25.4 Microscopic Anatomy of the Kidney
5 min read•june 18, 2024
The kidney's microscopic anatomy is a marvel of intricate structures working in harmony. From the renal cortex's filtering units to the medulla's concentrating tubules, each component plays a crucial role in maintaining our body's balance.
Understanding the kidney's microstructures is key to grasping how our bodies filter blood, regulate fluids, and remove waste. The , tubules, and capillaries work together in a complex dance of , , and to keep us healthy.
Microscopic Anatomy of the Kidney
Renal cortex vs medulla histology
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- Renal cortex
- Contains renal corpuscles (Bowman's capsules and glomeruli) and convoluted tubules (proximal and distal)
- Appears lighter in color due to the numerous renal corpuscles scattered throughout the cortical tissue
- Houses the initial and final segments of the , the (PCT) and (DCT)
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- Consists of renal pyramids composed of straight tubules (loops of Henle) and collecting ducts arranged in parallel
- Appears darker and striated due to the parallel arrangement of the tubules and blood vessels ()
- Contains the intermediate segments of the nephron, the descending and ascending limbs of the , as well as the collecting ducts that converge to form the
Structure of filtration membrane
- membrane components
- of glomerular capillaries allows passage of small molecules (ions, glucose) and fluid while restricting larger particles (blood cells, proteins)
- Basement membrane provides structural support and acts as a selective barrier based on molecular size and charge
- between interdigitating podocyte foot processes regulate the passage of molecules, preventing the loss of essential proteins (albumin) while allowing filtration of waste products (urea, creatinine)
- Functions of the filtration membrane
- Selectively filters blood to form an containing water, ions, glucose, amino acids, and small waste products
- Retains larger molecules such as proteins (hemoglobin) and blood cells within the capillaries, maintaining the colloid osmotic pressure of blood
- Maintains the balance of electrolytes and fluid in the body by regulating the composition of the filtrate entering the renal tubules
- Determines the (GFR), which is the volume of fluid filtered from the glomerular capillaries into per unit time
Renal Microstructures and Their Functions
Components of renal structures
- Renal corpuscle
- Bowman's capsule: Double-walled epithelial structure surrounding the glomerulus, with the forming the outer wall and the () in contact with the glomerular capillaries
- Glomerulus: Tuft-like network of capillaries arising from the and draining into the , responsible for the filtration of blood
- : Specialized epithelial cells with long, interdigitating foot processes that form the , contributing to the selective permeability of the filtration barrier
- Renal tubules
- Proximal convoluted tubule (PCT) reabsorbs the majority of the filtrate components, including nutrients (glucose, amino acids), ions (sodium, chloride), and water
- consists of the (permeable to water) and (impermeable to water, active transport of ions), creating a concentration gradient in the medulla
- Distal convoluted tubule (DCT) fine-tunes the composition of the filtrate by selectively reabsorbing or secreting ions (sodium, potassium, calcium) and regulating acid-base balance
- receives filtrate from multiple and conducts it to the renal pelvis, responsive to hormones (ADH, aldosterone) for final adjustments of water and electrolyte balance
- Renal capillaries
- surround the renal tubules, participating in the of essential substances and secretion of waste products and excess ions
- are parallel arrangements of capillaries in the medulla that maintain the osmotic gradient necessary for
Roles of renal capillaries
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- Reabsorb essential substances from the renal tubules, such as glucose, amino acids, and ions (sodium, potassium, calcium), returning them to the bloodstream
- Secrete waste products and excess substances into the renal tubules, such as hydrogen ions, potassium, and organic acids (uric acid), for elimination in the urine
- Maintain the concentration gradient necessary for efficient reabsorption and secretion processes by equilibrating with the interstitial fluid
- Vasa recta
- Countercurrent exchange system that preserves the medullary osmotic gradient established by the loop of Henle
- Descending vasa recta passively loses water and gains solutes (sodium, chloride, urea) as it enters the hyperosmotic medulla
- Ascending vasa recta passively gains water and loses solutes as it returns to the cortex, maintaining the concentration gradient
- Helps maintain the high osmolarity of the medullary interstitium, which is essential for the production of concentrated urine
Juxtaglomerular apparatus composition
- Location: is found at the point where the DCT comes in close contact with the afferent and efferent arterioles of its own nephron
- Cellular composition
- : Specialized cells of the DCT that sense changes in the tubular fluid composition (sodium chloride concentration) and transmit signals to the JG cells
- (JG cells): Modified smooth muscle cells in the walls of the afferent arteriole that secrete in response to decreased blood pressure, reduced sodium chloride delivery to the , or sympathetic stimulation (β1-adrenergic receptors)
- : Cells located between the macula densa and the JG cells that provide structural support and help regulate glomerular filtration rate through contraction and relaxation
- Function: Participates in , a mechanism that adjusts and GFR based on the composition of the tubular fluid
Histology of renal tubule segments
- Proximal convoluted tubule (PCT)
- Simple cuboidal epithelium with prominent microvilli forming a , increasing the surface area for reabsorption
- Acidophilic cytoplasm due to the presence of numerous mitochondria, reflecting the high energy demand for active transport processes
- Loop of Henle
- Descending thin limb: Simple squamous epithelium, highly permeable to water due to the presence of channels, allowing passive water reabsorption
- Ascending thin limb: Simple squamous epithelium, impermeable to water but permeable to ions, contributing to the generation of the medullary concentration gradient
- Thick ascending limb: Simple cuboidal epithelium with tight junctions, actively transports ions (sodium, chloride, potassium) out of the tubule, diluting the filtrate and generating the medullary osmotic gradient
- Distal convoluted tubule (DCT)
- Simple cuboidal epithelium with less prominent microvilli compared to PCT, reflecting its role in fine-tuning the composition of the filtrate
- Lighter staining cytoplasm compared to PCT due to fewer mitochondria, as the DCT has lower energy requirements for its transport processes
- Collecting duct
- Simple cuboidal epithelium in the cortex, transitioning to columnar epithelium in the medulla, allowing for greater water permeability in the presence of
- Presence of two cell types: reabsorb water and secrete potassium in response to ADH and aldosterone, while (α and β subtypes) regulate acid-base balance by secreting hydrogen ions or bicarbonate
Renal Physiology and Regulation
- Urine concentration: Process by which the kidneys produce concentrated urine through the system in the loop of Henle and the action of ADH on the collecting ducts
- : The kidney's role in maintaining fluid and electrolyte balance through selective reabsorption and secretion of water and solutes
- : Measure of how efficiently the kidneys remove substances from the blood, used to assess kidney function and determine the for various substances
Key Terms to Review (70)
Afferent Arteriole: The afferent arteriole is a small blood vessel that carries blood from the renal artery into the glomerulus of the nephron, the functional unit of the kidney. It plays a crucial role in the microscopic anatomy of the kidney and the physiology of urine formation.
Angiotensin I: Angiotensin I is an inactive precursor peptide hormone in the renin-angiotensin system that is converted to angiotensin II by the angiotensin-converting enzyme (ACE) primarily in the lungs. It plays a crucial role in regulating blood pressure and electrolyte balance.
Antidiuretic Hormone: Antidiuretic hormone (ADH), also known as vasopressin, is a peptide hormone produced by the hypothalamus and released by the posterior pituitary gland. It plays a crucial role in the regulation of fluid balance and osmotic homeostasis within the body by promoting water reabsorption in the kidneys, thereby reducing urine output and increasing water retention.
Antidiuretic hormone (ADH): Antidiuretic hormone (ADH) is a peptide hormone produced primarily by the hypothalamus and released by the posterior pituitary gland, which regulates water balance in the body by increasing the absorption of water in the kidneys. This process decreases urine volume, conserves body water, and increases blood pressure.
Aquaporin: Aquaporins are protein channels in the cell membrane that facilitate the transport of water molecules in and out of cells. They play a crucial role in regulating water balance within various tissues, including the kidneys.
Aquaporin: Aquaporins are specialized water channel proteins found in the cell membranes of various tissues, including the kidneys, which facilitate the rapid and selective movement of water molecules across the cell membrane, playing a crucial role in water homeostasis and the regulation of fluid balance within the body.
Ascending Limb: The ascending limb is a segment of the nephron, the functional unit of the kidney, that plays a crucial role in the process of urine formation and regulation of fluid and electrolyte balance within the body.
Bowman's Capsule: Bowman's capsule is a cup-shaped structure that forms the initial part of the nephron in the kidney. It is a crucial component in the process of urine formation, serving as the site where the filtration of blood takes place to produce the primary filtrate that will eventually become urine.
Brush border: A brush border is a microvilli-covered surface found on the epithelial cells of the small intestine and kidney, which significantly increases the surface area for absorption and secretion. These densely packed microvilli give the cell's surface a brush-like appearance under a microscope.
Brush Border: The brush border refers to the densely packed microvilli found on the apical surface of certain epithelial cells, particularly in the small intestine and renal proximal convoluted tubules. This specialized structure increases the surface area for more efficient absorption and reabsorption of nutrients and substances.
Collecting Duct: The collecting duct is a key component of the kidney's nephron, the functional unit responsible for filtering blood, reabsorbing useful substances, and excreting waste. It serves as the final pathway for urine before it is expelled from the body, playing a crucial role in the regulation of fluid volume and electrolyte balance.
Countercurrent multiplication: Countercurrent multiplication is a mechanism in the nephron of the kidney that enhances the concentration of urine by utilizing the opposing flow of fluid in the loops of Henle. This process allows for the efficient reabsorption of water and solutes, playing a crucial role in regulating body fluid balance and urine concentration.
Descending Limb: The descending limb is a crucial component of the nephron, the functional unit of the kidney, responsible for the reabsorption and concentration of filtrate as it travels through the kidney's microscopic structures.
Distal Convoluted Tubule: The distal convoluted tubule is a segment of the nephron, the functional unit of the kidney, located between the loop of Henle and the collecting duct. It plays a crucial role in the kidney's ability to regulate fluid volume and composition within the body.
Distal convoluted tubules: The distal convoluted tubule is a portion of the kidney nephron that lies between the loop of Henle and the collecting duct, playing a crucial role in the reabsorption of water and ions to maintain body fluid balance. It regulates pH and electrolyte levels by selective reabsorption and secretion.
Efferent arteriole: The efferent arteriole is a small blood vessel that carries blood away from the glomerulus of the kidney after filtration has occurred. It plays a critical role in regulating blood pressure within the glomerulus and influencing the rate of filtration.
Efferent Arteriole: The efferent arteriole is a small blood vessel that carries blood away from the glomerulus, the filtration unit in the kidney's nephron. It plays a crucial role in the process of urine formation by regulating the rate of blood flow and the amount of filtration that occurs in the kidney.
Erythropoietin: Erythropoietin is a glycoprotein hormone produced primarily by the kidneys that stimulates the production of red blood cells in the bone marrow. It plays a crucial role in regulating erythropoiesis, especially in response to low oxygen levels in the blood, which can be caused by various factors such as high altitudes, anemia, or respiratory diseases.
Extraglomerular Mesangial Cells: Extraglomerular mesangial cells are specialized cells located outside the glomerular tuft within the kidney's renal corpuscle. These cells play a crucial role in regulating the function and structure of the glomerulus, the filtration unit of the nephron.
Fenestrated Endothelium: Fenestrated endothelium is a type of endothelial cell lining found in certain organs that allows for the selective passage of materials between the bloodstream and surrounding tissues. It is characterized by the presence of small, circular openings or 'fenestrations' that facilitate the exchange of substances across the endothelial barrier.
Fenestrations: Fenestrations are small pores or openings in the endothelial cells of the glomerular capillaries within the kidneys. They allow for the filtration of blood while blocking larger molecules such as proteins and blood cells.
Filtration: Filtration in the context of capillary exchange is the process by which blood pressure forces fluid and small solutes through the capillary wall, leaving larger components like proteins and blood cells behind. It is a crucial mechanism for the movement of water and nutrients from blood into tissues.
Filtration: Filtration is the process by which substances are separated from a solution or mixture by passing it through a filter or membrane that allows the desired components to pass through while retaining the unwanted ones. This process is essential in various physiological contexts, including the function of epithelial tissues, capillary exchange, and the regulation of fluid volume and composition in the urinary system.
Filtration slits: Filtration slits are narrow gaps between the podocytes in the glomerulus of the kidney, acting as a barrier that allows the passage of water and small molecules into the Bowman's capsule while blocking larger molecules like proteins. These structures play a critical role in the kidney's ability to filter blood and form urine.
Filtration Slits: Filtration slits are small gaps or pores found between the foot processes of podocytes in the glomerular capillary tuft of the kidney. These slits act as a selective barrier, allowing the filtration of small molecules and water from the blood into the Bowman's capsule while retaining larger molecules and cells.
Forming urine: Forming urine is the physiological process by which the kidneys filter waste products, excess substances, and water from the bloodstream to produce urine. This intricate process involves various stages including filtration, reabsorption, secretion, and excretion to maintain fluid and electrolyte balance in the body.
Glomerular Filtration Rate: Glomerular filtration rate (GFR) is a measure of the kidney's ability to filter waste and excess water from the blood. It represents the volume of fluid filtered from the renal (kidney) glomerular capillaries into the Bowman's capsule per unit of time and is a key indicator of kidney function.
Glomerulonephritis: Glomerulonephritis is a group of kidney disorders characterized by inflammation of the glomeruli, the tiny blood vessel clusters in the kidneys responsible for filtering waste and excess fluid from the bloodstream. This condition can lead to impaired kidney function and potentially progress to kidney failure if left untreated.
Intercalated Cells: Intercalated cells are a specialized type of epithelial cell found in the collecting ducts of the kidney. They play a crucial role in the regulation of acid-base balance and electrolyte homeostasis within the body.
Juxtaglomerular Apparatus: The juxtaglomerular apparatus is a specialized structure located at the junction between the afferent arteriole and the distal convoluted tubule of the nephron in the kidney. It plays a crucial role in the regulation of renal blood flow and the maintenance of electrolyte balance within the body.
Juxtaglomerular apparatus (JGA): The juxtaglomerular apparatus is a specialized structure formed by the distal convoluted tubule and the glomerular afferent arteriole in the kidney. It plays a crucial role in regulating blood pressure and the filtration rate of the kidneys.
Juxtaglomerular cell: Juxtaglomerular cells are specialized smooth muscle cells located in the walls of the afferent arterioles that supply blood to the glomerulus in the kidneys. These cells play a critical role in regulating blood pressure by releasing renin, an enzyme that influences blood vessel constriction and sodium balance.
Juxtaglomerular cells: Juxtaglomerular cells are specialized smooth muscle cells located in the walls of the afferent arterioles of the nephron, primarily responsible for the regulation of blood pressure and fluid balance in the body. These cells play a crucial role in the renin-angiotensin-aldosterone system, contributing to the homeostatic control of renal blood flow and systemic vascular resistance.
Loop of Henle: The loop of Henle is a U-shaped tube in the nephron, within the kidney, that plays a crucial role in concentrating urine and conserving water. It accomplishes this by creating a gradient of salinity that allows for the reabsorption of water from the filtrate.
Loop of Henle: The loop of Henle is a U-shaped structure within the nephron of the kidney that plays a crucial role in the reabsorption and concentration of urine. It connects the proximal convoluted tubule to the distal convoluted tubule, and its unique structure and function are integral to the kidney's ability to maintain fluid and electrolyte balance in the body.
Macula densa: The macula densa is a group of specialized cells located in the distal convoluted tubule of the nephron in the kidneys, which senses sodium chloride (NaCl) concentration and regulates blood pressure and volume by signaling the juxtaglomerular cells to release renin. These cells play a crucial role in the kidney's ability to regulate blood pressure and filtrate formation through feedback mechanisms.
Macula Densa: The macula densa is a specialized group of cells located in the distal convoluted tubule of the nephron within the kidney. It plays a crucial role in the regulation of kidney function and blood pressure.
Merocrine secretion: Merocrine secretion is a process by which cells release their products (like enzymes or sweat) through exocytosis without losing any part of the cell's structure. It is one of the most common types of secretion in the body, involving secretory vesicles moving to the cell surface and releasing their contents into the extracellular space.
Mesangial: Mesangial cells are specialized cells located in the kidneys that provide structural support to the glomeruli, the tiny filters that remove waste and excess substances from the blood. They also regulate blood flow within the glomeruli by contracting and relaxing.
Mesangial Cells: Mesangial cells are specialized cells found within the glomerular tuft of the kidney's nephrons. They play a crucial role in maintaining the structural and functional integrity of the glomerular filtration barrier, which is essential for the kidney's ability to filter blood and produce urine.
Nephron: The nephron is the functional unit of the kidney, responsible for the filtration, reabsorption, and secretion of substances to form urine. It is the basic structural and functional unit that carries out the essential processes of urine formation, fluid and electrolyte balance, and the maintenance of homeostasis within the body.
Nephrons: Nephrons are the functional units of the kidney, each consisting of a glomerulus and its associated tubule, where blood filtration and urine formation occur. They play a critical role in the body's ability to regulate water and soluble substances by filtering the blood, reabsorbing what is needed, and excreting the rest as urine.
Osmoregulation: Osmoregulation is the biological process of maintaining the appropriate balance of water and solutes, such as salts and nutrients, within the body's fluids. It is a critical homeostatic mechanism that ensures the proper functioning of cells, tissues, and organs across various physiological systems.
Parietal layer: The parietal layer is a thin, flattened layer of cells that forms the outer lining of a body cavity or organ. It is a key component in the microscopic anatomy of the kidney and is also referenced in the context of anatomical terminology.
Pedicels: Pedicels are tiny, finger-like projections on the podocytes in the Bowman's capsule within the kidneys that play a crucial role in filtering blood. They wrap around the capillaries of the glomerulus, forming filtration slits that allow certain substances to pass while blocking others based on size and charge.
Peritubular capillaries: Peritubular capillaries are a network of tiny blood vessels that surround the tubules of the nephron in the kidneys, where substances are exchanged between blood and the tubular fluid. They play a crucial role in reabsorbing water and solutes back into the bloodstream from the filtrate produced in the glomerulus.
Peritubular Capillaries: Peritubular capillaries are a network of small blood vessels that surround the renal tubules in the kidneys. They play a crucial role in the filtration and reabsorption processes that occur within the nephrons, the functional units of the kidneys.
Podocytes: Podocytes are specialized cells in the kidneys that wrap around capillaries of the glomerulus, forming a crucial part of the filtration barrier that filters blood to form urine. They have foot-like extensions called foot processes or pedicels, which interlock with those of adjacent podocytes to prevent the passage of large molecules.
Podocytes: Podocytes are specialized epithelial cells that play a critical role in the filtration barrier of the kidney's glomerulus. They have unique foot-like extensions called pedicels that interdigitate with each other, forming filtration slits that help regulate the passage of substances from blood into urine.
Principal Cells: Principal cells are specialized epithelial cells found in the collecting ducts of the kidney, playing a crucial role in the regulation of water and sodium balance in the body. These cells are responsible for the reabsorption of sodium ions and the secretion of potassium ions, influenced by hormones such as aldosterone and antidiuretic hormone (ADH). Their function is vital for maintaining homeostasis, particularly in the urinary system, where they help regulate blood volume and pressure.
Proximal convoluted tubule: The proximal convoluted tubule (PCT) is a highly coiled section of the nephron in the kidney, situated between the Bowman’s capsule and the loop of Henle. It plays a crucial role in reabsorbing essential nutrients, ions, and water from the filtrate back into the bloodstream, making it vital for maintaining homeostasis and fluid balance.
Pyelonephritis: Pyelonephritis is a type of kidney infection that typically occurs when bacteria from the lower urinary tract, such as the bladder, spread to the kidneys. It is characterized by inflammation of the renal pelvis and kidney tissue.
Reabsorption: Reabsorption is the process by which water and solutes that have been filtered out of the blood capillaries are taken back into the bloodstream. It primarily occurs in the capillaries surrounding nephrons in the kidneys, ensuring essential nutrients and water are retained by the body.
Reabsorption: Reabsorption is the process by which substances, such as water, glucose, amino acids, and ions, are selectively taken back into the bloodstream from the filtrate in the kidneys. It is a crucial mechanism that allows the body to maintain fluid balance and regulate the composition of the blood and urine.
Renal Blood Flow: Renal blood flow refers to the volume of blood that flows through the kidneys per unit of time. It is a crucial factor in the kidney's ability to filter waste and maintain homeostasis within the body.
Renal Clearance: Renal clearance is the volume of blood or plasma that is completely cleared of a particular substance by the kidneys per unit of time. It is a measure of the kidney's ability to remove substances from the body through filtration and secretion processes.
Renal Corpuscles: Renal corpuscles are the fundamental structural and functional units of the kidney's filtration system. They are responsible for the initial filtration of blood to form the primary urine, which is then further processed and modified as it travels through the nephron.
Renal medulla: The renal medulla is the innermost part of the kidney, consisting of renal pyramids and playing a crucial role in urine formation. It contains the structures responsible for concentrating urine and regulating water and electrolyte balance, linking it closely to both the gross and microscopic anatomy of the kidney, as well as its physiological functions in maintaining homeostasis.
Renal Papilla: The renal papilla is the innermost part of the renal pyramid, the conical projection of the kidney's medulla that extends into the renal pelvis. It is the site where the collecting ducts of the kidney converge and empty urine into the minor calyces of the renal pelvis.
Renal Plasma Threshold: The renal plasma threshold refers to the maximum concentration of a substance in the blood plasma that the kidneys can efficiently filter and excrete before the substance begins to appear in the urine. It is an important concept in understanding the microscopic anatomy and function of the kidney.
Renin: Renin is an enzyme secreted by the juxtaglomerular cells of the kidneys in response to low blood pressure or decreased sodium chloride concentration. It plays a critical role in the body's regulation of blood pressure, electrolyte balance, and fluid volume.
Renin: Renin is an enzyme produced by the kidneys that plays a crucial role in the regulation of blood pressure and fluid balance within the body. It is a key component of the renin-angiotensin-aldosterone system, which is responsible for maintaining homeostasis in the cardiovascular and renal systems.
Secretion: Secretion is the process by which cells release various substances, such as hormones, enzymes, or other molecules, to the exterior of the cell or into the extracellular space. This process is essential for various physiological functions, including digestion, excretion, and maintaining homeostasis within the body.
Tubuloglomerular feedback: Tubuloglomerular feedback is a regulatory mechanism in the kidneys that adjusts the rate of glomerular filtration in response to the sodium chloride (NaCl) concentration in the fluid flowing through the distal convoluted tubule. It balances fluid and electrolyte levels by altering arteriole diameter and therefore renal blood flow.
Tubuloglomerular Feedback: Tubuloglomerular feedback is a homeostatic mechanism that regulates glomerular filtration rate and renal blood flow by adjusting the contractile state of the afferent arteriole based on the composition of the tubular fluid reaching the macula densa cells in the distal convoluted tubule.
Ultrafiltrate: Ultrafiltrate is the fluid that is initially filtered from the blood plasma as it passes through the glomerular capillaries in the renal (kidney) corpuscles. This filtered fluid contains water, small molecules, and waste products, but lacks the larger molecules and cells that remain in the blood.
Urine Concentration: Urine concentration refers to the process by which the kidneys regulate the amount of water and solutes in the urine, allowing the body to maintain proper fluid balance and conserve essential nutrients. This process is closely tied to the microscopic anatomy and function of the kidney.
Vasa recta: The vasa recta are a series of straight capillaries in the medulla of the kidney that run parallel to the loop of Henle. They play a critical role in the concentration and dilution of urine by participating in the counter-current exchange mechanism.
Vasa Recta: The vasa recta are a network of blood vessels found in the renal medulla of the kidney. They play a crucial role in the kidney's ability to concentrate urine and maintain the appropriate osmotic environment for reabsorption and secretion processes.
Visceral layer: The visceral layer is a thin, specialized layer of cells that forms the inner lining of an organ or body cavity. It is a key component in the structure and function of various organ systems, particularly in the context of anatomical terminology and the microscopic anatomy of the kidney.