# Kidney Structure and Function

> CIE A-Level Biology · Unit 15: Homeostasis
> Source: https://www.owlsprep.com/study/cie-9700-u15-kidney-structure-and-function/

This module covers the gross structure of the mammalian kidney, the detailed structure of the nephron (the kidney's functional unit), and how structural adaptations enable the key processes of ultrafiltration and selective reabsorption for excretion and osmoregulation.

**Prerequisites:** [Homeostasis principles](https://www.owlsprep.com/study/cie-9700-u15-homeostasis-principles/); Structure of mammalian circulatory system; Osmosis and water potential

## Learning objectives

- Identify the gross and microscopic structure of the mammalian kidney
- Relate nephron structure to the functions of ultrafiltration and selective reabsorption
- Explain the kidney's role in excretion and osmoregulation
- Recognise common exam questions on structural adaptations of the kidney

## Gross Structure of the Mammalian Kidney

The kidneys are paired bean-shaped organs located in the abdominal cavity, attached to the back wall. Each kidney receives oxygenated blood from a renal artery and drains deoxygenated blood via a renal vein, and connects to the bladder via a ureter that carries urine away for excretion.

**Renal Capsule** — Outer tough connective tissue layer that protects the kidney from damage

Internally, the kidney is divided into two main regions: the outer renal cortex and the inner renal medulla. The medulla is split into several cone-shaped renal pyramids, which drain into the central renal pelvis before urine enters the ureter.

**Worked example:** Identify which region of the kidney contains the highest density of glomeruli, and explain why.

1. Glomeruli, the site of ultrafiltration, are almost entirely located in the renal cortex.
2. The cortex receives the highest proportion of renal blood flow, which provides the high hydrostatic pressure required for ultrafiltration to occur.

> **Exam tip:** When asked to label a kidney diagram, always label cortex, medulla, renal artery, renal vein, ureter and pelvis: these are common 2-3 mark exam questions.

## Structure of the Nephron

The nephron is the functional unit of the kidney, with each human kidney containing approximately 1 million nephrons. Each nephron is a long tubule with distinct specialised regions, each adapted for a specific role in processing filtrate.

**Nephron** — The microscopic functional unit of the kidney responsible for forming urine, consisting of a renal corpuscle and a segmented renal tubule

- **Renal corpuscle**: Made of the Bowman's capsule (cup-shaped tubule end) surrounding a capillary knot called the glomerulus, site of ultrafiltration
- **Proximal Convoluted Tubule (PCT)**: First twisted region after Bowman's capsule, site of most selective reabsorption
- **Loop of Henle**: U-shaped tubule extending into the medulla, creates a salt gradient for water reabsorption
- **Distal Convoluted Tubule (DCT)**: Twisted region in the cortex, fine-tunes ion and water reabsorption under hormonal control
- **Collecting Duct**: Receives filtrate from multiple nephrons, passes through the medulla to the pelvis, final adjustment of urine concentration

**Worked example:** Explain how the structure of the glomerulus is adapted for ultrafiltration.

1. Three key structural adaptations support ultrafiltration:
2. 1. Glomerular capillaries have small fenestrations (pores) ~80nm wide that let small molecules (water, glucose, ions, urea) through, but block blood cells and large proteins.
3. 2. The inner Bowman's capsule lining is made of podocytes, with foot-like processes that leave narrow slit pores to further filter large molecules.
4. 3. The wider afferent arteriole entering the glomerulus creates higher hydrostatic pressure than the narrower efferent arteriole leaving, driving filtration.

## Ultrafiltration

Ultrafiltration is the first step in urine formation, where small molecules are forced out of glomerular blood into Bowman's capsule down a hydrostatic pressure gradient. The filtration barrier prevents large molecules and cells from entering the filtrate.

**Ultrafiltration** — Pressure-driven filtration of blood in the renal corpuscle that produces cell- and protein-free filtrate of blood plasma

**Check your understanding**

Which of the following substances would you not find in Bowman's capsule filtrate in a healthy person?

1. Select the correct substance that is absent from normal filtrate

   - A: Urea
   - B: Haemoglobin
   - C: Glucose
   - D: Sodium ions

   *Why:* Haemoglobin is a large protein contained inside red blood cells. Neither red blood cells nor large plasma proteins can pass through the healthy filtration barrier, so they do not enter the filtrate.

**Worked example:** Explain why hydrostatic pressure in the glomerulus is higher than in other systemic capillaries.

1. Blood enters the glomerulus via the afferent arteriole, which has a larger lumen diameter than the efferent arteriole that carries blood away from the glomerulus.
2. The narrower lumen of the efferent arteriole creates greater resistance to blood flow out of the glomerulus.
3. This results in higher hydrostatic pressure building inside glomerular capillaries, which provides the force required for ultrafiltration.

## Selective Reabsorption

After ultrafiltration, the filtrate contains many useful substances that the body needs to retain, including all glucose, most water and most mineral ions. These are reabsorbed back into the blood via selective reabsorption, with most occurring in the proximal convoluted tubule.

**Selective Reabsorption** — Process by which useful molecules and water are reabsorbed from filtrate back into blood, leaving waste products like urea in urine

The PCT is structurally adapted for reabsorption: its epithelial cells have a brush border of microvilli to increase surface area, many mitochondria to provide ATP for active transport, and co-transporter proteins for glucose and sodium uptake. All glucose and amino acids are reabsorbed here, along with ~65% of water and sodium ions.

**Worked example:** Explain how glucose is reabsorbed from filtrate in the PCT into blood.

1. 1. Sodium ions are actively transported out of PCT epithelial cells into surrounding capillaries, lowering sodium concentration inside the cell.
2. 2. Sodium ions and glucose molecules diffuse into the epithelial cell from filtrate via co-transporter proteins on the lumen-facing membrane.
3. 3. Glucose diffuses out of the epithelial cell into blood via facilitated diffusion through channel proteins.
4. All glucose is reabsorbed in a healthy kidney, so no glucose is normally present in urine.

## Common pitfalls

- **Wrong:** Stating that large proteins like albumin pass into Bowman's capsule in a healthy kidney
  - Why it fails: The filtration barrier blocks molecules larger than ~69,000 relative molecular mass, so plasma proteins remain in the blood
  - Correct: Only small molecules (glucose, urea, ions, water) enter the filtrate in a healthy kidney
- **Wrong:** Confusing afferent and efferent arterioles: claiming the efferent arteriole has a wider lumen
  - Why it fails: The difference in lumen diameter is what maintains high glomerular pressure; mixing this reverses the effect
  - Correct: The afferent arteriole (entering the glomerulus) has a wider lumen, and the efferent (leaving) is narrower, creating high hydrostatic pressure
- **Wrong:** Claiming all urea is excreted in urine
  - Why it fails: Around 50% of filtered urea is reabsorbed into blood, and some diffuses back into the medulla to maintain the osmotic gradient
  - Correct: Urea is a waste product, but only a portion of filtered urea is excreted in final urine
- **Wrong:** Stating ultrafiltration is active transport that requires ATP
  - Why it fails: Ultrafiltration is driven by hydrostatic pressure, not active transport, so it does not require ATP energy
  - Correct: Ultrafiltration is a passive, pressure-driven process; active transport is only used in selective reabsorption

## Cheatsheet

| Nephron Structure | Location | Main Function |
| --- | --- | --- |
| Renal corpuscle | Cortex | Ultrafiltration |
| Proximal Convoluted Tubule | Cortex | Main site of selective reabsorption |
| Loop of Henle | Medulla | Maintains medullary osmotic gradient |
| Distal Convoluted Tubule | Cortex | Regulates ion/water balance |
| Collecting duct | Cortex → Medulla | Final adjustment of urine concentration |

## What's next

Understanding kidney structure and function is the foundation for learning about hormonal control of osmoregulation, the next key topic in CIE A-Level homeostasis. This knowledge is also required to understand kidney failure and its different treatments, which is a common extended response question in Paper 4. Mastering the link between structure and function here will help you answer a wide range of exam questions that ask you to relate adaptations to their roles in excretion and osmoregulation.

- [Control and Coordination](https://www.owlsprep.com/study/cie-9700-u16-overview/)
- [Neurone structure](https://www.owlsprep.com/study/cie-9700-u16-neurone-structure/)
- [Nerve impulse propagation](https://www.owlsprep.com/study/cie-9700-u16-nerve-impulse-propagation/)

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