The Human Kidney and Dialysis [H]

This lesson covers the structure and function of the human kidney and how it plays a vital role in homeostasis. It also covers kidney failure and the treatments available, including dialysis and kidney transplantation. This is Higher Tier content on the AQA GCSE Biology specification.

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The Role of the Kidneys

The body has two kidneys, located at the back of the abdominal cavity. They perform three essential functions:

1. Filtering the blood — removing waste products such as urea.
2. Osmoregulation — controlling the water and ion balance of the blood.
3. Excretion — producing urine to remove waste substances from the body.

Substance	Where It Comes From	What the Kidney Does With It
Urea	Produced in the liver from the breakdown of excess amino acids (deamination)	Filtered out of the blood and excreted in urine
Excess water	From food and drink	Removed in urine to maintain correct blood water concentration
Excess ions (e.g. sodium, potassium)	From food and drink	Filtered out and excreted in urine to maintain ion balance
Glucose	From digestion of carbohydrates	Filtered out then completely reabsorbed back into the blood (none should appear in urine)

Exam Tip: A common mistake is to say that the kidneys "clean" the blood. Be more specific: the kidneys filter the blood to remove urea, excess water, and excess ions while reabsorbing useful substances like glucose and some water.

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Structure of the Kidney

Each kidney contains approximately one million tiny filtering units called nephrons. Each nephron carries out the processes of filtration, selective reabsorption, and excretion.

Key Structures in the Kidney

Structure	Location	Function
Renal artery	Enters the kidney	Brings blood containing waste products to the kidney
Renal vein	Leaves the kidney	Carries filtered (clean) blood away from the kidney
Ureter	Exits bottom of kidney	Carries urine from the kidney to the bladder
Cortex	Outer region of kidney	Contains the glomeruli and Bowman's capsules (where filtration occurs)
Medulla	Inner region of kidney	Contains the loops of Henle and collecting ducts
Pelvis	Central cavity	Collects urine before it enters the ureter

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How the Kidney Filters Blood

The process of urine formation involves three stages:

Stage 1: Ultrafiltration (in the Glomerulus)

1. Blood enters the kidney through the renal artery, which branches into smaller vessels.
2. Each small vessel forms a tight ball of capillaries called a glomerulus, which sits inside a cup-shaped structure called Bowman's capsule.
3. The blood in the glomerulus is under high pressure because the blood vessel leaving the glomerulus (efferent arteriole) is narrower than the one entering it (afferent arteriole).
4. This high pressure forces small molecules out of the blood and into Bowman's capsule. These include water, glucose, urea, ions, and amino acids.
5. Large molecules (such as proteins and blood cells) are too big to pass through the capillary walls and remain in the blood.

The liquid that passes into Bowman's capsule is called the filtrate (or glomerular filtrate).

Stage 2: Selective Reabsorption (in the Proximal Convoluted Tubule and Loop of Henle)

Not everything in the filtrate is waste — the body needs to reabsorb useful substances. This happens by selective reabsorption:

Substance	What Happens	How
All glucose	Reabsorbed back into the blood	Active transport
Most water	Reabsorbed back into the blood	Osmosis
Some ions	Reabsorbed as needed	Active transport
Urea	Mostly remains in the filtrate (some is reabsorbed)	Diffusion

Selective reabsorption occurs mainly in the proximal convoluted tubule, which has adaptations for efficient reabsorption:

• Long and coiled to increase surface area.
• Lined with cells that have microvilli to further increase surface area.
• Rich blood supply to maintain a concentration gradient.
• Many mitochondria to provide energy for active transport.

Stage 3: Formation of Urine (Collecting Duct)

The remaining liquid — containing urea, excess water, and excess ions — passes through the collecting duct and becomes urine. Urine flows into the renal pelvis, then down the ureter to the bladder, where it is stored before being excreted through the urethra.

flowchart TD
    A[Blood enters kidney via renal artery] --> B[Ultrafiltration in glomerulus]
    B --> C[Filtrate enters Bowman's capsule]
    C --> D[Selective reabsorption in tubules]
    D --> D1[Glucose reabsorbed - active transport]
    D --> D2[Water reabsorbed - osmosis]
    D --> D3[Some ions reabsorbed - active transport]
    D --> E[Remaining liquid becomes urine]
    E --> F[Urine flows to bladder via ureter]
    F --> G[Urine excreted via urethra]

Exam Tip: If glucose is found in a person's urine, it suggests they may have diabetes. In a healthy person, all glucose is reabsorbed by active transport in the kidney tubules. In diabetes, blood glucose is so high that the kidneys cannot reabsorb it all.

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Water Balance and ADH

The kidneys control the water content of the blood through a process called osmoregulation. This involves the hormone ADH (antidiuretic hormone), produced by the pituitary gland.

When the Body Is Dehydrated (Blood Too Concentrated)

1. Receptors in the hypothalamus detect that blood is too concentrated (water potential is too low).
2. The pituitary gland releases more ADH into the blood.
3. ADH travels to the kidneys and acts on the collecting ducts.
4. ADH makes the walls of the collecting ducts more permeable to water.
5. More water is reabsorbed from the filtrate back into the blood by osmosis.
6. A small volume of concentrated (dark) urine is produced.
7. Blood water concentration returns to normal.

When the Body Has Excess Water (Blood Too Dilute)

1. Receptors in the hypothalamus detect that blood is too dilute (water potential is too high).
2. The pituitary gland releases less ADH.
3. The collecting duct walls become less permeable to water.
4. Less water is reabsorbed from the filtrate.
5. A large volume of dilute (pale) urine is produced.
6. Blood water concentration returns to normal.

Condition	ADH Level	Collecting Duct Permeability	Water Reabsorbed	Urine Volume	Urine Concentration
Dehydrated	High	More permeable	More	Small	Concentrated (dark)
Over-hydrated	Low	Less permeable	Less	Large	Dilute (pale)

flowchart TD
    A{Blood water level} -->|Too low - dehydrated| B[Hypothalamus detects]
    B --> C[Pituitary releases more ADH]
    C --> D[Collecting ducts more permeable]
    D --> E[More water reabsorbed]
    E --> F[Small volume of concentrated urine]
    A -->|Too high - over-hydrated| G[Hypothalamus detects]
    G --> H[Pituitary releases less ADH]
    H --> I[Collecting ducts less permeable]
    I --> J[Less water reabsorbed]
    J --> K[Large volume of dilute urine]
    F --> L[Blood water level returns to normal]
    K --> L