Kidney Structure and the Nephron

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).

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: