Urinary system

• Components of the urinary system

  • Two kidneys
  • Two ureters
  • One urinary bladder
  • One urethra
• Urine


  The remaining water and solutes after the kidneys filter blood and return most of the water and many solutes to the blood
• Functions of the urinary system

  • Remove wastes, toxins, water through process of excretion
  • Maintain homeostasis of many factors (blood pH, electrolytes, and blood pressure)
  • Participate in production of erythrocytes (synthesise erythropoietin)
  • Transport, store, and eliminate urine from the body
• Functions of the kidneys


  • Regulation of ion levels in the blood
  • Regulation of blood volume and blood pressure
  • Regulation of blood pH
  • Production of hormones
  • Excretion of wastes
• Renal capsule


  The fibrous capsule that encloses each kidney
• Main areas of the kidney

  • Renal cortex
  • Renal medulla
  • Renal pelvis
• Renal cortex


  Contains the glomerulus, Bowman's capsule, proximal convoluted tubule, and distal convoluted tubule of the nephrons
• Renal medulla


  Contains the loop of Henle and collecting duct of the nephrons; renal pyramids, renal columns, major and minor calyces
• Renal pelvis


  Where the ureter joins the kidney
• Renal artery


  The blood vessel that enters the kidney
• Renal vein


  The blood vessel that leaves the kidney
• Nephrons

  The functional units of the kidney, numbering about a million in each kidney
• Functions of the nephron

  • Filtration
  • Reabsorption
  • Secretion
• Renal corpuscle


  The part of the nephron where blood plasma is filtered
• Renal tubule


  The part of the nephron into which the filtered fluid passes
• Parts of the renal tubule

  • Proximal convoluted tubule
  • Nephron loop
  • Distal convoluted tubule
• Collecting duct


  The duct that the distal convoluted tubules of several nephrons empty into
• Glomerulus


  A loop of capillaries twisted into a ball shape, surrounded by the Bowman's capsule, where ultrafiltration of blood occurs
• Bowman's capsule


  The capsule that surrounds the glomerulus
• Podocytes


  Specialised epithelial cells of Bowman's capsule which form the visceral layer of the capsule
• Components of the juxtaglomerular apparatus

  • Macula densa
  • Juxtaglomerular cells
  • Extraglomerular mesangial cells
• Macula densa


  Cells that detect low sodium concentration (during low blood pressure) in the tubular filtrate and release prostaglandins that stimulate renin secretion from juxtaglomerular cells
• Juxtaglomerular cells


  Cells found at the walls of the afferent arteriole that produce renin
• Extraglomerular mesangial cells


  Cells located in the space between the afferent and efferent arterioles, and the glomerular capillaries, that resemble smooth muscle cells and play a role in renal autoregulation of blood flow to the kidney and regulation of systemic blood pressure through the renin–angiotensin system
• Blood supply to the kidney

  • Renal artery -> Segmental artery -> Interlobar artery -> Arcuate artery -> Interlobular (cortical radiate) artery -> Afferent arteriole -> Glomerulus -> Efferent arteriole -> Peritubular capillaries
• Renal vein


  The vein that exits the kidney through the hilum and empties into the inferior vena cava
• Three basic functions performed by nephrons and collecting ducts

  • Glomerular filtration
  • Tubular reabsorption
  • Tubular secretion
• Glomerular filtration


  The first step of urine production where blood pressure forces water and most solutes in blood plasma across the wall of glomerular capillaries
• Tubular reabsorption


  The process of returning most of the filtered water and many of the filtered solutes to the blood
• Tubular secretion


  The process of removing substances, such as wastes, drugs, and excess ions, from blood in the peritubular capillaries and transporting them into the fluid in the renal tubules
• Glomerular filtration rate (GFR)


  The volume of glomerular filtrate formed per minute by the kidneys, an important indicator of kidney function
• Proximal convoluted tubule (PCT)


  The part of the renal tubule where most tubular reabsorption occurs
• Loop of Henle


  The U-shaped tube where filtrate enters the descending limb and exits the ascending limb
• Distal convoluted tubule (DCT)

  The part of the renal tubule where most tubular secretion occurs and where additional water reabsorption can take place
• Kidneys maintain an acid-base balance by secreting excess H+ ions
• Proteins and erythrocytes do not pass through the glomerular filtration barrier
• PCT


  Part of the renal tubule where almost all nutrients are reabsorbed, either by passive or active transport
• Sodium (Na+)


  The most abundant ion, most of which is reabsorbed by active transport
• Loop of Henle


  • A U-shaped tube
  • Filtrate enters the descending limb and exits the ascending limb
  • Descending limb is water-permeable, water travels to interstitial space
  • Ascending limb is permeable to sodium and chloride ions
• Tubular Reabsorption and Secretion

  1. By the time filtrate reaches the DCT, most of the urine and solutes have been reabsorbed

  2. If body requires additional water, all of it can be reabsorbed at this point

  3. Excretion of wastes occurs due to lack of reabsorption combined with tubular secretion

  4. Products like metabolic wastes, urea, uric acid, and certain drugs, are excreted by tubular secretion

  5. Most of the tubular secretion happens in the DCT, but some occurs in the early part of the collecting duct

  6. Kidneys also maintain an acid-base balance by secreting excess H+ ions