urinary

• Excretion


  The process of eliminating metabolic waste and other non-useful materials from the body
• Urinary system


  The system responsible for the production, storage, and elimination of urine from the body
• Learning objectives


  • Describe the excretory system
  • Explain the significance of the urinary system
  • Describe the anatomy of the kidney
  • Identify and describe the site of filtration in the kidney as the nephron
  • Describe the processes and sites in the nephron which produce urine
  • Explain how the body stores and excretes urine
  • Understand how the kidney and the endocrine system together controls hydration
  • Describe 3 ways in which the urinary system contributes to blood homeostasis
  • Give the anatomy and the physiology of the juxtaglomerular apparatus
  • Explain how the kidney regulates the body's blood pressure
  • Identify an example of a urinary system dysregulation
• Excretory system


  • Main functions: Maintain osmotic balance, Removal of metabolic waste
• Organs involved in removal of metabolic waste

  • Skin
  • Liver
  • Lungs
• Urea


  Nitrogenous waste product from protein metabolism
• Urinary system organs


  • Kidneys
  • Ureters
  • Bladder
  • Urethra
  • Sphincter
• Urinary system functions


  • Filters blood and removes waste products
  • Metabolises urea to urine for excretion
  • Balances electrolytes
  • Releases hormones to control blood pressure
  • Balances body fluids
  • Releases hormone to control red blood cell production
  • Helps with bone health by controlling calcium and phosphorus
• Urinary system anatomy


  1. 2 kidneys (filter blood & make urine)

  2. 1 bladder (storing urine)

  3. 2 ureters (connecting each kidney to the bladder)

  4. 1 Urethra (connecting the bladder to the body's surface)

  5. 2 sphincter muscles (stop urine leaking out)

  6. Nerve endings in bladder (to control stretching and emptying)
• Kidney location


  • Behind all other abdominal organs, on back abdomen wall behind the peritoneal cavity, between the thoracic and lumbar regions, retroperitoneal, 150g each, 12 x 6 x 3cm in size, right kidney slightly lower than left (pushed down by liver)
• Urinary system functions


  • Volume: maintains blood volume (and therefore blood pressure)
  • Concentration: maintains blood ion concentration, electrolyte balance
  • pH: Regulates pH by balancing H+ and HCO3-
  • Metabolic processes: gluconeogenesis, glycogenesis & glycolysis, Vitamin D synthesis
  • Excretion: of metabolic waste, drugs, and toxins
  • Endocrine functions: Renin (blood volume regulation) & Erythropoietin (red blood cell formation)
• Nephron


  The functional unit of the kidney that filters the blood and produces urine
• Path of urine


  Renal pyramid -> papilla -> minor and major calyx -> renal pelvis -> ureter -> bladder -> urethra -> out of body
• Ureter


  • Hollow tubes approx. 25 cm long, collects urine from the renal pelvis and carries it to the bladder, has outer adventitia layer, smooth muscle layer, and lumen surrounded by transitional epithelial tissue and lamina propria
• Bladder


  • Hollow, stretchy organ in the lower abdomen that stores urine before it leaves the body, can hold up to 700ml, has trigone area with stretch receptors, lined with urothelium and transitional epithelial tissue, has lamina propria and muscularis propria layers, has urethral sphincters to stop urine leaving
• Urethra


  • Duct from the bladder to the outside of the body, has internal and external sphincter muscles, different in males and females (female shorter at 4cm, male longer at 18-20cm and passes through prostate gland)
• Nephron


  Each kidney has about a million filtering units called nephrons, each with a glomerulus filter and a tubule
• Nephron filtration process


  Glomerulus filters blood, filtrate collected in Bowman's capsule, 99% of filtrate reabsorbed back into blood or lymph
• Proximal convoluted tubule


  • First segment of nephron, main role is reabsorption of solutes and water, reabsorbs 65% of water and NaCl, nearly 100% of nutrients, small amounts of urea, helps maintain extracellular fluid and electrolyte homeostasis
• Loop of Henle


  • Principal function is recovery of water and sodium chloride, 15% filtrate reabsorbed here, descending loop permeable to water, ascending loop permeable to electrolytes, establishes salt concentration gradient
• Distal convoluted tubule


  • Approx. 15% filtrate reabsorbed here, concerned with regulation of water, electrolytes, and pH, excretion/reabsorption of electrolytes, H+ ions, and ammonia
• Collecting duct


  • Approx. 5% filtrate reabsorbed here, collects urine from nephrons and moves it to renal pelvis and ureters, uses salt concentration from loop of Henle to reabsorb water, can secrete H+ ions, ammonia and medications
• The renal corpuscle (glomerulus + Bowman's capsule) is the site of filtration in the kidney
• Loop of Henle


  • Establishes salt concentration gradient between the inner medulla and outer cortex of the kidney (counter-current multiplier system)
  • Allows the nephron to reabsorb more water and concentrate the urine while at the same time using as little energy as possible, because of osmotic pressure
• Urea


  Can secrete into the descending Loop of Henle, while nothing secretes into the ascending part
• Distal Convoluted Tubule (DCT)

  • Approx. 15% filtrate reabsorbed here
  • Concerned with the detailed regulation of water, electrolytes, and hydrogen-ion balance, through controlled reabsorption
  • Excretion/reabsorption of electrolytes
  • Controls reabsorption of water
  • Reabsorption/excretion of hydrogen ions and bicarbonate ions
  • Controls blood pH
• Once filtrate passes through DCT

  It is now urine
• Collecting Duct


  • Approx. 5% filtrate reabsorbed here
  • Collects urine from the nephrons and moves it into the renal pelvis and ureters
  • Final component of the kidney to influence the body's electrolyte and fluid balance through reabsorption
  • Uses salt concentration established by loop of Henle to reabsorb water from urine
  • Epithelial cells of collecting duct must have aquaporins on their surface so that water will be reabsorbed by osmosis
• Excreting Urine


  1. The collecting duct dumps urine into the ureter via the minor and major calyx in the renal pelvis of the kidneys

  2. The ureters transfer the urine from each kidney to the bladder

  3. The urine is stored in the expanding bladder until it can be removed

  4. Urine exits the bladder via the internal urethral orifice when the urethral sphincter muscle opens

  5. Urine enters the urethra to be removed from the body
• Anti-diuretic hormone (ADH) is a chemical produced in the brain that causes the kidneys to release less water, decreasing the amount of urine produced
• ADH


  • Acts to maintain blood pressure, blood volume and tissue water content by controlling the amount of water
  • Stimulates the expression/action of aquaporins on the collecting duct cells
  • Secretion of ADH is stimulated when osmoreceptors on hypothalamus detect a high osmolarity (dehydration/reduced water in body)
  • Results in greater retention of water (reabsorption) and concentration of urine
• Hormones involved in water balance

  • Vasopressin (ADH)
  • Aldosterone
  • Renin
  • ANP (atrial natriuretic peptide)
• Vasopressin (ADH)


  • The water retaining hormone
  • Hypothalamic hormone secreted by the pituitary gland, in response to low blood volume or high plasma osmolarity
  • Increases the water permeability of the CT and more water is reabsorbed by body
• Aldosterone


  • The salt retaining hormone
  • Secreted by adrenal cortex in response to low blood sodium
  • Increases the Na+ permeability of the DCT and the CT and more sodium is reabsorbed by body
  • Water follows salt, water is also reabsorbed
• Renin


  • Produced by kidney in response to low blood pressure
  • Activates the RAAS system to produce angiotensin II
  • Angiotensin II causes vasoconstriction, the release of vasopressin and aldosterone, and activates thirst centres
  • Increases blood volume and flow rate, increases blood pressure
• ANP (atrial natriuretic peptide)


  • Secreted by myocardium in response to high blood pressure
  • Causes vasodilation, increased filtrate production rate, increased reabsorption of water
  • Inhibits renin secretion and sodium reabsorption
  • Reduces blood volume and, hence, pressure
• Blood pH control


  • Adjusts the amounts of excreted acids and reabsorbed bicarbonate to regulate pH changes
  • PCT and CD reabsorption of HCO3- (alkali, raises pH) & CD generates new HCO3- as needed
• Blood calcium level


  Low calcium levels detected, PTH secreted which causes the kidney to retain calcium, and increases the reabsorption of calcium
• EPO (erythropoietin)


  • More produced by kidneys when low blood oxygen level from a reduced red blood cell count detected
  • Stimulate hematopoietic stem cells in bone marrow to divide by mitosis and specialise to become red blood cells
• Juxtaglomerular Apparatus


  • Consists of granular cells and macula densa
  • Regulates renal blood flow and glomerular filtration rate
  • Responsible for the secretion of renin