OCR Specification focus:
‘Describe kidney gross structure and nephron histology, including associated blood vessels, from dissection and stained sections.’
The kidney plays a vital role in excretion and osmoregulation, filtering blood to remove metabolic waste. Its structure and histology demonstrate precise adaptations for efficient filtration, reabsorption, and secretion.
The Gross Structure of the Kidney
Each human has two bean-shaped kidneys, located retroperitoneally on either side of the spine. Their structure reflects both filtration and homeostatic control.
External Features
Renal capsule – a tough fibrous layer protecting the kidney from trauma and infection.
Renal hilum – the concave entry point for the renal artery, renal vein, and ureter.
Renal artery delivers oxygenated blood rich in nitrogenous waste.
Renal vein carries filtered blood back to systemic circulation.
Ureter transports urine to the bladder.
Internal Structure
The kidney can be divided into three main regions:
Frontal section of a human kidney showing the renal cortex, renal medulla (pyramids and columns), and the renal pelvis leading to the ureter. Labels also identify the hilum and major blood vessels, matching features described in the OCR specification. Some minor extras (e.g. adrenal inset) are present but do not distract from the core gross anatomy. Source.
Cortex – the outer layer containing the renal corpuscles and convoluted tubules.
Medulla – inner region composed of renal pyramids and loops of Henle.
Pelvis – a funnel-shaped cavity collecting urine into the ureter.
Blood flow and tissue arrangement in these regions underpin efficient filtration and reabsorption.
Blood Supply to the Kidney
The kidney receives about 20% of cardiac output, ensuring constant filtration.
Renal artery branches into afferent arterioles supplying each nephron.
Within the nephron, the arteriole forms the glomerulus, a dense capillary network for filtration.
Blood exits the glomerulus via the efferent arteriole, which maintains higher pressure to aid filtration.
The efferent arteriole then divides into:
Peritubular capillaries surrounding convoluted tubules.
Vasa recta in juxtamedullary nephrons, maintaining osmotic gradients in the medulla.
The Nephron: Structural and Functional Unit
Nephron: The microscopic structural and functional unit of the kidney responsible for filtration, reabsorption, and secretion to produce urine.
Each kidney contains approximately one million nephrons, each operating independently but coordinated for total renal function.
Labeled diagram of a nephron showing the renal corpuscle and tubular segments (PCT, loop of Henle, DCT) draining to a collecting duct. Arteriolar inflow and peritubular/vasa recta associations are indicated to contextualise microscopic structures. The figure includes simple flow arrows; these add minor functional context beyond the syllabus but remain helpful. Source.
Overview of Nephron Regions
Renal corpuscle (glomerulus + Bowman’s capsule): site of ultrafiltration.
Proximal convoluted tubule (PCT): reabsorbs the majority of filtered substances.
Loop of Henle: creates a concentration gradient in the medulla.
Distal convoluted tubule (DCT): fine-tunes ion and pH balance.
Collecting duct: final site for water reabsorption under hormonal control.
Histology of the Nephron
Histological examination of stained kidney sections reveals specialised epithelial adaptations.
The Renal Corpuscle
Located in the cortex, the renal corpuscle consists of:
Glomerulus – a network of fenestrated capillaries.
Bowman’s capsule – a double-layered capsule collecting filtrate.
Podocytes line the inner layer of Bowman’s capsule, forming filtration slits that restrict passage of large molecules like plasma proteins. The basement membrane acts as a selective barrier, allowing only small molecules such as water, glucose, and ions to pass.
Ultrafiltration: The process in which small molecules are forced from the blood in the glomerulus into the Bowman’s capsule under high hydrostatic pressure.
Histological slides show the glomerulus as a dense cluster of capillaries surrounded by a white, crescent-shaped Bowman’s space.
PAS-stained section of renal cortex highlighting the glomerular basement membranes and the renal corpuscle within Bowman’s space. Proximal and distal tubules are visible around the corpuscle. The emphasis on PAS-positive basement membranes is extra detail beyond the OCR essentials but enhances recognition. Source.
Proximal Convoluted Tubule (PCT)
The PCT, located in the cortex, is lined with cuboidal epithelial cells with microvilli forming a brush border, increasing surface area for reabsorption.
Features observed under the microscope include:
Prominent nucleus near the basal side.
Numerous mitochondria for active transport.
Narrow lumen due to dense microvilli.
The PCT reabsorbs approximately 85% of filtrate, including glucose, amino acids, and sodium ions, returning them to the blood in peritubular capillaries.
Loop of Henle
Found mainly in the medulla, this U-shaped section has distinct histological regions:
Descending limb: thin walls permeable to water but not solutes.
Ascending limb: thicker walls impermeable to water but actively transports sodium and chloride ions.
This countercurrent system establishes the medullary osmotic gradient, vital for water reabsorption in the collecting duct.
Distal Convoluted Tubule (DCT)
Located in the cortex, the DCT’s cuboidal epithelial cells lack microvilli, producing a clear lumen in stained sections. It fine-tunes ion balance through selective secretion and reabsorption.
The juxtaglomerular apparatus, adjacent to the glomerulus, contains macula densa cells that monitor sodium concentration and regulate blood pressure via renin secretion.
Juxtaglomerular apparatus: A structure where the DCT contacts its glomerulus, involved in regulating blood pressure and glomerular filtration rate.
Collecting Duct
Collecting ducts pass through the medulla and drain into the renal pelvis. They are lined with cuboidal to columnar epithelial cells, responsive to antidiuretic hormone (ADH).
Under ADH influence, aquaporins (water channels) insert into the membrane, allowing controlled water reabsorption. In histological sections, collecting ducts appear as large circular structures with distinct cell borders.
Associated Blood Vessels and Microscopic Features
Histological sections often reveal intricate vascular arrangements supporting nephron function:
Afferent arteriole entering each glomerulus is wider than the efferent arteriole, maintaining pressure for filtration.
Peritubular capillaries and vasa recta surround tubular structures, appearing as thin-walled vessels containing red blood cells.
In the medulla, vasa recta run parallel to the loops of Henle, crucial for maintaining osmotic equilibrium.
Microscopy also reveals the transition from dense cortical tissue to the lighter, striated medulla due to tubular and capillary orientation.
Practical Observation in Dissection and Microscopy
When dissecting a mammalian kidney:
The cortex appears granular due to the presence of corpuscles and convoluted tubules.
The medulla displays striations from parallel loops and collecting ducts.
The pelvis is smooth and pale, collecting urine.
Under stained histological slides:
Renal corpuscles are visible as round structures with dense capillaries.
PCTs show small lumina and thick walls.
DCTs appear clearer and more open.
Collecting ducts are easily distinguishable by their large lumina and columnar cells.
These observations support understanding of the kidney’s gross structure and nephron histology, directly aligning with the OCR specification’s emphasis on identifying and describing renal tissues and vessels from both dissection and microscopy.
FAQ
Cortical nephrons have their renal corpuscles in the outer cortex and have short loops of Henle that only dip slightly into the medulla.
Juxtamedullary nephrons have corpuscles near the corticomedullary junction and long loops of Henle that extend deep into the medulla.
This longer loop allows for a more effective countercurrent multiplier system, helping to produce concentrated urine. Juxtamedullary nephrons are fewer but are crucial for osmoregulation under conditions of limited water availability.
The afferent arteriole delivers blood into the glomerulus, while the efferent arteriole drains it.
Because the efferent arteriole has a narrower diameter, resistance to outflow increases.
This difference creates a high hydrostatic pressure within the glomerular capillaries, essential for driving ultrafiltration.
Without this pressure gradient, filtration of small solutes such as water, ions, and glucose into the Bowman’s capsule would not occur efficiently.
Podocytes not only form the final filtration barrier but also support the glomerular capillary structure.
Their interdigitating foot processes maintain capillary integrity against pressure changes.
They secrete components of the glomerular basement membrane, ensuring its renewal.
Podocytes also influence signalling between the glomerulus and the juxtaglomerular apparatus, helping regulate filtration rate.
Damage to podocytes can lead to proteinuria, where proteins leak into the filtrate — a sign of renal pathology.
In stained kidney sections:
The PCT has cuboidal cells with microvilli, giving a fuzzy appearance to the lumen (“brush border”).
The DCT lacks microvilli, producing a clear, open lumen.
Additionally, PCT cells have more mitochondria for active transport, appearing darker under staining. DCT cells appear lighter and are often located near the renal corpuscle, forming part of the juxtaglomerular apparatus.
These histological cues help students and pathologists identify nephron segments in microscopy.
The vasa recta are capillary loops running parallel to the loops of Henle in juxtamedullary nephrons.
They preserve the osmotic gradient of the medulla by:
Allowing slow blood flow to reduce solute washout.
Acting as a countercurrent exchanger, where solutes and water move between descending and ascending limbs of the vessels.
This mechanism ensures that reabsorbed water and solutes return to circulation without disrupting medullary concentration gradients, which are vital for urine concentration.
Practice Questions
Question 1 (2 marks)
Identify two structural features of the proximal convoluted tubule (PCT) that enable it to carry out its function effectively.
Mark scheme:
Award 1 mark for each correct feature, up to 2 marks.
Presence of microvilli forming a brush border to increase the surface area for reabsorption (1 mark).
Numerous mitochondria to supply ATP for active transport of solutes (1 mark).
Basal membrane infoldings to increase area for ion transport (credit as alternative to above) (1 mark).
Question 2 (5 marks)
Describe and explain how the structure of the nephron and its associated blood vessels enable the process of ultrafiltration in the kidney.
Mark scheme:
Award up to 5 marks for a clear, structured explanation.
Afferent arteriole is wider than the efferent arteriole, creating high hydrostatic pressure in the glomerulus (1 mark).
High pressure forces water, glucose, ions and small molecules out of the blood into the Bowman’s capsule (1 mark).
Endothelium of glomerular capillaries has fenestrations (pores) that allow small molecules to pass through (1 mark).
The basement membrane acts as a selective barrier, preventing large proteins and blood cells from being filtered (1 mark).
Podocytes of the Bowman’s capsule have filtration slits between their foot processes, aiding the selective movement of small solutes into the capsular space (1 mark).
