Kidney Structure Overview

Excretion Overview

• Excretion removes waste products from metabolic reactions.
• Carbon dioxide (CO2) and urea are the major waste products.
• CO2 is removed via the lungs during aerobic respiration.
• Urea is produced in the liver from excess amino acids.

Deamination

• Excess amino acids are broken down.
• The liver deaminates amino acids, removing the amine group (-NH2) and a hydrogen atom.
• This forms ammonia (NH3).
• The remaining keto acid can enter the Krebs cycle or be converted to glucose or glycogen.

Kidney Structure

• Kidneys receive blood from the renal artery (aorta branch).
• Blood exits via the renal veins (inferior vena cava).
• Ureters carry urine from the kidneys to the bladder.
• The urethra carries urine from the bladder to the exterior.

Kidney Macroscopic Structure

• The kidney has three main regions:
  • Capsule (outer covering)
  • Cortex (beneath the capsule)
  • Medulla (central region)

Kidney Microscopic Structure

• Nephrons are the functional units of the kidney, numerous small tubules.
• Each nephron has associated blood vessels:
  • Glomerulus (network of capillaries)
  • Bowman's capsule (surrounds glomerulus)
  • Proximal convoluted tubule (PCT)
  • Loop of Henle (long U-shaped loop in the medulla)
    • Descending limb
    • Ascending limb
  • Distal convoluted tubule (DCT)
  • Collecting duct

Blood Vessels and Nephrons

• Nephrons are supplied by branches of the renal artery (afferent arterioles).
• Blood from the glomerulus passes into the efferent arteriole.
• This blood flows into peritubular capillaries (surrounding nephron).
• Eventually, blood exits the kidney via the renal vein.

Urine Formation

• Urine is formed in two stages:
  • Ultrafiltration: Small molecules from blood are filtered into Bowman's capsule.
  • Selective Reabsorption: Useful molecules are reabsorbed from the filtrate as it moves through the nephron.

Ultrafiltration

• Blood in the glomerulus is separated from Bowman's capsule by a filter (endothelium of capillary + basement membrane + podocytes).
• Small molecules (water, urea, glucose, salts) pass through this filter.
• Larger molecules (proteins, blood cells) are retained.
• Filtrate contains useful and waste substances.

Glomerular Filtration Rate (GFR)

• GFR is the rate of fluid filtration from the glomerulus into Bowman's capsule.
• Normal GFR is approximately 125cm³/min.
• High glomerular blood pressure (afferent vs. efferent arteriole size) drives ultrafiltration.

Selective Reabsorption in the Proximal Convoluted Tubule (PCT)

• Active transport reabsorbs glucose and amino acids.
• Passive transport reabsorbs water and ions (sodium, chloride).
• A large portion of the filtrate is reabsorbed in the PCT.

Reabsorption in the Loop of Henle and Collecting Duct

• The loop of Henle establishes a concentration gradient in the kidney medulla.
• Water is reabsorbed in the descending loop due to the high salt concentration.
• Sodium and chloride are actively reabsorbed in the ascending loop, maintaining the concentration gradient.
• The collecting duct fine-tunes water reabsorption based on the body's hydration needs. Active transport of sodium contributes to creating a gradient for water reabsorption.

Reabsorption in Distal Convoluted Tubule (DCT) and Collecting Duct

• Active transport pumps out sodium ions.
• Potassium ions are passively transported into the tubule.