Kidney Anatomy and Glomerular Filtration - Study Notes

Anatomy of the Kidney and Nephron

• Location and basic anatomy
  • Kidneys are located on either side of the spinal column in the posterior abdominal wall.
  • Adrenal glands sit on top of the kidneys.
  • In a dissection view, you would go through the skin and muscle to expose digestive organs (stomach, liver, small intestine).
  • In the retroperitoneal space behind these organs lie the two kidneys, each held in place by connective tissue.
• Internal structure and organization
  • The renal lobe is composed of the medullary pyramid and the outer cortex.
  • The renal hilum is the entrance/exit point for vessels and the renal pelvis; the pyramids drain toward the hilum into the renal pelvis (funnel-shaped area).
  • Cortex (outer layer) and medulla (inner layer) contain the functional units and collecting structures.
  • Blood is delivered to the kidneys by the renal arteries, which branch into smaller vessels until they reach the nephron.
  • The nephron is the functional unit of the kidney.
• Nephron structure and parts
  • Each nephron consists of:
  • Renal corpuscle: glomerulus + Bowman's capsule.
  • Renal tubule: proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), collecting duct.
  • Afferent arteriole supplies the glomerulus; efferent arteriole drains it.
  • The diameter of the afferent arteriole is larger than that of the efferent arteriole, producing high pressure within the glomerulus.
• Functional goals of the kidney
  • Filter blood to remove nitrogenous wastes.
  • Balance water and electrolytes.
  • Maintain homeostasis through regulated filtration, reabsorption, and secretion.
• Transitional anatomy perspective during dissection
  • If the kidneys were removed and viewed in frontal section, two layers would be visible: outer cortex and inner medulla with pyramids pointing toward the hilum.
  • The renal pyramids are the urine-producing areas; urine drains into the renal pelvis via the calyces.
  • Blood flow pathway: renal arteries → smaller vessels → glomerulus → peritubular capillaries and other microvasculature → renal veins → vena cava.

Glomerular Filtration: Pressures and Filtrate

• First step of urine production: glomerular filtration in the renal corpuscle
  • Blood enters the capillary bed called the glomerulus and exits into Bowman's capsule via the glomerular capillaries.
  • Filtrate at this stage is plasma-like, not urine.
• Filtrate composition (plasma-like) in Bowman's capsule
  • Water
  • Ions: K\, Na\, Cl\
  • Nitrogenous wastes: urea, uric acid, creatinine
  • Organic molecules: glucose, amino acids
• Why substances filter from blood into Bowman's capsule
  • Forces at work in capillary beds:
  • Hydrostatic pressure pushes fluid out of the capillaries.
  • Osmotic (colloid osmotic) pressure pulls fluid back in due to plasma proteins.
  • In the glomerulus, an additional force is the glomerular hydrostatic pressure, i.e., the pressure of fluid already inside the glomerulus.
  • Net filtration pressure (NFP) describes the combined effect of these forces:
  • \text{NFP} = P{\text{GHP}} - (\pi{\text{GOP}} + P_{\text{BCP}})
    • where $P{\text{GHP}}$ is glomerular hydrostatic pressure, $\pi{\text{GOP}}$ is glomerular oncotic (colloid osmotic) pressure, and $P_{\text{BCP}}$ is Bowman's capsule hydrostatic pressure.
  • If $P{\text{GHP}}$ > $\pi{\text{GOP}} + P_{\text{BCP}}$, fluid moves from the capillary into Bowman's capsule (filtration).
  • Conversely, if these forces are not favorable, filtration would decrease and renal function could be impaired.
• Glomerular filtration rate (GFR)
  • Normal rate of glomerular filtration for both kidneys is approximately 120\ \text{mL/min}.
  • GFR is driven by hydrostatic pressure; changes in blood pressure can alter GFR.
  • Mechanisms exist to regulate a constant flow and keep GFR stable.
• Autoregulation and the Juxtaglomerular Apparatus (JGA)
  • Local regulatory system that compares pressure between the afferent arterioles and efferent arterioles.
  • The JGA modulates afferent arteriole tone to maintain constant GFR.
  • Response to increased blood pressure: afferent arteriole constriction reduces incoming blood, helping to re-establish 120 mL/min.
  • Response to decreased blood pressure: afferent arteriole dilation increases incoming blood, helping to restore GFR.
  • Overall function: afferent arteriole adjusts its diameter to dictate the amount of force arriving at the glomerulus, maintaining a steady filtration rate regardless of activity state (rest, exercise, sleep).

Tubular Processes: Reabsorption and Secretion

• Tubular reabsorption (in the renal tubule)
  • As filtrate passes through the PCT, loop of Henle, DCT, and collecting duct, useful substances are reabsorbed into the peritubular capillaries.
  • Substances reabsorbed via active transport include glucose, amino acids, and some Na+ and Ca+ ions.
  • The peritubular capillaries reabsorb water by osmosis.
• Tubular secretion (in the distal convoluted tubule and beyond)
  • Secretion removes larger nitrogenous wastes, excess hydrogen ions (H+), and excess potassium ions (K+) from the peritubular capillaries into the tubular fluid.
  • This process adds wastes to the filtrate to be eliminated as urine.
• Outcome: filtrate is progressively modified to become urine, containing the wastes and excess ions that must be excreted while reclaiming useful substances back into the bloodstream.

Functional Implications and Integration

• Purpose of filtration, reabsorption, and secretion
  • Maintain homeostasis by removing wastes while preserving water and electrolyte balance.
  • Keep plasma composition within narrow limits despite varying intake and physiological states.
• Clinical and physiological relevance
  • GFR stability is essential for consistent waste removal and fluid balance.
  • Dysregulation of glomerular pressures or JGA function can impair filtration, leading to renal dysfunction.

Key Terms and Concepts to Remember

• Afferent arteriole vs. efferent arteriole: diameter differences create high glomerular pressure.
• Glomerulus and Bowman's capsule: site of filtration forming the initial filtrate.
• Glomerular hydrostatic pressure ($P_{\text{GHP}}$): drives filtration.
• Bowman's capsule hydrostatic pressure ($P{\text{BCP}}$) and glomerular oncotic pressure ($\pi{\text{GOP}}$): opposing forces.
• Net filtration pressure (NFP): balance of forces determining filtration.
• Renal tubule segments: PCT, loop of Henle, DCT, collecting duct.
• Peritubular capillaries: reabsorb water and solutes from the tubule.
• Juxtaglomerular apparatus (JGA): regulatory mechanism controlling afferent arteriole tone and GFR.
• Glomerular filtration rate (GFR): ~ 120\ \text{mL/min} for the kidneys as a whole.
• Filtrate vs urine: filtrate is plasma-like; urine is the excreted fluid after processing.