CHAPTER 18 : URINARY

• Urinary system


  The major excretory system of the body
• Some organs in other systems also eliminate wastes, but they are not able to compensate in the case of kidney failure
• Urinary system functions


  • Excretion
  • Regulation of blood volume and pressure
  • Regulation of blood solute concentration
  • Regulation of extracellular fluid pH
  • Regulation of red blood cell synthesis
  • Regulation of Vitamin D synthesis
• Components of the urinary system

  • Two kidneys
  • Two ureters
  • One urinary bladder
  • One urethra
• Kidney characteristics


  • Bilateral retroperitoneal organs
  • Bean shaped
  • Weigh 5 ounces (bar of soap or size of fist)
  • Located between 12th thoracic and 3rd lumbar vertebra
• Renal capsule


  Connective tissue around each kidney that protects and acts as a barrier
• Hilum


  Indentation that contains renal artery, veins, nerves, ureter
• Renal sinus


  Contains renal pelvis, blood vessels, fat
• Renal cortex


  Outer portion of the kidney
• Renal medulla


  Inner portion of the kidney
• Renal pyramid


  Cone shaped structures in the medulla whose bases project into the cortex
• Renal papillae


  Tip of pyramids which drain into calyces
• Renal pelvis


  Where calyces join together and narrows to form ureter
• Nephron


  The functional unit of the kidney, each kidney has over one million nephrons
• Types of nephrons


  • Juxtamedullary nephrons
  • Cortical nephrons
• Juxtamedullary nephrons


  Renal corpuscles are deep in the cortex near the medulla, long loops of Henle extend deep into the medulla, well adapted for water conservation, about 15% of nephrons
• Cortical nephrons


  Renal corpuscles distributed throughout the cortex, loops of Henle are shorter and closer to the outer edge of the cortex than juxtamedullary nephrons
• Renal corpuscle


  The filtration portion of the nephron
• Glomerulus


  A network of capillaries twisted around each other like a ball of yarn
• Bowman's capsule


  Enlarged end of nephron that surrounds glomerulus, opens into proximal convoluted tubule, contains podocytes (specialized cells around glomerular capillaries)
• Characteristics of renal corpuscle

  • Porous capillaries - highly permeable due to the presence of pores, neither large proteins nor blood cells can fit through them
  • Porous inner layer of Bowman capsule - A basement membrane lies between the endothelial cells of the glomerular capillaries and the podocytes of the Bowman capsule
  • High pressure
  • An afferent arteriole supplies blood to the glomerulus for filtration, an efferent arteriole transports the filtered blood away from the glomerulus, efferent arteriole has smaller diameter than afferent arteriole creating a high pressure in the capillaries
• Filtration membrane


  Consists of capillary endothelium, the basement membrane, and the podocytes of the Bowman capsule, filtrate is the fluid filtered from the glomerular capillaries that enters the lumen inside the Bowman capsule
• Juxtaglomerular apparatus


  Specialized cells of the afferent arteriole and distal convoluted tubule in close contact with each other, juxtaglomerular cells are specialized smooth muscle cells located where the afferent arteriole enters the renal corpuscle, macula densa is part of the distal convoluted tubule that lies between the afferent and efferent arterioles next to the renal corpuscle, secretion of the enzyme renin by the juxtaglomerular apparatus plays an important role in the regulation of filtrate formation and blood pressure
• Flow of filtrate through nephron

  1. Renal corpuscle

  2. Proximal convoluted tubule

  3. Descending loop of Henle

  4. Ascending loop of Henle

  5. Distal convoluted tubule

  6. Collecting duct

  7. Papillary duct
• Blood flow through kidney

  1. Renal artery

  2. Interlobar artery

  3. Arcuate artery

  4. Interlobular artery

  5. Afferent arteriole

  6. Glomerulus

  7. Efferent arteriole

  8. Peritubular capillaries

  9. Vasa recta

  10. Interlobular vein

  11. Arcuate vein

  12. Interlobar vein
• Urine formation


  Involves three processes: filtration, tubular reabsorption, and secretion
• Filtration


  Occurs in the renal corpuscle, blood plasma leaves glomerulus and enters Bowman space
• Tubular reabsorption


  Involves removing substances from the filtrate and placing them back into the blood
• Secretion


  Involves taking substances from the blood at a nephron area other than the renal corpuscle and putting back into the nephron tubule
• Urine formation - filtration

  • Movement of water, ions, small molecules through filtration membrane into Bowman's capsule, 19% of plasma becomes filtrate, 180 Liters of filtrate are produced by the nephrons each day, 1% of filtrate (1.8 liters) become urine, the rest is reabsorbed, only small molecules are able to pass through filtration membrane, formation of filtrate depends on filtration pressure, filtration pressure forces fluid across filtration membrane, filtration pressure is influenced by blood and interstitial fluid pressures, and osmotic pressures of plasma and interstitial fluid
• Urine production - reabsorption


  99% of filtrate is reabsorbed and reenters circulation, the proximal convoluted tubule is the primary site for reabsorption of solutes and water, the descending Loop of Henle concentrates filtrate, reabsorption of water and solutes from distal convoluted tubule and collecting duct is controlled by hormones
• Urine production - secretion

  Tubular secretion removes some substances from the blood, including by-products of metabolism that become toxic in high concentrations and drugs or other molecules not normally produced by the body, secretion occurs through either active or passive mechanisms, ammonia secretion is passive, secretion of H+, K+, creatinine, histamine, and penicillin is by active transport, the secretion of H+ plays an important role in regulating the body fluid pH
• Urine concentration mechanism


  The kidneys regulate blood composition and are able to produce very dilute or very concentrated urine to maintain the extracellular fluid concentration close to 300 mOsm/L, the ability to control the volume and concentration of the urine depends on: (1) countercurrent mechanisms, (2) a medullary concentration gradient, and (3) hormonal mechanisms
• Countercurrent mechanism


  Fluids in separate structures flow in opposite directions relative to each other, as they pass by each other, materials can be exchanged between them
• Medullary concentration gradient


  The countercurrent mechanism creates a very high solute concentration in the medulla compared to the cortex, the concentration of solutes in the medulla increases from 300 mOsm/L to 1200 mOsm/L deep in the medulla at the tip of the renal pyramid
• Descending limb of the loop of Henle

  A critical site for water reabsorption, the filtrate leaving the proximal convoluted tubule is further concentrated as it passes through the descending limb, the wall of the thin segment is highly permeable to water, as the filtrate moves through the medulla containing the highly concentrated interstitial fluid, water is reabsorbed out of the nephron by osmosis
• Ascending limb of the loop of Henle


  Dilutes the filtrate by removing solutes, the thin segment is not permeable to water, but it is permeable to solutes so solutes diffuse out of the nephron
• Distal convoluted tubule and collecting duct


  The permeability varies depending upon hormonal control, concentrated urine is produced when the permeability increases and water leaves filtrate by osmosis
• Regulation of urine concentration and volume

  • Renin-angiotensin-aldosterone system
  • Antidiuretic hormone (ADH)
  • Atrial natriuretic peptide (ANP)
• Loop of Henle


  • Ascending limb dilutes the filtrate by removing solutes
  • Thin segment of ascending limb is not permeable to water but permeable to solutes so solutes diffuse out of the nephron