The Nephron Structure and Function

The Nephron

• The kidney is composed of over 1 million nephrons, each consisting of a renal corpuscle and a renal tubule.
• The renal corpuscle is the site of plasma filtration, and it is composed of:
  • Glomerulus: capillaries where filtration occurs.
  • Glomerular (Bowman's) capsule: a double-walled epithelial cup that collects the filtrate.
• The renal tubule is composed of:
  • Proximal convoluted tubule.
  • Loop of Henle: dips down into the medulla.
  • Distal convoluted tubule.

Juxtamedullary Nephron

• 15-20% of nephrons are juxtamedullary nephrons, which are located near the medulla and have long loops of Henle that extend into the deepest medulla.
• These nephrons enable the excretion of dilute or concentrated urine.

Cortical Nephron

• 80-85% of nephrons are cortical nephrons, which are located in the outer cortex and have loops of Henle that lie mainly in the cortex.

Histology of the Nephron and Collecting Duct

• The nephron and collecting duct are composed of a single layer of epithelial cells.
• Each region has distinctive features due to its function:
  • Microvilli in the proximal convoluted tubule.
  • Cuboidal versus simple epithelial cells in the distal convoluted tubule.

Glomerular Capsule

• The glomerular capsule is composed of visceral and parietal layers.
• The visceral layer is composed of modified simple squamous epithelial cells called podocytes.
• The parietal layer is composed of simple squamous epithelium and forms the outer wall of the capsule.

Structure of Renal Corpuscle

• The renal corpuscle is surrounded by Bowman's capsule.
• The visceral layer is composed of podocytes that cover the capillaries.
• The parietal layer is composed of simple squamous cells.
• Glomerular capillaries arise from the afferent arteriole and form a ball before emptying into the efferent arteriole.

Blood and Nerve Supply of the Kidney

• The kidney is abundantly supplied with blood vessels, receiving approximately 20% of the resting cardiac output via the renal arteries.
• Functions of different capillary beds:
  • Glomerular capillaries: where filtration of blood occurs.
  • Peritubular capillaries: carry away reabsorbed substances from the filtrate.
  • Vasa recta: supplies nutrients to the medulla without disrupting its osmolarity.
• The nerve supply to the kidney is derived from the renal plexus (sympathetic division of the autonomic nervous system).
• Sympathetic vasomotor nerves regulate blood flow and renal resistance by altering arterioles.

Nephron Function

• Nephrons perform three basic functions:
  • Glomerular filtration.
  • Tubular reabsorption.
  • Tubular secretion.
• A renal tubule consists of:
  • Proximal convoluted tubule.
  • Loop of Henle.
  • Distal convoluted tubule.

Blood Vessels Around the Nephron

• Glomerular capillaries are formed between the afferent and efferent arterioles.
• Efferent arterioles give rise to the peritubular capillaries and vasa recta.

Nephron Types

• There are two types of nephrons:
  • Cortical nephrons: have a short loop of Henle that penetrates only into the outer region of the medulla.
  • Juxtamedullary nephrons: have a long loop of Henle that stretches through the medulla and almost reaches the renal papilla.

Renal System

• The urinary system consists of:
  • Two kidneys.
  • Two ureters.
  • One urinary bladder.
  • One urethra.
• Urine is excreted from each kidney through its ureter and is stored in the urinary bladder until it is expelled from the body through the urethra.

Kidney Functions

• Regulation of blood ionic composition (Na+, K+, Ca++, Cl-, and phosphate ions).
• Regulation of blood pH, osmolarity, and glucose.
• Regulation of blood volume by conserving or eliminating water.
• Regulation of blood pressure by secreting the enzyme renin and adjusting renal resistance.
• Release of erythropoietin and calcitriol.
• Excretion of wastes and foreign substances.

Anatomy and Histology of the Kidneys

• The paired kidneys are retroperitoneal organs.
• The kidneys are surrounded by three layers of tissue:
  • Renal capsule.
  • Adipose capsule.
  • Renal fascia.

Internal Anatomy of the Kidney

• The kidneys consist of:
  • Cortex.
  • Medulla.
  • Pyramids.
  • Papillae.
  • Columns.
  • Calyces.
  • Pelves.
• The renal cortex and renal pyramids constitute the functional portion or parenchyma of the kidney.

Glomerular Filtration Rate

• The amount of filtrate formed in all renal corpuscles of both kidneys per minute.
• Average adult male rate is 125 mL/min.
• Homeostasis requires a constant GFR.
• Changes in net filtration pressure affect GFR.

Juxtaglomerular Apparatus

• A structure where the afferent arteriole makes contact with the ascending limb of the loop of Henle.
• Macula densa is a thickened part of the ascending limb.
• Juxtaglomerular cells are modified muscle cells in the arteriole.