14.05 Structure and Histology of the Kidney

Overview of Kidney Structure and Position

• Kidney Location: Positioned in the lower back, each kidney receives oxygenated blood through a renal artery and drains deoxygenated blood through a renal vein.
• Urine Pathway:
  • Ureter: Carries urine from each kidney to the bladder.
  • Bladder: Stores urine until it is excreted.
  • Urethra: Releases urine from the bladder to outside the body.

External Structure of the Kidney

• Fibrous Capsule: A tough outer covering that protects each kidney.
• Renal Cortex: The outermost region, beneath the capsule, containing glomeruli and Bowman’s capsules.
• Renal Medulla: The inner region with tubules and collecting ducts.
• Renal Pelvis: A funnel-shaped cavity where urine collects before entering the ureter.

Internal Structure (Microscopic Anatomy)

1. Nephron: Functional unit of the kidney, responsible for filtering blood and forming urine. Each kidney contains thousands of nephrons with three main sections:
   • Bowman’s Capsule: Cup-shaped structure surrounding the glomerulus, located in the cortex, where ultrafiltration occurs.
   • Proximal Convoluted Tubule (PCT): A twisted tube immediately after Bowman’s capsule, responsible for selective reabsorption of water, ions, and nutrients.
   • Loop of Henle: A U-shaped tube in the medulla that creates a concentration gradient essential for water reabsorption.
   • Distal Convoluted Tubule (DCT): A twisted segment in the cortex following the loop of Henle, adjusting ion and pH balance.
   • Collecting Duct: Runs through the medulla, carrying urine from multiple nephrons to the renal pelvis.

Blood Supply to the Nephron

• Afferent Arteriole: Brings blood to the glomerulus (a capillary network within Bowman’s capsule) for filtration.
• Glomerulus: Site of ultrafiltration due to high blood pressure, where small molecules are forced into Bowman’s capsule.
• Efferent Arteriole: Carries filtered blood away from the glomerulus to a network of capillaries surrounding the nephron, eventually leading to the renal vein.

Processes in Urine Formation

1. Ultrafiltration:
   • Location: Glomerulus within Bowman’s capsule.
   • Process: High blood pressure forces water, glucose, urea, and other small molecules from the blood through the glomerular filtration membrane into Bowman’s capsule, forming glomerular filtrate.
   • Selective Retention: Large molecules, like proteins, remain in the blood as they cannot pass through the filtration membrane.
2. Selective Reabsorption:
   • Location: Primarily in the proximal convoluted tubule.
   • Process: As filtrate flows through the nephron, essential molecules (e.g., glucose, ions, and water) are reabsorbed back into the blood via capillaries surrounding the nephron.
   • Formation of Urine: Remaining waste substances and excess ions stay in the filtrate, forming urine as it passes through the nephron to the collecting duct.

Summary of Key Terminology

• Nephron: The structural and functional unit of the kidney, crucial for filtration and urine formation.
• Bowman’s Capsule: Collects filtrate from the glomerulus.
• Proximal Convoluted Tubule (PCT): Initial nephron region where reabsorption occurs.
• Loop of Henle: Creates a concentration gradient for water reabsorption.
• Distal Convoluted Tubule (DCT): Regulates ion and pH balance.
• Collecting Duct: Carries final urine to the renal pelvis.
• Afferent and Efferent Arterioles: Supply and drain blood from the glomerulus, respectively.
• Ultrafiltration: Molecular-level filtration of blood in the glomerulus, separating small molecules from larger ones.

Complete Urine Formation Pathway

Excretion: Final urine flows from the collecting ducts to the renal pelvis, then through the ureter to the bladder for excretion.

Ultrafiltration: Blood pressure forces water and small molecules from the glomerulus into Bowman’s capsule, forming the filtrate.

Selective Reabsorption: Vital molecules are reabsorbed as filtrate moves through the nephron.