Renal Circulation and Kidney Anatomy

• Kidneys purify, filter, and reabsorb, each 10-12 cm long.
• Renal circulation originates from the renal artery, a branch of the abdominal aorta.
• Renal artery carries unpurified blood; the renal vein returns purified blood to circulation

Renal Circulation

• Segmental artery pierces renal columns towards the corticomedullary junction.
• Renal columns are extensions of the cortex between renal lobes (cortex and medulla).
• Interlobar arteries run through renal columns between lobes.
• Arcuate arteries arch over interlobar arteries at the corticomedullary junction.
• Interlobular arteries arise from arcuate arteries, running toward the cortex.
• Presence of interlobular arteries confirms location in the cortex.
• Afferent arterioles supply glomeruli for filtration.
• Glomerular capillaries are the main filtration site in Bowman's capsule.
• Filtrate drains through the proximal convoluted tubule, Loop of Henle, distal convoluted tubule, and collecting duct.
• Efferent arterioles receive filtered blood from the glomerulus, branching into peritubular capillaries or vasa recta.
• Peritubular capillaries accompany cortical convoluted tubules, draining into the renal vein.
• Vasa recta accompany the Loop of Henle in the medulla, also draining into the renal vein.
• Interlobular veins "complement" the arterial system, running parallel.

Nephron and Collecting Tubule

• Nephron: kidney's main functional unit
• Cortical nephrons are located in the renal cortex.
• Juxtamedullary nephrons span the renal cortex and medulla

Renal Corpuscle

• Consists of Bowman's capsule and the glomerulus
• Bowman's space is located between the two.

Glomerulus

• Filtration unit within the kidney
• Glomerular filtration barrier includes:
  • Endothelial layer: Innermost layer with fenestrated cells, permeable to water, ions, and small molecules but not cells.
  • Glomerular Basement Membrane (GBM): Negatively charged layer of collagen IV, laminin, and proteoglycans restricting large proteins like albumin.
  • Podocyte Layer: Outermost layer of podocytes forming filtration slits covered by a slit diaphragm to regulate filtration and prevent protein leakage.

Juxtaglomerular Apparatus (JGA)

• It regulates blood pressure and glomerular filtration rate.
• Components include:
  • Macula Densa: Located at the junction of the vascular pole and distal tubule, osmoreceptors detecting filtrate osmolarity and recognizing hypo-osmolarity.
  • Juxtaglomerular Granular (JG) Cells: Located in the afferent arteriole, baroreceptors detecting low blood pressure or osmolarity, release renin to regulate blood pressure and sodium balance.
  • Extraglomerular Mesangial Cells: Located between the macula densa and afferent/efferent arterioles.

Proximal Convoluted Tubules (PCT)

• Reabsorb 60-65% of filtrate, secreting urea, and certain drugs
• Eosinophilic due to many mitochondria.
• Characterized by a prominent brush border for reabsorption and fuzzy lumen from pinocytosis.

Loop of Henle

• Divided into thick and thin descending limbs and a thick ascending limb.
• Thin descending limb is simple squamous epithelium and very permeable to water.
• Thick ascending limb includes active Na+-K+ ATPase maintaining the hyperosmolar medullary interstitium.

Distal Convoluted Tubule (DCT)

• Has a clearer lumen and more basophilic cuboidal cells compared to PCT.

Collecting Duct

• Contains principal cells for sodium reabsorption and potassium secretion, rich in aquaporins for water reabsorption.
• Consists of intercalated cells that secrete or reabsorb hydrogen or bicarbonate ions to maintain acid-base balance.
• Heavily influenced by antidiuretic hormone (ADH), promoting aquaporin opening for water reabsorption

Urine Formation - Glomerular Filtration

• High blood pressure drives filtration through glomerular capillaries, the glomerular basement membrane, and filtration slits.
• Glomerular capillaries: Fenestrated, serving as a size barrier to exclude large molecules.
• Glomerular basement membrane: Negatively charged, serving as a charge barrier.
• Filtration slits: Formed by podocyte secondary processes, permeable to small substances.

Renin-Angiotensin-Aldosterone System

• If the urine is too dilute, it indicates decreased GFR.
• Low salt stimulates macula densa, and decreased blood pressure stimulates JG cells, triggering renin release.
• Renin converts angiotensinogen to angiotensin I, then ACE converts it to angiotensin II
• Angiotensin II, a vasoconstrictor, increases blood pressure and stimulates aldosterone release.
• Aldosterone stimulates DCT to reabsorb sodium.

Urine Excretion

• Urine flows from minor to major calyces, to the renal pelvis, ureter, and urinary bladder.
• Bladder capacity ranges from 500cc to 1L.
• Urothelium protects urinary tract, contains umbrella cells
• Uroplakins on umbrella cells prevent hyperosmotic urine ingress.