Kidney Structure and the Nephron

Spec Mapping — OCR H420 Module 5.1.2 — Excretion, content statements covering the macroscopic anatomy of the mammalian kidney (cortex, medulla, pyramids, pelvis, ureter), the microscopic structure of the nephron (Bowman's capsule, PCT, loop of Henle, DCT, collecting duct), and the associated renal blood supply (afferent and efferent arterioles, glomerulus, peritubular capillaries, vasa recta) (refer to the official OCR H420 specification document for exact wording).

The mammalian kidney is an organ of astonishing precision. In a resting human, the two kidneys filter around 180 L of plasma every day, reabsorb 99 % of it, and produce only about 1.5 L of urine, the composition of which is finely tuned to the body's needs. Understanding how this happens requires a detailed grasp of kidney anatomy at both macroscopic and microscopic levels, with particular attention to the nephron — the functional unit of the kidney. This lesson covers gross kidney structure, the nephron in detail, and the blood supply, matching OCR A-Level Biology A specification module 5.1.2(e).

Nephron anatomy was established by twentieth-century histologists; the functional understanding of how each segment contributes to urine formation came from twentieth-century renal physiology, particularly the work of Homer Smith and others on the principles of clearance and filtration. The countercurrent multiplier mechanism in the loop of Henle was elucidated in the 1950s and remains one of the most elegant pieces of comparative anatomy in vertebrate physiology — the loop's length correlates directly with an animal's ability to concentrate urine, ranging from short loops in beaver (which never need to conserve water) to extreme loops in desert rodents such as the kangaroo rat. This work is paraphrased here for context; original publications are not quoted verbatim.

Key Definitions:

• Nephron — the functional unit of the kidney; a long tubule along which the filtrate is modified to form urine.
• Cortex — the outer region of the kidney where Bowman's capsules, proximal and distal convoluted tubules are located.
• Medulla — the inner region of the kidney containing the loops of Henle and collecting ducts.
• Glomerulus — a capillary knot inside the Bowman's capsule where ultrafiltration occurs.

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Gross Anatomy of the Kidney

In a longitudinal section of a mammalian kidney you can see three main regions:

• Cortex — the outer layer, darker and grainy in appearance, containing the Bowman's capsules, the glomeruli, and the proximal and distal convoluted tubules.
• Medulla — the inner region, paler and more striated, organised into pyramid-shaped regions. It contains the loops of Henle and the collecting ducts.
• Pelvis — the central, hollow, funnel-shaped cavity that collects urine from the collecting ducts and passes it into the ureter.

Each kidney receives blood via the renal artery (branching from the abdominal aorta) and returns it via the renal vein to the inferior vena cava. The ureter carries urine from the pelvis to the bladder.

flowchart LR
    A["Renal artery<br/>from aorta"] --> B[Kidney]
    B --> C["Renal vein<br/>to vena cava"]
    B --> D["Ureter<br/>to bladder"]

Blood Supply Within the Kidney

Blood from the renal artery branches into smaller arteries, then into afferent arterioles, each of which feeds one glomerulus. Blood leaves each glomerulus via an efferent arteriole — not a venule — which is a crucial fact. The efferent arteriole is narrower than the afferent, maintaining a high hydrostatic pressure in the glomerulus and driving ultrafiltration.

After leaving the glomerulus, blood in the efferent arteriole enters a second capillary network that wraps around the nephron tubule: the peritubular capillaries (and the vasa recta around the loops of Henle in juxtamedullary nephrons). Only after this second capillary network does blood enter venules and return to the renal vein.

flowchart LR
    A[Renal artery] --> B[Afferent arteriole]
    B --> C[Glomerulus]
    C --> D["Efferent arteriole<br/>narrower"]
    D --> E[Peritubular capillaries / vasa recta]
    E --> F[Renal vein]

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The Nephron: Functional Unit of the Kidney

Each human kidney contains around 1 million nephrons. Each nephron is a narrow, convoluted tubule about 3–5 cm long, arranged so that part lies in the cortex and part in the medulla. The tubule has several distinct regions, each specialised for a particular function.

Parts of the Nephron

1. Bowman's capsule — cup-shaped structure in the cortex, surrounding a glomerulus. The site of ultrafiltration.
2. Proximal convoluted tubule (PCT) — a long, winding tubule in the cortex, just after the Bowman's capsule. The site of most selective reabsorption.
3. Loop of Henle — a hairpin-shaped loop extending into the medulla. It consists of a descending limb (permeable to water but not salts) and an ascending limb (permeable to salts but not water). It creates the high salt concentration in the medulla needed to reabsorb water from the collecting duct.
4. Distal convoluted tubule (DCT) — a shorter, winding tubule in the cortex, between the loop of Henle and the collecting duct. The site of fine-tuned ion and pH regulation.
5. Collecting duct — passes from the cortex through the medulla to the pelvis. The site of final water reabsorption under the control of ADH.

Many nephrons drain into each collecting duct, which delivers the final urine into the kidney pelvis and onward to the ureter.

Flow of Filtrate

flowchart LR
    A[Glomerulus] --> B[Bowman's capsule]
    B --> C[Proximal convoluted tubule]
    C --> D[Loop of Henle descending]
    D --> E[Loop of Henle ascending]
    E --> F[Distal convoluted tubule]
    F --> G[Collecting duct]
    G --> H[Renal pelvis]
    H --> I[Ureter]

Types of Nephron

Two types of nephron exist in the mammalian kidney:

• Cortical nephrons — about 80 % of nephrons. Their glomeruli lie in the outer cortex and their loops of Henle are short, dipping only slightly into the medulla.
• Juxtamedullary nephrons — about 20 % of nephrons. Their glomeruli lie close to the cortex–medulla boundary and their loops of Henle are long, reaching deep into the medulla. These are the nephrons responsible for producing very concentrated urine in water-stressed mammals.

Animals adapted to dry environments (e.g., kangaroo rats) have a higher proportion of juxtamedullary nephrons and longer loops of Henle — they produce extremely concentrated urine.

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Microscopic Structure of Key Regions

Bowman's Capsule

A double-walled cup-shaped structure. The outer wall is a simple squamous epithelium. The inner wall is modified into specialised cells called podocytes, which wrap around the glomerular capillaries and have finger-like processes creating filtration slits (see next lesson for details). Between the walls lies the capsular space, into which the glomerular filtrate collects.

Proximal Convoluted Tubule

The PCT cells are cuboidal epithelial cells with:

• Many microvilli on their apical (luminal) surface, forming a brush border that greatly increases the surface area for reabsorption.
• Many mitochondria, providing ATP for active transport of ions and co-transport of glucose and amino acids.
• Tight junctions between adjacent cells that prevent solutes leaking back into the tubule.
• Basal infoldings on the basolateral (blood-facing) surface to increase the area for Na⁺/K⁺ pumps that drive reabsorption.

Loop of Henle

• Descending limb — thin epithelium, permeable to water but not to ions. Water leaves as the loop dips into the increasingly salty medulla.
• Ascending limb — thicker epithelium in its distal portion, impermeable to water, with active transport of Na⁺, K⁺ and Cl⁻ out of the tubule.

Distal Convoluted Tubule

Cuboidal cells similar to PCT cells but with fewer microvilli and fewer mitochondria. Under hormonal control, DCT cells fine-tune reabsorption of Na⁺ (controlled by aldosterone) and H⁺/HCO₃⁻ for pH regulation.

Collecting Duct

Cuboidal cells whose permeability to water is controlled by ADH (vasopressin) from the posterior pituitary. The collecting duct passes through the hyperosmotic medulla established by the loop of Henle, allowing final concentration of the urine.

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Blood Flow and the Nephron Working Together

The architecture of the kidney is designed so that every nephron has its own close capillary supply:

• Afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries / vasa recta → renal venules.

This arrangement allows the nephron to filter large volumes of plasma under high pressure in the glomerulus, and then to reabsorb (and secrete) substances back into the blood in the peritubular capillaries, which run slowly past the tubule at low pressure.

Structure	Location	Function
Bowman's capsule + glomerulus	Cortex	Ultrafiltration
PCT	Cortex	Selective reabsorption of most filtered substances
Loop of Henle	Medulla	Creates hyperosmotic medulla (countercurrent multiplier)
DCT	Cortex	Fine-tuned reabsorption of ions; pH regulation
Collecting duct	Medulla	Final water reabsorption under ADH control

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Exam Tip

When asked to describe the nephron, use the full sequence Bowman's capsule → PCT → descending loop → ascending loop → DCT → collecting duct → pelvis and mention the cortex/medulla location of each. OCR often awards one mark for the sequence and another for locations.

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Common Exam Mistakes

• Saying blood leaves the glomerulus via a venule. It leaves via the efferent arteriole, which is narrower than the afferent.
• Forgetting the peritubular capillaries. Blood passes through a second capillary bed around the nephron, essential for reabsorption.
• Confusing cortical and juxtamedullary nephrons. Only juxtamedullary nephrons have long loops of Henle reaching deep into the medulla.
• Mislocating the loop of Henle. It is in the medulla, not the cortex.
• Drawing the collecting duct draining into a blood vessel. It drains into the pelvis, then the ureter.
• Missing the podocytes. These specialised cells are essential to the filtration barrier and are a common exam question.
• Saying "filtrate becomes urine in the Bowman's capsule". Urine is only produced after passage through all five regions of the nephron and the collecting duct.

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Quick Recap

• The kidney consists of cortex, medulla and pelvis.
• Blood enters via the renal artery and exits via the renal vein; urine leaves via the ureter.
• The functional unit is the nephron: Bowman's capsule → PCT → loop of Henle → DCT → collecting duct → pelvis.
• The glomerulus is supplied by an afferent arteriole and drained by a narrower efferent arteriole, which maintains high pressure for ultrafiltration.
• Peritubular capillaries (and vasa recta around the loop of Henle) carry out reabsorption and secretion along the length of the nephron.
• Juxtamedullary nephrons have long loops of Henle and allow the production of concentrated urine.

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SVG — The Mammalian Kidney in Longitudinal Section

The OCR canonical diagram has three regions (cortex, medulla, pelvis), three pyramids visible in cross-section, and three tube-and-vessel connections to the kidney (artery, vein, ureter). Memorise all three triplets.

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SVG — A Single Nephron with Vascular Supply