The Kidney and Osmoregulation

This lesson covers the structure and function of the kidney, including how it filters blood, removes waste, and regulates water balance through osmoregulation. Understanding the role of ADH (antidiuretic hormone) in controlling water levels is essential for Edexcel GCSE Biology (1BI0) Topic 7.

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The Functions of the Kidneys

The kidneys are two bean-shaped organs located in the back of the abdominal cavity, one on each side of the spine. They perform three vital functions:

Function	Detail
Filtering the blood	Removing waste products (especially urea) from the blood
Regulating water levels	Controlling the amount of water in the blood and body fluids (osmoregulation)
Regulating ion levels	Controlling the concentration of ions (such as sodium) in the blood

What Is Urea?

Urea is a waste product formed in the liver from the breakdown of excess amino acids (from protein digestion). This process is called deamination.

• Proteins are digested into amino acids in the gut.
• Amino acids cannot be stored in the body.
• Excess amino acids are transported to the liver.
• In the liver, the amino group is removed (deamination), producing ammonia.
• Ammonia is toxic, so the liver converts it into urea, which is less toxic.
• Urea is released into the blood and transported to the kidneys for removal.

Exam Tip: Urea is made in the liver, not the kidneys. The kidneys remove urea from the blood. This is a very common misconception — examiners will test this.

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Kidney Structure

Each kidney contains approximately one million tiny filtering units called nephrons. The nephron is the functional unit of the kidney.

Structure of a Nephron

A nephron consists of the following parts (in order of blood flow through the nephron):

Part	Location	Function
Bowman's capsule	Surrounds the glomerulus in the cortex	Collects the filtrate from ultrafiltration
Proximal convoluted tubule	In the cortex (coiled tube)	Reabsorbs useful substances (all glucose, most water, some ions)
Loop of Henle	Extends from cortex down into medulla and back	Creates a concentration gradient; enables water reabsorption
Distal convoluted tubule	In the cortex (coiled tube)	Fine-tuning of ion and pH balance
Collecting duct	Passes through the medulla	Final water reabsorption (controlled by ADH); urine collects here

Blood Supply to the Nephron

• The renal artery brings blood to the kidney.
• Blood enters a tight ball of capillaries called the glomerulus, which sits inside the Bowman's capsule.
• The blood vessel leading into the glomerulus (the afferent arteriole) is wider than the vessel leaving it (the efferent arteriole). This creates high pressure inside the glomerulus.

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Step 1: Ultrafiltration

Ultrafiltration is the first stage of kidney function. It takes place in the glomerulus and Bowman's capsule.

How Ultrafiltration Works

1. Blood enters the glomerulus under high pressure (because the afferent arteriole is wider than the efferent arteriole).
2. The high pressure forces small molecules out of the blood and through the walls of the glomerular capillaries into the Bowman's capsule.
3. The walls of the capillaries and Bowman's capsule act as a filter.

What Is Filtered Out (Into Bowman's Capsule)

Substance	Filtered?	Why?
Water	Yes	Small molecule, passes through the filter
Glucose	Yes	Small molecule, passes through the filter
Urea	Yes	Small molecule, passes through the filter
Ions (sodium, potassium, chloride)	Yes	Small molecules/ions, pass through the filter
Amino acids	Yes	Small molecules, pass through the filter

What Is NOT Filtered (Stays in the Blood)

Substance	Filtered?	Why?
Blood cells (red and white)	No	Too large to pass through the filter
Platelets	No	Too large
Large proteins	No	Too large to pass through the filter

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

Exam Tip: Ultrafiltration is a non-selective process — everything small enough to pass through the filter will be filtered out, including useful substances like glucose. This is why reabsorption is needed in the next step.

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Step 2: Selective Reabsorption

Selective reabsorption takes place mainly in the proximal convoluted tubule. It is the process by which useful substances are reabsorbed back into the blood from the filtrate.

What Is Reabsorbed?

Substance	Amount Reabsorbed	How
All glucose	100% reabsorbed	By active transport (requires energy from respiration)
Most water	~99% reabsorbed	By osmosis
Some ions	Variable; depends on the body's needs	By active transport
Some amino acids	Reabsorbed as needed	By active transport

What Is NOT Reabsorbed (Remains in the Tubule)

Substance	Why?
Urea	It is a waste product; the body needs to excrete it
Excess water	Only the water the body needs is reabsorbed; the rest stays
Excess ions	Only the ions the body needs are reabsorbed; the rest stays

The substances that are not reabsorbed continue along the nephron and eventually form urine.

Exam Tip: The fact that all glucose is reabsorbed is very important. If glucose appears in the urine, it is a sign that something is wrong — this can be an indicator of diabetes (where blood glucose is so high that the kidneys cannot reabsorb all of it).

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Urine Formation

After ultrafiltration and selective reabsorption, the remaining fluid is urine. Urine contains:

• Urea — the main waste product
• Excess water — water the body does not need
• Excess ions — ions the body does not need
• Small amounts of other waste substances

Urine flows from the collecting duct → into the renal pelvis → down the ureter → into the bladder (where it is stored) → and out through the urethra during urination.

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Osmoregulation — Controlling Water Levels

Osmoregulation is the control of water levels in the blood and body fluids. The kidneys are the main organs involved.

Water balance is critical because:

• If blood is too dilute (too much water), cells may absorb too much water by osmosis and swell or burst (lysis).
• If blood is too concentrated (too little water), cells may lose water by osmosis and shrink (crenation).

The Role of ADH

ADH (antidiuretic hormone) is a hormone produced by the hypothalamus and released by the pituitary gland. It controls how much water is reabsorbed in the collecting duct of the nephron.

When You Are Dehydrated (Blood Too Concentrated)

1. The hypothalamus detects that the blood is too concentrated (osmoreceptors detect high solute concentration).
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 collecting duct back into the blood (by osmosis).
6. A smaller volume of concentrated (dark yellow) urine is produced.

When You Are Over-Hydrated (Blood Too Dilute)

1. The hypothalamus detects that the blood is too dilute (osmoreceptors detect low solute concentration).
2. The pituitary gland releases less ADH into the blood.
3. With less ADH, the walls of the collecting ducts become less permeable to water.
4. Less water is reabsorbed from the collecting duct.
5. A larger volume of dilute (pale/clear) urine is produced.

ADH — Negative Feedback Summary

Condition	ADH Level	Effect on Collecting Duct	Urine Produced
Dehydrated (blood too concentrated)	High	More permeable to water → more water reabsorbed	Small volume, concentrated (dark)
Over-hydrated (blood too dilute)	Low	Less permeable to water → less water reabsorbed	Large volume, dilute (pale)

Exam Tip: Think of ADH as the "Anti-Dilute Hormone" — when ADH levels are high, the body holds on to water and urine is concentrated. When ADH levels are low, more water is lost as dilute urine.

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Factors That Affect Water Balance