The Urinary System Functions

Components of the Urinary System

• The urinary system consists of 2 kidneys (right and left), 2 ureters (right and left), urinary bladder, and urethra.

Kidney Functions

• Regulate blood ionic composition by:
  • Reabsorbing Na+, K+, Ca+2, Cl-, and phosphate ions when needed
  • Eliminating excess ions through secretion
• Regulate blood pH by:
  • Secreting H+ ions
  • Reabsorbing HCO3-
• Regulate blood osmolarity by controlling electrolyte concentrations
• Regulate blood glucose levels
• Regulate blood volume by:
  • Conserving water when blood volume is low
  • Eliminating water when blood volume is high
• Regulate blood pressure by:
  • Secreting renin to activate the Renin-Angiotensin-Aldosterone System (RAAS)
  • Adjusting renal resistance
• Produce erythropoietin for red blood cell synthesis and calcitriol for calcium absorption
• Remove wastes and foreign substances from the body### Internal Anatomy of the Kidney
• The renal cortex and renal pyramids form the functional portion of the kidney
• The nephron is the functional unit of the kidney, located in the renal cortex and renal pyramids, and consists of a vascular component and a tubular component
• Each kidney contains approximately 1 million nephrons

Blood and Nerve Supply of the Kidney

• The renal artery supplies the kidney with oxygen and nutrients, and filters blood
• The renal vein carries filtered blood back to the circulation
• At rest, the kidneys receive 25% of cardiac output (1250 ml/min) via the renal arteries
• The nerve supply to the kidney is derived from the renal plexus (sympathetic division of the autonomic nervous system), which regulates blood flow and renal resistance

Components of the Nephron

• Vascular component:
  • Afferent arteriole: carries blood to the glomerulus and controls blood flow
  • Glomerulus: a ball-like tuft of capillaries that filters water and solutes from the blood
  • Efferent arteriole: transports blood from the glomerulus and controls the amount of blood leaving the glomerulus
  • Peritubular capillaries: surround the tubular part of the nephron and are involved in tubular changes
  • Vasa recta: surrounds the loop of Henle
• Tubular component:
  • Bowman's/glomerular capsule: collects glomerular filtrate
  • Proximal convoluted tubule (PCT): filters fluid from the Bowman's capsule, and is the site of most reabsorption
  • Loop of Henle: a U-shaped structure that reabsorbs and secretes solutes
  • Distal convoluted tubule (DCT): continues reabsorption and secretion of solutes
  • Collecting ducts: drain urine to the renal pelvis and then to the ureter

Types of Nephrons

• Cortical nephrons: 80-85% of nephrons, with a short extension of the loop of Henle
• Juxtamedullary nephrons: 15-20% of nephrons, with a long extension of the loop of Henle, important for maintaining osmolarity of the urine

Juxtaglomerular Apparatus (JGA)

• Structure formed where the afferent arteriole makes contact with the ascending limb of the loop of Henle
• Helps regulate blood pressure and the rate of blood filtration by the kidneys
• Two types of contacted cells:
  • Macula densa: senses Na+ concentration in the filtrate and provides feedback to the afferent and efferent arterioles
  • Juxtaglomerular cells: modified smooth muscle cells that secrete renin

Glomerular Filtration

• The first step in the formation of urine
• Filtration of a protein-free plasma from the glomerulus into the Bowman's capsule
• Glomerular filtrate is produced at a rate of 125 ml/min
• Structure of the renal corpuscle:
  • Glomerular capsule: consists of visceral and parietal layers
  • Filtration membrane: consists of fenestrated endothelium, basal lamina, and slit membrane
• Glomerular filtration occurs by the interaction of forces:
  • Glomerular capillary pressure (favoring filtration)
  • Glomerular capillary oncotic pressure (opposing filtration)
  • Bowman's capsule hydrostatic pressure (opposing filtration)
  • Net filtration pressure (favoring filtration)

Regulation of Glomerular Filtration Rate (GFR)

• Three principal mechanisms:
  • Renal autoregulation: kidneys regulate GFR through factors within the organs
  • Neural regulation: sympathetic nerve fibers supply the afferent and efferent arterioles
  • Hormonal regulation: two hormones regulate GFR, atrial natriuretic peptide (ANP) and angiotensin II

Tubular Reabsorption and Secretion

• The second and third processes in the formation of urine
• Filtered substances are transferred from the tubular lumen to the peritubular capillaries
• Highly selective and variable process
• Reabsorption rates are high, with 124 of 125 ml of filtered fluid per minute being reabsorbed
• Routes:
  • Paracellular reabsorption
  • Transcellular reabsorption