## Learning Outcomes

- Identify the main parts of the renal system and the gross anatomy of the kidney
- Describe the structure of a nephron and trace the path of blood, filtrate and urine
- Explain how ultrafiltration forms filtrate in Bowman's capsule
- Distinguish between selective reabsorption, tubular secretion and urine formation
- Understand how the loop of Henle and collecting duct help control water balance
- Explain how ADH changes urine concentration and why this matters for homeostasis
- Recognise what should and should not normally be present in urine

## T Level Health (Core) Syllabus

For the T Level Health (Core), you are expected to understand kidney anatomy, nephron structure and the formation of urine, with a focus on the following syllabus points:

- Know that the renal system removes urea and helps regulate water balance
- Identify the main parts of the urinary system: kidneys, ureters, bladder and urethra
- Describe the kidney regions: renal cortex, renal medulla and renal pelvis
- Identify the main parts of a nephron, including the glomerulus, Bowman's capsule, PCT, loop of Henle, DCT and collecting duct
- Explain ultrafiltration at the glomerulus and why blood cells and large proteins are normally not filtered out
- Explain selective reabsorption of glucose, amino acids, ions and water
- Understand the role of the loop of Henle and collecting duct in concentrating urine
- Explain how ADH increases water reabsorption and leads to more concentrated urine

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Name the main regions of the kidney and state where urine collects before entering the ureter.
2. What is the nephron, and why is it important?
3. How does ultrafiltration happen at the glomerulus?
4. What is the difference between selective reabsorption and tubular secretion?
5. How does ADH affect the concentration of urine?

## Introduction

The renal system is a high-yield topic because it links anatomy, physiology and homeostasis. In exam questions, you are often expected to move beyond naming structures and explain how kidney structure allows urine to form, how useful substances are kept in the body, and how water balance is controlled.

A common weakness is treating urine formation as one process. It is not. The kidney first filters blood, then reclaims useful substances, then fine-tunes water and ion balance. If you keep those stages separate, the topic becomes much easier to understand and explain clearly in an exam.

> **Key Term: renal system**
> The body system that removes nitrogenous waste in urine and helps regulate water and mineral ion balance.

### Gross anatomy of the kidney and the urinary pathway

The renal or urinary system includes the **kidneys**, **ureters**, **bladder** and **urethra**. The kidneys do the main physiological work. The ureters transport urine to the bladder, the bladder stores it, and the urethra carries it out of the body during urination.

Each kidney is supplied with blood by a **renal artery** and drained by a **renal vein**. This matters because kidney function depends on processing blood. Urine is not made from food directly in the gut or from water in the bladder. It is made by filtering the blood.

The kidney has several visible regions.

- The **renal cortex** is the outer region.
- The **renal medulla** is the inner region.
- The **renal pelvis** is the central funnel-shaped region that collects urine before it enters the ureter.

These regions are not just labels. They relate directly to function. The cortex contains the start of the nephron, including the glomeruli and convoluted tubules. The medulla contains loops of Henle and collecting ducts, which are vital for concentrating urine. The renal pelvis is a collecting space, not a site of filtration.

> **Key Term: renal cortex**
> The outer region of the kidney, containing glomeruli, Bowman's capsules and convoluted tubules.
>
> **Key Term: renal medulla**
> The inner region of the kidney, containing loops of Henle and collecting ducts.
>
> **Key Term: renal pelvis**
> The central collecting region of the kidney where urine gathers before passing into the ureter.

A common exam error is saying that urine is formed in the renal pelvis. This is incorrect. Urine is formed in nephrons, then drains into collecting ducts, then into the renal pelvis.

> **Test Tip:** If a question asks you to trace urine flow, make sure you start in the nephron or collecting duct, not the renal pelvis. The renal pelvis collects urine; it does not make it.

### The nephron as the functional unit of the kidney

Each kidney contains about **1 million nephrons**. The nephron is the kidney's functional unit because it is the structure where blood is filtered and where the filtrate is modified into urine.

A nephron includes the following parts, in order:

- afferent arteriole
- glomerulus
- Bowman's capsule
- proximal convoluted tubule (PCT)
- loop of Henle
- distal convoluted tubule (DCT)
- collecting duct

Blood enters the glomerulus through the **afferent arteriole** and leaves through the **efferent arteriole**. The filtrate formed in Bowman's capsule then travels along the tubule.

> **Key Term: nephron**
> The functional unit of the kidney where filtration, reabsorption and secretion occur to form urine.
>
> **Key Term: glomerulus**
> A knot of capillaries inside Bowman's capsule where ultrafiltration takes place.
>
> **Key Term: Bowman's capsule**
> The cup-shaped start of the nephron that collects filtrate forced out of the glomerulus.

The nephron is an excellent exchange structure for three main reasons.

First, it is long, giving a large surface area for movement of substances.

Second, nephron walls are thin, so diffusion distances are short.

Third, it is closely associated with capillaries, so substances can move efficiently between the filtrate and the blood.

This is why the kidney can remove waste while also keeping valuable substances. Without reabsorption, huge amounts of water, glucose and amino acids would be lost in urine.

### Urine formation: ultrafiltration, selective reabsorption and tubular secretion

Urine formation is easiest to revise as three linked stages:

1. **ultrafiltration**
2. **selective reabsorption**
3. **tubular secretion**

These are related, but they are not the same.

> **Key Term: ultrafiltration**
> Filtration under pressure at the glomerulus that forces water and small solutes into Bowman's capsule.
>
> **Key Term: selective reabsorption**
> The process by which useful substances are taken back from the filtrate into the blood.
>
> **Key Term: tubular secretion**
> The movement of extra substances from the blood into the nephron tubule.

Ultrafiltration happens at the glomerulus. Blood arrives under high pressure, and this pressure pushes water and small dissolved substances out of the capillaries and into Bowman's capsule. The high pressure is possible because the afferent arteriole is wider than the efferent arteriole. Blood enters more easily than it leaves, so pressure builds inside the glomerulus.

That cause-and-effect chain is very useful in exams:

wider afferent arteriole + narrower efferent arteriole -> high hydrostatic pressure in glomerulus -> filtration of small molecules into Bowman's capsule

The filtrate contains:

- water
- glucose
- amino acids
- mineral ions such as sodium, potassium and chloride
- urea

It should not normally contain:

- red blood cells
- most large plasma proteins

This is a key distinction. If blood cells and large proteins entered the filtrate in large amounts, they would usually remain in the tubule and appear in urine, which suggests damage.

After ultrafiltration, the filtrate moves into the PCT. This is where most selective reabsorption occurs. Glucose and amino acids are valuable nutrients, so the body reclaims them. Many ions are also reabsorbed, and most water follows.

This explains why filtrate and urine are not the same thing. Filtrate starts off containing many useful substances. Urine is what remains after the kidney has taken back what the body still needs.

Tubular secretion is the opposite direction to reabsorption. Instead of moving substances from filtrate to blood, the nephron moves additional substances from blood into the tubule. These include **H+**, **K+** and some drugs or toxins. This helps with waste removal and ion balance.

Students often confuse secretion with excretion. Secretion into the tubule is one step in urine formation. Excretion is the later removal of urine from the body.

> **Test Tip:** If asked to compare ultrafiltration and selective reabsorption, make the contrast explicit. Ultrafiltration moves many small substances into filtrate at the start. Selective reabsorption takes useful substances back into the blood later.

### The loop of Henle, collecting duct and hormonal control of urine concentration

The loop of Henle is important because it helps produce concentrated urine. It does this by helping set up a concentration gradient in the medulla.

The two limbs behave differently.

- The **descending limb** is more permeable to water.
- The **ascending limb** is not permeable to water and allows salts to leave.

This difference is easy to memorise but needs explaining. As filtrate moves down the descending limb, water can move out by osmosis into the surrounding medulla. As filtrate moves up the ascending limb, salts leave but water does not follow. This helps make the medulla increasingly concentrated.

That matters because the collecting duct passes through the medulla. Since the medulla has a lower water potential, water can leave the collecting duct by osmosis. The more water that leaves, the more concentrated the urine becomes.

> **Key Term: loop of Henle**
> The U-shaped part of the nephron that helps create the medullary concentration gradient needed to concentrate urine.
>
> **Key Term: collecting duct**
> The final part of the nephron system where water reabsorption is adjusted before urine passes to the renal pelvis.

The hormone **ADH** controls how much water is reabsorbed in the distal nephron and collecting duct. When the body is dehydrated, ADH release increases. ADH makes the collecting duct more permeable to water, so more water is reabsorbed into the blood. This produces a smaller volume of more concentrated urine.

When the body has excess water, less ADH is released. The collecting duct becomes less permeable to water, less water is reabsorbed, and a larger volume of more dilute urine is produced.

> **Key Term: ADH**
> Antidiuretic hormone; a hormone that increases water reabsorption in the distal nephron and collecting duct, making urine more concentrated.

A related hormone is **aldosterone**. Students often mix this up with ADH. ADH mainly controls water reabsorption directly. Aldosterone mainly increases sodium reabsorption in the distal nephron, and water follows sodium by osmosis. So both can increase water retention, but they do it in different ways.

Finally, normal urine usually contains water, urea, creatinine and variable mineral ions. It should not normally contain glucose, large proteins or blood cells. If these appear, it suggests something abnormal in blood glucose control or kidney filtration.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- The renal system removes urea and helps regulate water and ion balance
- The urinary pathway is kidney to ureter to bladder to urethra
- The main kidney regions are the renal cortex, renal medulla and renal pelvis
- The nephron is the functional unit of the kidney, and each kidney contains about 1 million nephrons
- The nephron includes the afferent arteriole, glomerulus, Bowman's capsule, PCT, loop of Henle, DCT and collecting duct
- Ultrafiltration occurs at the glomerulus because pressure is high
- The afferent arteriole is wider than the efferent arteriole, helping create filtration pressure
- Filtrate contains water, glucose, amino acids, ions and urea, but not normally blood cells or large proteins
- Selective reabsorption returns useful substances, especially in the proximal convoluted tubule
- Tubular secretion adds extra substances such as H+ and K+ into the tubule
- The loop of Henle helps create the medullary concentration gradient needed to concentrate urine
- ADH increases water reabsorption in the collecting duct, causing more concentrated urine

## Key Terms and Concepts

- renal system
- renal cortex
- renal medulla
- renal pelvis
- nephron
- glomerulus
- Bowman's capsule
- ultrafiltration
- selective reabsorption
- tubular secretion
- loop of Henle
- collecting duct
- ADH


Renal system - Kidney anatomy, nephron, and urine formation