: Define excretion. Excretion is the removal of metabolic waste products from the body. These are substances made by chemical reactions inside cells.

  2. 2

    Step 2: Provide an example for excretion. A key example is urea, which is produced in the liver from excess amino acids and removed from the blood by the kidneys.

  3. 3

    Step 3: Define egestion. Egestion is the discharge of undigested or unabsorbed food material from the body. This material has never been inside the body's cells.

  4. 4

    Step 4: Provide an example for egestion. An example is dietary fibre, which passes through the digestive system and is eliminated as part of the faeces.

Recap

  • Excretion is the removal of metabolic waste from the body.
  • The main human excretory products are urea, carbon dioxide, and excess salts and water.
  • The primary excretory organs are the kidneys, lungs, and skin.
  • Excretion is a chemical process of removing waste made by cells, while egestion is a physical process of removing undigested food.

Quick check

  1. Name two substances excreted by the kidneys.2 marks
  2. Which organ excretes carbon dioxide?1 mark

2. Structure of the Urinary System

The human urinary system is responsible for producing, storing, and eliminating urine. It consists of four main parts. The two kidneys are bean-shaped organs that filter waste products from the blood to produce urine. Blood enters each kidney via the renal artery and leaves via the renal vein. The two ureters are tubes that carry urine from each kidney to the bladder. The bladder is a muscular, elastic sac that stores urine, allowing urination to be infrequent and controlled. The urethra is the tube through which urine passes from the bladder out of the body.

Key term

Urea: The main nitrogenous waste product in mammals, formed in the liver from the breakdown of excess amino acids and excreted by the kidneys.

Examiner insight

Examiners expect students to be able to label the main parts of the urinary system and state the function of each part in a clear, sequential manner.

Worked example 14 marks

A diagram shows the human urinary system. A student has labelled the kidney, ureter, bladder, and urethra. Describe the function of each of these four structures.

  1. 1

    Step 1: Describe the kidney's function. The kidney filters blood to remove waste products like urea and excess substances like water and salts, producing urine.

  2. 2

    Step 2: Describe the ureter's function. The ureter is a tube that transports urine from the kidney to the bladder using waves of muscular contraction (peristalsis).

  3. 3

    Step 3: Describe the bladder's function. The bladder is a muscular bag that stores urine before it is expelled from the body.

  4. 4

    Step 4: Describe the urethra's function. The urethra is a tube that carries urine from the bladder to the outside of the body.

Recap

  • The urinary system consists of the kidneys, ureters, bladder, and urethra.
  • Kidneys filter blood and produce urine.
  • Ureters carry urine from the kidneys to the bladder.
  • The bladder stores urine.
  • The urethra expels urine from the body.
  • The renal artery supplies blood to the kidney, and the renal vein carries blood away from it.

Quick check

  1. What is the name of the blood vessel that brings blood to the kidney?1 mark
  2. What is the function of the bladder?1 mark

3. Inside the Kidney: The Nephron

The kidney has three main regions: the outer cortex, the inner medulla, and the central pelvis which collects urine. The real work of filtration happens in millions of microscopic tubules called nephrons, which span the cortex and medulla. Each nephron begins with a cup-shaped structure called the Bowman's capsule, which surrounds a knot of capillaries called the glomerulus. Leading from the Bowman's capsule is a long, coiled tubule consisting of the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. This tubule eventually joins a collecting duct, which carries urine down through the medulla to the renal pelvis.

Key term

Nephron: The microscopic functional and structural unit of the kidney, responsible for filtering blood and forming urine.

Fun fact

Each of your kidneys contains about one million nephrons. If you stretched them all out end-to-end, they would cover a distance of over 80 kilometres (50 miles)!

Worked example 15 marks

Trace the path of a water molecule from the blood in the glomerulus until it becomes part of the urine in the collecting duct, naming the key parts of the nephron it passes through.

  1. 1

    Step 1: Start at the glomerulus. The water molecule is in the blood within the glomerulus.

  2. 2

    Step 2: Filtration. Under high pressure, the water molecule is forced out of the glomerulus and into the Bowman's capsule, becoming part of the glomerular filtrate.

  3. 3

    Step 3: Proximal Convoluted Tubule. The molecule then flows into the proximal convoluted tubule.

  4. 4

    Step 4: Loop of Henle. Next, it passes down and then up the loop of Henle.

  5. 5

    Step 5: Distal Convoluted Tubule. The molecule then enters the distal convoluted tubule.

  6. 6

    Step 6: Collecting Duct. Finally, if it has not been reabsorbed, it flows into the collecting duct as part of the urine.

Recap

  • The kidney is composed of a cortex, medulla, and pelvis.
  • The nephron is the functional filtering unit of the kidney.
  • A nephron consists of the glomerulus, Bowman's capsule, convoluted tubules, and the loop of Henle.
  • Multiple nephrons drain into a single collecting duct.
  • The collecting ducts transport urine to the renal pelvis.

Quick check

  1. What is the name of the ball of capillaries inside the Bowman's capsule?1 mark
  2. Which part of the kidney collects urine before it passes to the ureter?1 mark

4. Step 1: Ultrafiltration in the Bowman's Capsule

Urine formation begins with ultrafiltration. Blood enters the glomerulus at high pressure. This is because the afferent arteriole (leading in) is wider than the efferent arteriole (leading out), creating a bottleneck that builds up pressure. This high hydrostatic pressure forces water and small solutes from the blood through the capillary walls and into the Bowman's capsule. The resulting fluid is called glomerular filtrate. It contains water, glucose, salts, amino acids, and urea. However, large components like red blood cells, white blood cells, platelets, and large proteins (like albumin) are too big to pass through the filtration membrane and remain in the blood.

Key term

Ultrafiltration: The filtration of blood at a molecular level under high pressure in the glomerulus, where small molecules pass into the Bowman's capsule but large molecules and cells are retained.

Common pitfall

Forgetting to mention that high pressure is the driving force for ultrafiltration, or that large molecules like proteins and red blood cells are too big to pass through the filter.

Worked example 13 marks

Explain why red blood cells are not found in the glomerular filtrate of a healthy person.

  1. 1

    Step 1: State the process. Filtration occurs in the glomerulus, where substances move from the blood into the Bowman's capsule.

  2. 2

    Step 2: Explain the mechanism. The walls of the glomerular capillaries and the inner wall of the Bowman's capsule act as a filter.

  3. 3

    Step 3: Relate size to filtration. Red blood cells are large cells. They are too large to pass through the pores of the filtration membrane.

  4. 4

    Step 4: Conclude. Therefore, red blood cells are retained in the blood within the glomerulus and do not enter the Bowman's capsule to become part of the filtrate.

Recap

  • Ultrafiltration is the first step in urine formation.
  • It is a non-selective process driven by high pressure in the glomerulus.
  • Small molecules like water, glucose, urea, and salts are forced into the Bowman's capsule.
  • Large molecules like proteins and all blood cells remain in the blood.
  • The fluid formed in the Bowman's capsule is called glomerular filtrate.

Quick check

  1. What provides the force for ultrafiltration?1 mark
  2. Name one substance found in blood plasma but not in glomerular filtrate.1 mark

5. Step 2: Selective Reabsorption

After ultrafiltration, the glomerular filtrate contains many useful substances that the body cannot afford to lose. Selective reabsorption is the process of reclaiming these substances from the filtrate and returning them to the blood. This occurs all along the nephron tubule. All glucose is reabsorbed in the proximal convoluted tubule by active transport, which requires energy. Most of the water is reabsorbed by osmosis, following the movement of solutes. Many of the salts (ions) are also reabsorbed, some by active transport and some by diffusion. By the end of the tubule, only waste products (like urea), excess salts, and excess water remain, forming urine.

Key term

Selective Reabsorption: The process by which the nephron removes useful solutes and water from the glomerular filtrate and returns them to the blood.

Examiner insight

High marks are awarded for specifying *where* certain substances are reabsorbed (e.g., all glucose in the proximal tubule) and by what mechanism (e.g., active transport for glucose, osmosis for water).

Worked example 14 marks

Explain why glucose is found in the glomerular filtrate but not normally in the urine of a healthy person.

  1. 1

    Step 1: Explain presence in filtrate. Glucose is a small molecule, so during ultrafiltration in the glomerulus, it easily passes from the blood into the Bowman's capsule, becoming part of the glomerular filtrate.

  2. 2

    Step 2: Explain absence in urine. As the filtrate passes along the nephron, specifically in the proximal convoluted tubule, all the glucose is reabsorbed back into the blood.

  3. 3

    Step 3: State the mechanism. This reabsorption occurs via active transport, a process that uses energy to move glucose against its concentration gradient.

  4. 4

    Step 4: Conclude. Because all the glucose is selectively reabsorbed, none is left in the fluid that becomes urine.

Recap

  • Selective reabsorption reclaims useful substances from the filtrate.
  • All glucose is reabsorbed in the proximal convoluted tubule by active transport.
  • Most water is reabsorbed by osmosis.
  • Some salts are reabsorbed by active transport and diffusion.
  • This process ensures that essential substances are not lost in the urine.

Quick check

  1. By what process is glucose reabsorbed from the filtrate?1 mark
  2. In which part of the nephron does most selective reabsorption occur?1 mark

6. Water Balance and Osmoregulation

Osmoregulation is the control of the water potential of the blood. This is a crucial homeostatic process managed by a negative feedback loop involving a hormone called ADH (antidiuretic hormone). If the brain's hypothalamus detects that the blood is too concentrated (e.g., due to dehydration), the pituitary gland is stimulated to release more ADH. ADH travels in the blood to the kidneys and makes the walls of the distal convoluted tubules and collecting ducts more permeable to water. This allows more water to be reabsorbed from the filtrate back into the blood. The result is a small volume of concentrated urine. Conversely, if the blood is too dilute, less ADH is released, the tubules become less permeable, less water is reabsorbed, and a large volume of dilute urine is produced.

Key term

Osmoregulation: The homeostatic control of the water potential of the body's fluids, such as blood, by regulating water and salt concentrations.

Common pitfall

Mixing up the effects of ADH. A good way to remember is: 'Add Da H2O' (to the blood). More ADH means more water is added back to the blood.

Worked example 16 marks

A person runs a marathon on a hot day and becomes dehydrated. Describe and explain the hormonal changes that occur to help their body conserve water.

  1. 1

    Step 1: Identify the initial stimulus. Due to sweating and not drinking enough, the water potential of the person's blood decreases (it becomes more concentrated).

  2. 2

    Step 2: Describe detection. This change is detected by the hypothalamus in the brain.

  3. 3

    Step 3: Describe the hormonal response. The hypothalamus stimulates the pituitary gland to secrete more antidiuretic hormone (ADH) into the bloodstream.

  4. 4

    Step 4: Explain the effect of the hormone. ADH travels to the kidneys and increases the permeability of the collecting ducts and distal tubules to water.

  5. 5

    Step 5: State the result in the kidney. More water moves by osmosis from the filtrate in the collecting ducts back into the blood.

  6. 6

    Step 6: Describe the final outcome. The body conserves water, and a small volume of highly concentrated urine is produced. The blood water potential returns towards the normal level.

Recap

  • Osmoregulation controls the water potential of the blood.
  • It is a negative feedback system involving the hormone ADH.
  • ADH is released from the pituitary gland when the blood is too concentrated.
  • ADH makes the collecting ducts more permeable to water.
  • More ADH leads to more water reabsorption and concentrated urine.
  • Less ADH leads to less water reabsorption and dilute urine.

Quick check

  1. What does the acronym ADH stand for?1 mark
  2. If you drink a large volume of water, will your pituitary gland release more or less ADH?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term excretion.2 marks
  2. Name the tube that carries urine from the kidney to the bladder.1 mark
  3. Describe the process of ultrafiltration that takes place in the kidney.4 marks
  4. Explain why the urine of a healthy person does not contain protein or glucose.4 marks
  5. The concentration of urea in the blood plasma is 0.03 g per 100 cm³, but in urine it can be 2.0 g per 100 cm³. Explain this large increase in concentration.3 marks
  6. Describe the roles of the kidneys, ureters, bladder and urethra in the removal of urea from the body.4 marks
  7. Describe the role of ADH in osmoregulation on a hot day when a person has not drunk much water.6 marks
  8. A person with untreated diabetes mellitus often has a high concentration of glucose in their urine. Suggest an explanation for this.3 marks
  9. State three differences in the composition of blood in the renal artery compared with blood in the renal vein.3 marks
  10. Explain how the structure of a nephron, including the glomerulus and the long tubule, is related to its function.5 marks

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