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Urinary system microscopic anatomy of the kidneys

Nephron: Structural overview

  • The nephron is the structural and functional unit of the kidney.
  • It consists of:
    • Renal corpuscle (glomerulus + Bowman's capsule) — the beginning portion where filtration occurs.
    • Tubular system: proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.
  • Two main nephron types:
    • Cortical nephrons: corpuscle and most of the tubule in the renal cortex (outer region).
    • Juxtamedullary nephrons: corpuscle in cortex but with a long loop of Henle extending into the medulla.
  • Blood vessels associated with the nephron:
    • Afferent glomerular arteriole: brings blood into the glomerulus for filtration.
    • Efferent glomerular arteriole: exits the glomerulus and gives rise to the peritubular capillaries.
    • Peritubular capillaries: wrap around the renal tubules and are key for tubular reabsorption.
  • Urine flow pathway following filtration:
    • From tubules to papillary duct → minor calyx → major calyx → renal pelvis → ureter → bladder.
  • Nephron count: ~1{,}000{,}000 ext{ to }1{,}300{,}000 nephrons per kidney.

Renal corpuscle and Bowman's capsule

  • Glomerulus: a tuft of highly porous fenestrated capillaries that allow filtration of plasma
    • Fenestrations enable passage of water, electrolytes, and small solutes while restricting larger components.
  • Bowman's capsule (Glomerular capsule): cup-shaped hollow structure surrounding the glomerulus with two layers:
    • Parietal layer: outer, simple squamous epithelium.
    • Visceral layer: inner layer containing podocytes (branched epithelial cells).
    • Podocytes have foot processes (pedicels) that wrap around capillaries.
    • Filtration slits exist between pedicels, contributing to the filtration barrier.
  • The capsule contains a capsular (Bowman’s) space receiving filtrate from the glomerulus.
  • Filtration entry point: filtrate moves from the capillary lumen through the glomerular basement membrane and filtration slits into Bowman's space.
  • Blood enters via the afferent arteriole and is filtered at the glomerulus.

Nephron tubules: structure and function

  • Proximal convoluted tubule (PCT)
    • Location: cortex, closest to Bowman's capsule.
    • Epithelium: cuboidal cells with dense microvilli forming a brush border.
    • Function: major site of reabsorption and secretion; high surface area supports extensive solute movement.
    • Surface area emphasis: brush border increases reabsorption efficiency for solutes and water.
    • Substances reabsorbed (examples from transcript): glucose, amino acids, proteins, vitamins, lactate, urea, uric acid; Na extsuperscript{+}, K extsuperscript{+}, Ca extsuperscript{2+}, Mg extsuperscript{2+}, Cl extsuperscript{−}, HCO ext{3} extsuperscript{−}, and water.
  • Loop of Henle (nephron loop)
    • Segments: descending limb (toward medulla) and ascending limb (toward cortex), with thinner vs thicker portions.
    • Descending limb: permeable to water; contributes to medullary osmolality gradient.
    • Ascending limb: less permeable to water; active/secondary active transport of Na extsuperscript{+}, K extsuperscript{+}, and Cl extsuperscript{−} (especially in the thick ascending limb via NKCC cotransporter).
    • Thickness differences create regional permeability differences and help establish concentration gradients for urine concentration.
    • Note on terminology in transcript: mentions proximal descending, distal descending, and thick descending regions; standard anatomy features descending (thin) and ascending (thick) limbs with corresponding permeabilities.
  • Distal convoluted tubule (DCT)
    • Location: cortex; cuboidal cells with few microvilli.
    • Function: less surface area than PCT; primarily involved in selective reabsorption and secretion; balance of electrolytes and pH.
    • Regulated reabsorption/secretion: aldosterone influences Na extsuperscript{+} reabsorption and K extsuperscript{+} secretion in distal segments.
  • Collecting duct
    • Location: runs through the renal cortex and medulla within renal pyramids; ultimately feeds into the minor papilla.
    • Relationship to nephron: continuous with the nephron but not technically part of it.
    • Function: principal site of water reabsorption via aquaporin channels; water reabsorption is regulated by antidiuretic hormone (ADH, vasopressin).
    • Aquaporins: channel proteins that allow selective water reabsorption.
  • Peritubular capillaries (associated with tubules)
    • Play a key role in reabsorption from tubules back into blood.

Filtration: pressures and glomerular filtration rate

  • Filtration is driven by a pressure gradient that moves water and small solutes from the glomerulus into Bowman's capsule; plasma proteins largely stay in the blood, maintaining colloid osmotic pressure.
  • Glomerular filtration rate (GFR): the volume process rate across the filtration barrier.
    • Normal adult GFR ~125\ \text{mL/min}.
    • Daily filtration: ~180\ \text{L/day} of fluid filtered by both kidneys.
  • Forces contributing to filtration: glomerular hydrostatic pressure, capsular hydrostatic pressure, and glomerular oncotic pressure (colloid osmotic pressure) of the plasma.
  • Net filtration pressure (NFP): the driving force for filtration, commonly expressed as
    • \text{NFP} = P{GHP} - P{CAP} - P_{GOP}
    • where:
    • P_{GHP} = glomerular hydrostatic pressure (blood pressure within glomerular capillaries)
    • P_{CAP} = capsular hydrostatic pressure (pressure within Bowman's capsule)
    • P_{GOP} = glomerular oncotic pressure (colloid osmotic pressure of the plasma)
  • Effective filtration rate is governed by these pressures and the filtration coefficient (permeability and surface area of the filtration barrier).
  • In the transcript’s diagram description, there are arrows representing the opposing forces (glomerular hydrostatic and capsular osmotic pressures) and contravening forces (glomerular osmotic pressure and capsular hydrostatic pressure) that determine the net flow. The standard representation is the NFP above.

Tubular reabsorption and secretion: overview

  • Definitions:
    • Reabsorption: selective movement of solutes from the renal tubule back into the peritubular blood.
    • Secretion: selective movement of solutes from the peritubular blood into the renal tubule for excretion.
  • Reabsorption and secretion occur throughout the nephron, with the majority of reabsorption happening in the PCT and fine-tuning throughout the distal parts.
  • Proximal tubule: major site of reabsorption; significant water and solute reclamation; active and passive processes.
  • Collecting duct: water reabsorption via ADH-regulated aquaporins; final adjustments to urine concentration.
  • The peritubular capillaries reclaim most solutes and water that are filtered, returning them to systemic circulation.
  • Urine is formed from what remains after reabsorption and secretion.

Segments: specific reabsorption and secretion patterns

  • Proximal convoluted tubule (PCT) – reabsorption emphasis:
    • Reabsorbs glucose, amino acids, proteins, vitamins, lactate, urea, uric acid.
    • Reabsorbs Na extsuperscript{+}, K extsuperscript{+}, Ca extsuperscript{2+}, Mg extsuperscript{2+}, Cl extsuperscript{−}, HCO ext{3} extsuperscript{−}, and water.
  • Loop of Henle:
    • Descending limb: water reabsorption (contributes to medullary osmotic gradient).
    • Ascending limb: Na extsuperscript{+}, K extsuperscript{+}, and Cl extsuperscript{−} reabsorption (thick ascending limb actively transports ions; limb is relatively impermeable to water).
  • Distal convoluted tubule (DCT):
    • Reabsorbs NaCl and bicarbonate; water reabsorption less extensive and more hormonally regulated.
  • Collecting duct:
    • Reabsorbs water largely via aquaporins regulated by ADH.
    • Reabsorbs or secretes urea as part of urine concentration; water reabsorption is a key feature.
  • Secretion (yellow boxes in the transcript’s illustration):
    • Substances secreted into the tubule include uric acid, creatinine, some drugs, hydrogen ion (H extsuperscript{+}), ammonium (NH4 extsuperscript{+}), and certain organic acids and bases.
  • Reabsorption (purple boxes):
    • Substances reabsorbed from tubular fluid back into blood include glucose, amino acids, proteins, vitamins, lactate, urea, uric acid; ions like Na extsuperscript{+}, K extsuperscript{+}, Ca ext{2+}, Mg ext{2+}, Cl extsuperscript{−}, HCO ext{3} extsuperscript{−}, and water; later segments focus on NaCl, bicarbonate, and water in varying amounts.
  • Collecting duct secretion/reabsorption specifics:
    • Hydrogen ion and ammonium secretion contribute to acid-base balance.
    • Potassium handling in distal parts and collecting ducts is hormonally regulated (aldosterone).

Normal urine composition

  • Nitrogenous wastes: breakdown products of protein/amino acids
    • Urea, uric acid, ammonia, creatinine.
  • Electrolytes: Na extsuperscript{+}, K extsuperscript{+}, NH4 extsuperscript{+}, Cl extsuperscript{−}, bicarbonate (HCO ext{3} extsuperscript{−}), phosphate, sulfate, Ca ext{2+}, Mg ext{2+} (in balanced amounts).
  • Toxins: bacterial toxins or other toxins processed by liver/metabolic pathways may appear in urine.
  • Pigments: urochromes (yellow pigmentation from red blood cell breakdown).
  • Hormones: in excess, hormones may be detectable in urine.

Hormonal and regulatory control of nephron function

  • Antidiuretic hormone (ADH, vasopressin)
    • Increases water reabsorption in the collecting duct by increasing aquaporin channel insertion in the apical membrane.
    • Also referred to as vasopressin in some texts.
  • Renin-angiotensin-aldosterone system (RAS)
    • Increases sodium reabsorption (via aldosterone, a mineralocorticoid) to help conserve volume and raise blood pressure.
  • Atrial natriuretic peptide (ANP)
    • Promotes sodium loss (natriuresis) and acts to oppose Na extsuperscript{+} reabsorption, aiding in volume regulation.
  • Tubular-Glomerular feedback (TGF)
    • Mechanism linking tubular flow and NaCl concentration to glomerular filtration rate (GFR) via juxtaglomerular apparatus.
    • If blood Na extsuperscript{+} levels are too high: juxtaglomerular cells contract, decreasing GFR to conserve water by reducing filtration.
    • If blood Na extsuperscript{+} levels are too low: juxtaglomerular cells relax, vasodilate, increase GFR, and increase filtrate flow.
  • Sympathetic nervous system (SNS) regulation
    • Fight-or-flight context affects renal hemodynamics.
    • The transcript describes reduced sympathetic stimulation leading to afferent arteriole vasodilation and increased GFR.
    • There is also a discussion of sympathetic activity contributing to vasoconstriction under stress in standard physiology; the transcript emphasizes the concept of switching between states depending on context.
  • Myogenic mechanism (intrinsic autoregulation)
    • High blood pressure stretches the afferent arteriole, causing it to contract to maintain relatively constant blood flow into the glomerulus (constant GFR).
    • Low blood pressure causes relaxation of the afferent arteriole, preserving GFR when overall perfusion is reduced.

Key numerical references and concepts to remember

  • Normal GFR: 125\ \text{mL/min}
  • Total filtration per day: \approx 180\ \text{L/day}
  • Nephrons per kidney: \approx 1.0 \times 10^6 \text{ to } 1.3 \times 10^6
  • Net filtration pressure (NFP) formulation (standard):
    • \text{NFP} = P{GHP} - P{GOP} - P_{CAP}
    • where P{GHP} is glomerular hydrostatic pressure, P{GOP} is glomerular oncotic pressure, and P_{CAP} is capsular hydrostatic pressure.
  • GFR is influenced by NFP and the filtration coefficient Kf:\text{GFR} = Kf \cdot \text{NFP}

Connections to broader physiology and clinical relevance

  • The nephron’s selective reabsorption and secretion enable precise regulation of body fluid volume, electrolyte balance, and acid-base status, supporting homeostasis.
  • Hormonal control (ADH, RAS, ANP) integrates kidney function with systemic blood pressure and volume status.
  • Tubuloglomerular feedback ensures that changes in tubular flow or NaCl delivery adjust GFR to stabilize filtration and protect the kidneys.
  • The collecting duct’s water permeability, governed by ADH via aquaporins, is a major determinant of urine concentration and body water balance.
  • The urine contains diagnostic markers (nitrogenous wastes, electrolytes, pigments) that reflect the kidney’s filtration and reabsorption performance and systemic metabolism.