urinary system

• Anatomy of the kidney 


  Lie either side of the vertebral, column on the posterior abdominal cavity wall. The liver is superior to the right kidney thus it lies slightly lower than the left. Kidneys are surrounded by several layers:

  • Renal capsule - layer of fibrous connective tissue
  • Layer of adipose tissue
  • Renal fascia - layer of connective tissue
  • Final layer of adipose tissue
• Anatomy of the kidney

  Hilum: opening for the renal artery and nerves to enter. Renal vein and ureter to exit.

  Renal sinus: the cavity which opens after the hilum, filled with connective and adipose tissue.

  Kidney is made up of an outer cortex and inner medulla.
• Renal pyramids

  Base form the boundary between cortex and medulla.
• Renal papillae


  Point of the pyramids.
• Minor calyces


  Renal papillae extend into this funnel.
• Major calyces


  Several pyramids minor calyces merge into major calyces.
• Renal pelvis


  Major calyces merge to form.
• Ureter


  Renal pelvis forms a small diameter tube the utter which extends to the bladder.
• Renal columns


  In between renal pyramids.
• Anatomy of nephron


  Functional units in the kidneys. Made up of four components:

  • Renal corpuscle: filters blood
  • Proximal convoluted tubule: returns filtered substances back to the blood depending on the body's need
  • Loop of Henle: conserve water and solutes
  • Distal convoluted tubule: additional waste added to filtrate

  Collecting duct - connects to several DCT and carries fluid from cortex to medulla and empties into the papillary duct.

  Papillary duct - empties into the minor calyx.
• Flow through kidneys

  Renal corpuscle

  Proximal convoluted tubule

  Loop of Henle

  Distal convoluted tubule

  Collecting duct

  Papillary ducts

  Minor calyx

  Major calyx

  Renal pelvis

  Ureter

  Urinary bladder

  Urethra
• Renal corpuscle


  Comprised of:

  • Glomerulus: network of capillaries which receives blood from the afferent arteriole and exits at the efferent arteriole.
  • Bowman's capsule: capsule that surrounds the glomerulus and where fluid is filtered into from the capillaries and then flows to the PCT.

  Filtration membrane allows fluid to flow into the capsule.
• Juxtaglomerular apparatus


  Juxtaglomerular cells: smooth muscle cells forming an arrangement around the afferent arteriole at the entry point to the glomerulus.

  Macula densa: between the afferent and effect arteriole lies a section of the DCT with in this section contain specialised cells.

  Juxtaglomerular apparatus: the contact of the juxtaglomerular cells and macula densa.

  Function - secretes renin which assists with the regulation of blood pressure and formation of filtrate.
• Renal tubule


  Proximal convoluted tubule:

  • Longer than the DCT
  • Outer basement membrane with simple cuboidal epithelial cells
  • Inner surface has many microvilli projections

  Loop of Henle:

  • Thick portions are simple cuboidal epithelium cells
  • Thing portions are simple squamous epithelium cells
• Renal tubule


  Distal convoluted tubule:

  • Simple cuboidal epithelium cells
  • Smaller cells than the PCT
  • Less microvilli

  Collecting duct:

  • Simple cuboidal epithelium cells
  • Larger in diameter compared to the rest of the renal tubule
• Renal blood supply


  Aorta (systemic) -> renal artery (divides into arteries) -> segmental artery -> interloper artery -> arcuate artery -> cortical radiate artery -> afferent arteriole -> glomerulus -> efferent arteriole (divide again) -> peritubular capillaries or vasa recta -> cortical radiate vein -> arcuate vein -> interlobar vein -> renal vein -> inferior vena cava
• Urine formation

  Filtration: the process of non selectively forcing small molecules and water out of the blood into the Bowman's capsule where it is called filtrate.

  Tubular reabsorption: the process of returning water and solutes back into the blood as the filtrate flows through the renal tubule.

  Tubular secretion: the movement of drugs and toxic by-products from the blood into the filtrate.
• Filtration membrane


  Capillary endothelial layer - contains many pores

  Basement membrane - has spaces between fibres

  Bowman's capsule epithelial layer (podocyte) - has foot processes creating filtration slits
• Urine production


  Renal fraction = % of total cardiac output (how much blood/min) that enters the kidney

  • Resting, healthy adults = 21%

  Glomerular filtration rate (GFR) = the amount of filtrate (plasma) that enters the Bowman's capsule

  • Renal blood flow rate per min = cardiac output mL/min x renal fraction
  • Renal plasma flow rate = renal blood flow rate x amount of plasma in blood (55%)
  • GFR = renal plasma flow rate x 19% (filtration fraction)
• Filtration pressures


  Filtration pressure: the pressure gradient in the renal corpuscle.

  Glomerular capillary pressure:

  • Outward pressure of blood in the capillaries
  • Forces solutes and fluid out of capillaries and into the Bowman's capsule
  • Greater pressure in glomerulus compared to other capillaries
  • Efferent arteriole is smaller in diameter compared to afferent arteriole and capillaries
• Filtration pressures


  Capsular hydrostatic pressure:

  • Inward pressure of the filtrate in the Bowman's capsule pressing back on the capillaries

  Blood colloid osmotic pressure:

  • Inward pressure resulting form the osmotic force of plasma proteins in the glomerular capillaries
• Intrinsic mechanism of control

  Auto-regulation: direct regulation of GFR

  Myogenic mechanism - can work in reverse

  • Increase in afferent arteriole pressure (vasodilation) = increase in vessel stretch = constriction = decrees in GFR (less stretch)
  • Restricts blood flow and lower cap pressure more consistent with efferent arteriole.
• Intrinsic mechanisms of control

  Tubuloglomerular mechanism - can work in reverse

  Increase in GFR:

  = Increase in flow rate

  = Detection by macula densa cells of DCT

  = Secretion of paracrine hormone from macula densa

  • = increase constriction of afferent arteriole
  • = deceased GFR due to reduced flow = decreased capillary pressure
• Changes in GFR


  Afferent arteriole:

  Diameter increases, filtration increases = allows more in, increases pressue

  Diameter decreases, filtration decrease = less fluid in, less pressure, less filtrated

  Efferent arteriole:

  Diameter increases, filtration decrease = allows a lot to leave, less pressure

  Diameter decrease, filtration increases = blood backs up, more pressure
• PCT reabsorption


  PCT - active and selective reabsorption, filtrate reduced by 65% at the end of PCT and more reabsorption occurs here.
• Tubular reabsorption


  The basal membrane and apical membrane are the driving force which sodium ions concentration firstly set by its active transport across the basal membrane.
• Loop of Henle


  The long nephron loops of the juxtamedullary nephrons create the gradient. They act as a countercurrent multiplier. The vasa recta preserve the gradient. They act as countercurrent exchangers. The collecting ducts of all nephrons use the gradient to adjust urine osmolality.
• Countercurrent multiplier


  Fluid flows in the opposite direction through two adjacent parallel sections of a nephron loop. The descending limb is permeable to water but not to salt and water is able to flow out. The ascending limb is impermeable to water and pumps out salt, water can't escape and salt starts leaving.
• Loop of Henle


  Filtrate enters the nephron loop and is somatic to both blood plasma and cortical interstitial fluid. Water moves out of the filtrate in the descending limb down its osmotic gradient. This concentrates the filtrate. Filtrate reaches its highest concentration at the bend of the loop. Sodium ions and Chloride ions are pumped out of the filtrate and this increases the interstitial fluid osmolarity. Filtrate is at its most dilute s it leaves the nephron loop.
• Countercurrent multiplier


  More salt is continually added to the PCT. The high the osmolarity of the ECF, the more water leaves the descending limb by osmosis. The more water that leaves the descending limb, the saltier the fluid is that remains in the tubule. The saltier the fluid is in the ascending limb, the more salt the tubule pumps into the ECF. The more salt that is pumped out of the ascending limb, the saltier the ECF is in the renal medulla.
• DCT and collecting duct reabsorption

  Solute and water reabsorption are primarily under hormonal control.
• Tubule reabsorption and secretion

  Proxmial tubule:

  • Reabsorption: sodium, chloride, bicarbonate, potassium, water, glucose, amino acids
  • Secretion: hydrogen ions, organic acids and bases

  Thin descending loop of Henle:

  • Reabsorption: water

  Thick ascending loop of Henle:

  • Reabsorption: sodium, chloride, potassium, calcium, bicarbonate, magnesium
  • Secretion: hydrogen ions
• Tubular reabsorption and secretion

  Early distal tubule:

  • Reabsorption: sodium, chloride, calcium, magnesium

  Late distal tubule and collecting duct:

  Principal cells

  • Reabsorption: sodium, chloride
  • Secretion: potassium, ADH mediated water reabsorption

  Intercalated cells

  • Reabsorption: bicarbonate, potassium
  • Secretion: hydrogen ions
• Ureters 


  Run inferiorly and medially to the bladder and enter on the posterolateral surface. Peristaltic contractions moves urine through the ureters.
• Urinary bladder


  Hollow, muscular container, reservoir for urine. Urinary bladder is able to distend:

  • Large folds inside
  • Cells are transitional cells which stretch
  • Outer smooth muscle is able to stretch

  Smooth muscle and elastic connective tissue prevent urine from exiting

  Contraction of the smooth muscle assists with forcing the urine out

  External urinary sphincter - skeletal muscle which controls the urine flow through the urethra
• Micturition reflex


  Micturition: elimination of urine from the urinary bladder

  Stimuli:

  • Stretch of the bladder wall
  • Stretch receptors produce action potenitals

  Response:

  • Parasympathetic stimulation = contraction of the bladder's smooth muscle
  • Decrease somatic stimuli = relaxation of the external urinary sphincter
  • Higher centres of the brain can inhibit or stimulate the reflex
• Regulation of volume and concentration

  The DCT and collecting duct are regulate by hormonal mechanisms depending on the conditions of the body. If water needs to be retained, then water is reabsorbed and results in urine which is concentrates and of a small volume. If water needs to be lost, then the dilute filtrate can pass through the DCT and collecting duct with no change in concentration resulting in a large volume which is dilute. Mechanisms which work together and assist with this are:

  • Renin-angiotensin-aldosterone hormone mechanisms
  • Antidiuretic hormone mechnaism
• Anti-diurretic hormone (ADH)


  Sensitive to changes in blood osmolarity and blood pressure.

  Increase in blood osmolarity = increase in ADH secretion

  Response:

  • Increase water reabsorption at the kidneys

  = Decrease in urine output

  = Decrease in blood osmolality

  Decrease in blood pressure (and blood volume) = increase ADH secretions

  Response:

  • Increased water reabsorption

  = Decrease in urine output

  = Increase in blood pressure and volume
• Renin-angiotensin-aldosterone mechanism/system


  Renin is sensitive to changes in blood pressure.

  Renin is secreted in response to:

  • Reduced afferent arteriole stretch (reduced blood pressure)
  • Low sodium ion levels detected by the macula dense cells in the DCT

  Renin converts angiotensinogen (produced in the liver) to angiotensin 1

  Angiotensin 1 is converted to angiotensin 2. Response (target tissue = adrenal cortex) = vasoconstriction = increase in blood pressure. Increase aldosterone and ADH secretion

  Aldosterone increases the rate of sodium ion reabsorption
• Renin-angiotensin-aldosterone mechanism/system


  RAAS maintains GFR through:

  Angiotensin 2 = constriction of afferent and efferent arteriole = less renal blood flow = increasing systemic blood pressure. However, to maintain GFR, angiotensin has a great effect on efferent arteriole therefore more blood remain in glomerulus and GFR remains the same.