Urinary system

• What are the functions of the kidneys?


  1. filter and excrete
  2. Regulate key homeostatic systems
  3. Produce hormones
• What do the kidneys filter and excrete?


  Remove metabolic waste products and toxins from blood

  Excrete these into the urine
• What do the kidneys regulate in terms of the homeostatic systems?


  • body fluid status
  • body electrolyte balance
  • body acid-base balance
• What are the hormones produced by the kidneys involved in?


  • erythrogenesis
  • Ca2+ metabolism
  • Regulation of blood pressure and blood flow
• Where are the kidneys in relation to the rest of the body?


  • bean shaped organs
  • either side of the vertebral column in the retroperitoneal space
  • vertebral level L2
• How are kidneys and urinary system viewed outside the body?

  • intravenous pyelogram
  • iodinated contrast medium
  • imaged by multi-detector CT
  • allows images of urinary excretion via collecting system, ureters and bladder
• Describe the outside structure of the kidneys

  Ecompassed by renal capsule

  • superiorly - adrenal gland
  • Hilum of the kidneys - renal artery, renal vein, ureter
• Describe the internal structure of the kidneys


  outer layer - cortex

  part of the ureter that enters the kidney - pelvis

  Superior part of the pee tube - major calyx

  inferior part of the pee tube - minor calyx

  Parts of the area that filter the blood - pyramid

  Area of pyramid from the top to the edge of the pee tube - medulla
• How much of the cardiac output is received by the kidney?


  20%
• Describe the pathway of the renal arteries

  Renal artery

  -> segmental artery - branch off the main artery

  -> interlobar artery - go between the lobes of the pyramid

  -> arcuate artery - at the superior part of the pyramids in the kidneys

  -> interlobular artery - superior to the arcuate arteries on the pyramids of the kidney

  -> Afferent arteriole - artery that leads into the glomerulus of the nephron

  -> Glomerulus - big ball of artery in the nephron

  -> Efferent arteriole - artery leading out of the glomerulus

  -> Peritubular capillaries (vasa recta)
• Describe the pathway of the renal veins


  Interlobular vein - vein above the pyramid

  -> arcuate vein - on the top of the pyramid

  -> interlobular vein - inbetween pyramids

  -> renal vein
• What is the functional unit of the kidney?

  The nephron
• Describe the nephron


  ~ 1 million nephrons in each human kidney

  Each nephron composed of

  • renal corpuscle (filtration)
  • tubule (reabsorption/secretion)

  The kidney cannot regenerate nephrons

  • reduce in number with age, but remain functional until a dramatic loss
• Describe the renal corpuscle


  Part of the functional unit of the nephron - where majority of the filtration of the blood occurs in the kidney

  Composed of:

  • Glomerulus - ball of capillaries
  • afferent arteriole in / efferent arteriole out
  • Bowman's capsule - structure surrounding the glomerulus
  • Bowman's space - space between the capsule and the glomerulus

  Blood enters the glomerulus via afferent arteriole and is filtered accross glomerular capillaries
• Describe the tubule of the nephron

  Filtered blood from the glomerulus (known as ultrfiltrate) then enters the renal tubule

  Made from distinct segments;

  • proximal convoluted tubule
  • Loop of Henle
  • thin descending limb
  • thin ascending limb
  • thick ascending limb
  • Distal convoluted tubule
  • Collecting duct
• Describe the filter and excrete function of the kidneys

  1. blood is filtered at the glomerulus
  2. Ultrafiltrate enters the tubule and flows along distinct segments
  3. Reabsorption of ions, solutes and water and secretion of waste products occurs along the length of the tubule
  4. Remainder of tubular fluid flows to bladder and will leave as urine
• Describe the kidneys function as a regulator of homeostatic systems


  Through the same as process as filtration and secretion the kidneys can;

  • control the amount of water in your body
  • control what goes out as well as what stays in your blood
  • ions and sodium levels are being regulated - what comes out of your blood and what remains in your plasma
  • protons and bicarbonates - acid base balances
• Describe the function of the kidneys in terms of their involvement in the production of hormones


  renin - regulates blood pressure - found next to the afferent arteriole
• What are the different types of nephron?


  • Superficial (cortical) nephron
  • Juxtamedullary nephron
• Describe the most common type of nephron


  superficial (cortical) nephron 85% of all nephrons

  • glomerulus is in the outer region of the cortex
  • loop of Henle is short, terminates in the outer medulla
  • Efferent arteriole forms only the peritubular capillaries
• Describe the less common type of nephron

  Juxtamedullary nephron - 15 %

  • Glomerulus closer to the medulla
  • Loop of Henle is much longer, extending into the inner medulla
  • Efferent arteriole forms peritubular capillaries and the vasa recta
• Describe the epithelia found in the corpuscle


  Podocytes

  • specialised squamous epithelial cells
  • form part of the glomerular filter

  Parietal layer of the Bowman's capsule

  • simple squamous epithelia
• Describe the epithelia found in the tubule


  Whole tubule is lined with epithelia

  • almost all simple cuboidal epithelia
  • Except in the thin limbs of loop of Henle - simple squamous
• Describe the structure and function of the ureters

  Function - urine is conveyed from the renal pelvis to the bladder via the ureters

  Structure

  • muscular tubes, multiple layers
  • innermost layer is epithelium - urothelium
• How is urine moved from the renal pelvis to the ureters?


  As urine collects in the renal pelvis, it dilates

  • triggers action potentials in pacemaker cells of renal pelvis
  • stimulates peristaltic contractions that propel the urine
• Describe the bladders relation to the ureters 


  • Ureters enter the lower posterior portion of the bladder
  • The ureteral orifices, connected by a ridge of tissue, and the urethral orifice from the bladder trigone
• What forms the bladder trigone?


  • The 2 ureteral orifices
  • The urethral orifice
• What lines the bladder?

  • urothelium
  • Connective tissue
  • Smooth muscle layers
  • detrusor muscle
• How is the process of needing to urinate stimulated in the bladder?

  Bladder fills

  • the rugae (infoldings) smooth out and the volume increase without increasing the pressure
  • bladder continues to fill and once over 300ml pressure starts to increase
  • stimulates stretch receptors in muscle wall
  • Pressure closes ureteral orifices (no backflow to ureter)
• Describe the structure and function of the urethra


  Function - conducts urine from the bladder to be voided

  Structure;

  • initially lined by transitional epithelium, other epithelia beyond this
  • 2 sphincters - internal (involuntary) and external (voluntary)
• Describe the differences between the male and female urethra


  Female - short (5cm) - more susceptible to UTI

  Male - longer (20 - 25cm)

  • passes through the prostate gland into penis
  • Conduit for both urine and semen
• What is micturition?


  Urination - emptying of the urinary bladder
• What controls micturition?

  Autonomic and voluntary nervous system

  Bladders have 500ml capacity

  • further stretch receptor activation causes parasympathetic motor efferents to;
  • stimulate bladder (detrusor muscle) contraction
  • Relaxation of internal urethral sphincter
  • Emptying is prevented by CNS until the outer urethral sphincter is relaxed voluntarily