The Urinary System

I. Kidney Anatomy Diagram Diagram

A. External Anatomy

i. Three supportive tissue layers surrounding kidney: renal capsule, adipose capsule, and renal fascia

ii. Kidney contains a structure (hilus) where the renal arteries and vein are located as well as the beginning of the ureters

B. Internal Anatomy Diagram

i. Kidney is composed of three specific regions: cortex, medulla, and pelvis

ii Internal structures: renal column, renal pyramid, papilla, and minor and major calyx

C. Blood Supply Diagram

i. Renal arteries: segmental --> lobar --> interlobar --> arcurate --> interlobular (cortical radiate)

ii. Renal vein - no segmental or lobar <-- interlobar <-- arcuate <-- interlobular (cortical radiate)

D. Nephrons Diagram

• Functional unit of the kidney; tubule system that filters out wastes and other unnecessary chemicals circulating in the blood
• "filtering" is the result of tubule cell's ability to change its permeability
• 85% of nephrons are cortical; 15% are juxtamedullary
• Nephron structure: Diagram
  • glomerular (Bowman's) capsule
    • Note: the glomerular capsule + the glomerulus = renal corpuscle Diagram
  • proximal convoluted tubule (PCT)
  • loop of Henle (ascending and descending limb)
  • distal convoluted tubule (DCT)
  • collecting tubule - papillary duct - calyces
• Capillary beds (microvasculature) of nephron are composed of two capillary beds, glomerular (feeding nephron) and peritubular (surrounding nephron)
  
• Glomerulus is fed and drained by arterioles (afferent and efferent) which arise from interlobular arteries that run through the renal cortex; due to arterioles being high-resistance vessels and afferent arterioles having larger diameters than efferent, blood pressure is high, therefore fluid and solute are forced out of blood into the glomerular capsule; most filtrate is returned to blood via peritubular capillary bed surrounding the nephron.

E. Juxtaglomerular Apparatus Diagram

• Region where the coiling distal tubule lies against the afferent arteriole feeding the glomerulus
• Both arteriole and distal tubule has specialized cells:
  • juxtaglomerular cells (JG) - smooth muscle cells surrounding the afferent and efferent artioles and composed of granules containing renin; these cells act as mechanoreceptors that sense the blood pressure in the arteriole
  • macula densa - distal tubule cells that act as chemoreceptors or osmoreceptors that respond to changes in solute concentration of filtrate in distal tubule
• Therefore, both aid in the regulation systemic blood pressure and rate of filtration formation!

II. Kidney Physiology

A. Overview  Diagram Diagram

• 1000-1200 ml of blood passes through glomeruli each minute
• 650mls is plasma
• 120-125 ml is forced into the renal rubules
• Filtrate - everything plasma contains except proteins
• Urine - remnants of filtrate that percolated into the collecting ducts (therefore loss of water, nutrients, and some ions)
• Three processes: Glomerular filtration, tubular reabsorption, and secretion

B. Glomerular Filtration

• Nonselective, passive process in which fluids and solutes are forced through a membrane by hydrostatic pressure
  
• Glomerulus is more efficient filter than other capillary beds because.....

1. Filtration membrane is thousands of times more permeable to water and solutes than other capillary membranes
2. Glomerular blood pressure is higher than that of other capillary beds, resulting in a higher net filtration pressure

• Filtration membrane (filter), is composed of three layers:

1. Fenestrated capillary (glomerular) endothelium: capillary pores prevent passage of blood cells
2. Visceral membrane made of podocytes (branching epithelium); basement membrane restricts all but smallest proteins
3. Intervening basement membrane; podocyte's pedicles (filtration slits) provide no restriction in passage of molecules

• Glomerular Filtration Rate (GFR) - total amount of filtrate formed per minute by the kidneys
• Factors influencing GFR are
  • total surface area available for filtration
  • filtration membrane permeability
  • net filtration pressure (blood hydrostatic pressure - (blood osmotic pressure + capsular hydrostatic pressure))
  • Osmotic pressure - pressure resulting from the movement of water through a membrane and against its concentration gradient (i.e. the process of osmosis)
  • Hydrostatic pressure - pressure exerted to counteract the process of osmosis
    
  • Note: (1) albumins have large influence on blood osmotic pressure and (2) overal pressure within tubule is relatively low!
    
• Movment throught nephron tubule must be regulated
• If a high GFR occurs – then body would lose needed nutrients/chemicals because of membrane transporters not moving material out of tubule fast enough or not at all… What are some physiological examples and problems that may arise?
  
• If a low GFR occurs – then body would retain waste (i.e., can leak back into peritubular capillary)… What are some physiological examples and problems that may arise?
• Therefore, too slow or too fast has phsyiological repercussions
• Regulation of glomerular filtration (influence on blood pressure) Diagram
  • renal autoregulation (myogenic mechanism and JG apparatus)
  • Myogenic control of afferent arteriole
  • Increase in BP and increase in GFR causes a loss of nutrients which stimulate myogenic cells constrict afferent arteriole
  • Decrease in BP and decrease in GFR causes waste accumulates which signal myogenic cells to relax and cause the afferent arteriole to dilate 
  • JG Apparatus
  • Juxta-glomerular cell regulation (mechanoreceptors that detect vessel stretch)
  • Decrease in BP and decrease in GFR causes a release in rennin 
  • Renin-Angiotensin mechanism (sodium retention) increases BP and GFR
  • Macula densa cell regulation (osmoreceptors that sense sodium concentration)
  • Increase in BP and increase in GFR causes loss of sodium
  • GFR is too high will stimulate macula densa cells to release a vasopressor to cause afferent constriction as well as JG cell inhibition
  • neural controls (sympathetic nervous system can cause afferent arteriole constriction, deliver less blood into glomerulus and decrease GFR) Diagram
  • hormonal - renin-angiotensin mechanism and antidiuretic hormone
    

C. Tubular Reabsorption

• Tubular reabsorption - hormonally controlled transepithelium process in proximal tubules where water, nutrients, and ions are reclaimed
  
• Two processes
  • Active tubular reabsorption - diffusion by ATP-dependent carrier; transport of glucose, amino acids, and vitamins
    • there is cotransportation of molecules
    • molecules being carried depends upon another molecule's (e.g. Na) transport across basolateral membrane
  • Passive tubular reabsorption - diffusion, facilitated diffusion, and osmosis; substance moves along electrochemical gradients without the use of energy
    • sodium movement establishes osmotic gradient
    • water moves by osmosis into peritubular capillaries
    • causes obligatory water reabsorption
• Nonreabsorbed substances due to:
  • a lack of carriers
  • not lipid soluble
  • too large to pass through plasma membrane pores of tubular cells
  • examples: urea, creatine, and uric acid
• Reabsorption by region: 
  • PCT Diagram
    • 65% sodium ions and water 
    • 100% glucose, lactate, amino acids, vitamin
    • 90% bicarbonate ions
    • 50% chloride ions
    • 55% potassium ions
    • calcium, phosphate, and magnesium ion (hormonally controlled)
  • Loop of Henle Diagram
    • 25% sodium ions
    • 10% water
    • 35% chloride
    • 30% potassium
  • DCT (NOTE: not all that remains in DCT will be reabsorbed - review ADH, Aldosterone, and ANP) Diagram
    • 10% sodium ions
    • 10% chloride ions
    • 25 % water

D. Tubular Secretion Diagram

• Hydrogen and potassium ions, creatine, ammonia, and other substances move from blood of peritubular capillaries through the tubule cells into the filtrate; therefore urine is composed of filtered and secreted substances
• Urine characteristics Diagram
• Tubular secretion is important for

1. disposing of substances not already in the filtrate
2. eliminating undesirable substances (urea & uric acid) that have been reabsorbed by passive processes
3. ridding the body of excessive ions
4. controlling pH of blood

III. Ureters, Urinary Bladder, Urethra Diagram

A. Ureters

• Convey urine from kidneys to bladder; composed of three layers:

1. inner mucosa (covering lumen) composed of transitional epithelium and lamina propria
2. muscularis, composed of circular and longitudinal smooth muscle
3. adventitia, composed of fibrous connective tissue

• As urine enters the renal pelvis, peristaltic waves initiated there force urine into the ureter, distending it
• Distension of ureter stimulates its contraction, which propels urine into the bladder

B. Urinary Bladder

• Muscular sac located on pelvic floor
• Contains openings for urethra and ureters as well as triangular region outlying opening (trigone)
• Bladder consists of three layers:

1. mucosa with transitional epithelium
2. muscular layer (detrusor muscle) with longitudinal and circular muscles
3. fibrous adventitia

• Bladder is distensible -bladder collapses into a pyramidal shape, if full then it is pear-shaped

C. Urethra

• Thin-walled muscular tube drains urine from the bladder floor and conveys out of the body; composed mainly of pseudostratified columnar epithelium
• Two muscles control the flow of urine:
  • internal (involuntary) urethral sphincter (smooth muscle) controls leakage between voidings
  • external (voluntary) urethral sphincter (skeletal muscle) controls voluntary constriction of the urethra
    

IV. Clinical

• Glomerulonephritis - inflammation or scarring of nephrons
• Goodpasture syndrome - autoimmune disease -- antibodies attack lungs and kidneys
• Urination abnormalities Diagram
• Kidney stones - calcium, struvite, uric acid, cystine
• Neurogenic bladder - nerves don't work properly in messaging when to relax or contract
• Polycystic Kidney Disease (PKD) - genetic disorder characterized by growth of numerous cysts filled with fluid in the kidneys
• Urethritis, cystitis (bladder), pyelonephritis