urinary

• Urinary system


  Also known as the renal system, refers to the structures that produce and conduct urine to the point of excretion

  
• Functions of the urinary system

  • Excretion
  • Regulation of blood volume and pressure
  • Regulation of concentration of solutes in the blood
  • Regulation of extracellular fluid pH
  • Regulation of red blood cell synthesis
  • Regulation of vitamin D synthesis

  
• Kidney functions


  • Filter 200 liters of blood daily, allowing toxins, metabolic wastes, and excess ions to leave the body in urine
  • Regulate volume and chemical makeup of the blood
  • Maintain the proper balance between water and salts, and acids and bases

  
• Other urinary system organs

  • Urinary bladder
  • Paired ureters
  • Urethra
• Renal capsule


  Layer of collagen fibers that encloses the kidney, prevents kidney infection

  
• Adipose capsule


  Fatty mass that cushions/holds the kidney and helps attach it to the body wall

  
• Renal fascia


  Outer layer of dense fibrous connective tissue that binds the kidney to abdominal wall

  
• Internal anatomy of the kidney

  • Cortex - light colored, granular superficial region
  • Medulla - exhibits cone-shaped medullary (renal) pyramids separated by columns
  • Renal pelvis - flat funnel shaped tube lateral to the hilus within the renal sinus

  
• Parts of the renal pelvis

  • Major calyces
  • Minor calyces

  
• Nephron


  The structural and functional units that form urine, consisting of a glomerulus and a renal tubule

  
• Renal corpuscle


  • The glomerulus and its Bowman's capsule
  • Glomerular endothelium - fenestrated epithelium that allows solute-rich, virtually protein-free filtrate to pass from the blood into the glomerular capsule

  
• Proximal convoluted tubule (PCT)


  • Composed of cuboidal cells with numerous microvilli and mitochondria, reabsorbs water and solutes from filtrate and secretes substances into it
• Loop of Henle


  • A hairpin-shaped loop of the renal tubule, with a proximal part similar to the PCT, followed by the thin segment and the thick segment

  
• Distal convoluted tubule (DCT)


  • Cuboidal cells without microvilli that function more in secretion than reabsorption
• Types of nephrons


  • Cortical nephrons - 85% of nephrons, located in the cortex
  • Juxtamedullary nephrons - located at the cortex-medulla junction, have loops of Henle that deeply invade the medulla and have extensive thin segments, involved in the production of concentrated urine
• Capillary beds of the nephron

  • Every nephron has two capillary beds - the glomerulus and the peritubular capillaries
  • Each glomerulus is fed by an afferent arteriole and drained by an efferent arteriole

  
• Filtration membrane


  • Composed of three layers - fenestrated endothelium of the glomerular capillaries, visceral membrane of the glomerular capsule (podocytes), and basement membrane composed of fused basal laminae of the other layers
• Mechanisms of urine formation

  1. Glomerular filtration

  2. Tubular reabsorption

  3. Secretion

  
• Glomerular filtration rate (GFR)


  The total amount of filtrate formed per minute by the kidneys (Normal >60 mL/min)
• Factors governing filtration rate at the capillary bed
• Net filtration pressure (NFP)

  Glomerular blood hydrostatic pressure (GBHP) - [capsular hydrostatic pressure (CHP) + blood colloid osmotic pressure (BCOP)]

  
• If the GFR is too high

  Needed substances cannot be reabsorbed quickly enough and are lost in the urine
• If the GFR is too low

  Everything is reabsorbed, including wastes that are normally disposed of
• Tubular reabsorption


  • All organic nutrients are reabsorbed
  • Water and ion reabsorption is hormonally controlled (ADH)
  • Reabsorption may be an active (requiring ATP) or passive process
• Sodium reabsorption


  Almost always by active transport, with Na+ entering the tubule cells at the luminal membrane and actively transported out by a Na+-K+ ATPase pump
• Reabsorption by PCT cells

  • Active pumping of Na+ drives reabsorption of water by osmosis, aided by aquaporins, and reabsorption of cations, fat-soluble substances, organic nutrients and selected cations
• Creatinine


  A waste product made by muscles as part of regular, everyday activity
• Substances not reabsorbed
• Active pumping of Na+

  Drives reabsorption of:

  • Water by osmosis, aided by water-filled pores called aquaporins
  • Cations and fat-soluble substances by diffusion
  • Organic nutrients and selected cations by secondary active transport
• Substances not reabsorbed


  • Lack carriers
  • Are not lipid soluble
  • Are too large to pass through membrane pores
• Urea, creatinine, and uric acid are the most important nonreabsorbed substances
• Creatinine


  A waste product made by your muscles as part of regular, everyday activity
• Substances reabsorbed in PCT

  • Sodium, all nutrients, cations, anions, and water
  • Urea and lipid-soluble solutes
  • Small proteins
• Substances reabsorbed in Loop of Henle

  • H2O, Na+, Cl−, K+ in the descending limb
  • Ca2+, Mg2+, and Na+ in the ascending limb
• Substances reabsorbed in DCT

  • Ca2+, Na+, H+, K+, and water
  • HCO3- and Cl-
• Substances reabsorbed in Collecting duct


  • Water and urea
• Tubular secretion


  Substances move from peritubular capillaries or tubule cells into filtrate
• Tubular secretion is important for

  • Disposing of substances not already in the filtrate
  • Eliminating undesirable substances such as urea and uric acid
  • Controlling blood pH
• Anti-Diuretic Hormone (ADH)


  Helps regulate the amount of water in the body
• Anti-Diuretic Hormone (ADH) release

  Released by the posterior pituitary gland when there is a decreased in blood volume or low blood pressure (occurs during dehydration or hemorrhage), which is detected by the baroreceptors (sensors of the heart and large blood vessels)