Download Loop of Henle

STRUCTURE & FUNCTION OF THE
RENAL SYSTEM
AND
RENAL
DIAGNOSTIC PROCEDURES
• Kidneys (Fig. 28-1)
– Paired, encapsulated
• At hilus
– Renal artery enters, renal vein exits
– Approx. 25% cardiac output to kidney
• Two main functions of kidney:
• Filtration
– Removes metabolic wastes
Esp. urea, other N- containing wastes
• Regulation
– Electrolytes
– Intravascular volume
– Blood pH
• Renal blood flow from cortex to medulla
– Medulla has high metabolic rate
• Prone to hypoxia if ischemia
– Medulla arranged in wedges (Fig.28-2)
• Renal pyramids
• Columns
– Between pyramids from cortex through medulla
• Apices
– Extend to:
• Minor calyces
– Cup-shaped cavities; join  major calyces 
– Renal pelvis
• Ureter (Fig.28-1)
– Tube from kidney that carries waste fluid to
the:
• Bladder
– Urine storage site; attached to urethra 
outside body
Nephron
• Anatomic unit of kidney function (Fig.28-3)
– Approx. 106 nephrons/kidney
– Two anatomical regions:
• Glomerulus
– Tuft of capillaries loop into circular capsule
– In cortex of kidney
– Blood filtration site
• Renal tubule
– Begins as end of glomerulus
– Traverses cortex, medulla
– Site where water, solutes reclaimed following filtration
• Renal tubule – cont’d
– Anatomical areas:
– Proximal, convoluted tubule
» Impt to reabsorption of water, electrolytes
– Loop of Henle
» Hairpin loop
» Impt to urine concentration
– Distal tubule
» Both straight and convoluted
» Impt to reabsorption in response to hormonal signals
– Collecting ducts
» Impt to Afine tuning@ of water, electrolytes
Renal Tubule Summary
• Anatomical areas:
– Proximal, convoluted tubule
• Impt to reabsorption of water, electrolytes
– Loop of Henle
• Hairpin loop
• Impt to urine concentration
– Distal tubule
• Both straight and convoluted
• Impt to reabsorption responding to hormonal signals
– Collecting ducts
• Impt to fine tuning of water, electrolytes
Renal corpuscle
• Bowman's capsule + glomerular
capillary tuft (Fig.28-6)
• Bowman's capsule
– Circular
– Space inside = Bowman’s space
– Narrows to form proximal tubule
• Glomerular capillaries arranged in loops
• Glomerular capillary membranes (Fig.28-6)
– Blood components from artery filtered here
– Glomerular capillary tuft -- three membrane
layers
• Different than most other capillaries in body
– Cells of membrane layers -- unique w/ unique
structures in each layer
• Inner capillary endothelium
• Basement membrane
• Epithelium
– Primary urine forms as result of filtration
• Glomerular filtration
– Glomerulus permeable to
• Water
• Electrolytes
• Small organic molecules (ex: glucose, urea,
creatinine)
– Not permeable to
• Rbc’s
• Wbc;s
• Molecules w/ MW > 70,000 (so most proteins)
– Leave glomerulus (still in blood) through the efferent
arteriole
– Size and charge of molecules impt to whether
molecule will be filtered or not
– Capillary pressures important (Fig.28-11,
Table 28-1)
• Hydrostatic Pressure forces fluids through filter
– What is the filter?
• Forces opposing BHP:
– Colloid osmotic P
» Due to blood cells, proteins, as in “regular”
capillaries
– Hydrostatic P of fluid already in Bowman=s space
• Net filtration P =
(forces favoring filtration) - (forces opposing
filtration)(Table28-1)
– Positive or negative net filtration in a healthy system?
– Glomerular filtration rate (GFR)
• Volume of plasma filtered at glomerulus per unit time
• Approx. 180 L/day = 120 mL/min
– BUT only 1-2 L/day excreted
» About 99% of filtrate reabs’d in tubule
• GFR depends on factors affecting fluid pressures in
nephron and vasculature
– Arteriolar resistance changes capillary hydrostatic P in
glomerulus
– Decr’d blood flow to kidney from systemic circulation decr’s GFR
– Obstruction to urine outflow may incr back pressure at Bowman’s
capsule, so decr GFR
– Loss of protein-free fluid alters COP, so alters net filtration
– Renal disease
Hormonal Regulation of GFR
• Through renin
•  changes in renal blood flow, so
changes in GFR
– Occur and regulated locally (at kidney
tissue) and systemically
• Two impt hormones affect GFR:
– Aldosterone (regulates Na+)
– ADH (regulates water)
• Specialized cells sensitive to specific
hormones
– Juxtaglomerular aparatus
• Juxtaglomerular cells
– Lie around afferent arteriole
• Macula densa
– Portion of renal distal tubule
» Loops back up toward Bowman’s capsule
» Butts up against glomerulus
– Located in space between afferent and efferent
arterioles
– Together juxtaglomerular cells + macula
densa cells (or juxtaglomerular apparatus)
control:
• Blood flow into nephron at glomerulus, so
– Glomerular filtration
• Renal secretion
– Because sensitive to hormones such as
aldosterone and ADH
Renal Tubule Physiology
• Extend from Bowman’s capsule
• Receive and process filtrate from capsule
• Processing of fluid by two major forces:
tubular reabsorption and secretion
– Reabsorption
• Occurs through minute pores in tubules
• Compounds reabsorbed from tubule  peritubular
capillaries (surround the tubules)  blood
– Secretion
• Opposite of reabsorption
• Compounds secreted go from blood capillaries
 renal tubule  filtrate
– By the end of the proximal tubule
• Water, Na+
– 60-70% reabsorbed back into blood
• K+, glucose, bicarb, Ca+2, amino acids, uric
acid
– Approx 90% reabsorbed back into blood
– Loop of Henle
• Mostly Na+, Cl-, H2O move to adjust concentration of fluid
in tubule
• Loop has ascending, descending regions
– Cells in some regions permeable to water
» Here water can move out of tubule
– Cells in other regions not permeable to water
» Here water trapped in the tubule
– Cells in some regions permeable to Na+
» Here Na+ can move out of the tubule
– Cells in other regions not permeable to Na+
» Here Na+ is trapped in the tubule
• Resulting fluid through loop goes from isotonic
to hypertonic to hypotonic
– Due to differences in kidney cells from cortex to
medulla and different permeabilities of tubule to
different molecules
– As the fluid leaves loop of Henle, it is hypotonic (so
dilute, compared to other body fluids)
– Distal tubule - final regulation of water, and
acid-base balance
• Water, Na+, bicarb
– Reabsorbed here when ADH present
• K+, urea, H+, ammonia
– Secreted
• Hormones important to regulation here
– ADH (antidiuretic hormone)
»  incr’d water reabsorption
– Aldosterone
»  incr’d Na+ reabsorption
– Parathyroid hormone (PTH)
»  incr’d Ca+2 reabsorption
– Collecting duct
• Final urine concentration adjusted
• Final pH balance adjusted
• Final product = urine low in volume
(compared to what was filtered), and
high in osmotic concentration (the body
rids itself of unwanted molecules)
Renal diagnostic procedures
(Table 28-3 p.804)
• Urinalysis
– Non-invasive, inexpensive
– Normal urine properties well-known, easily measured
• Specific gravity
–
–
–
–
Solute concentration in urine
Correlates w/ osmolality in normal urine
Normal value: 1.025-1.032
Usually describes ADH
• Because ADH controls water reabsorption
• What ADH problem might you expect if specific gravity were
high? Low?
• Urine sediment can be examined
microscopically
– Red blood cells
• Hematuria = large number rbcs in the urine
– Should be few/none
– Why? What keeps rbc’s in the body?
– Casts
• Precipitates from cells lining the renal tubules
– Red cells suggest tubule bleeding
– White cells suggest tubule inflammation
– Epithelial cells suggest degeneration, necrosis of
tubule cells
– Crystals
• May form as urine cools
• Observed microscopically
• Indications
– Inflammation
– Infection
– Stones
– White blood cells (pyuria)
• From urinary tract infection
– Bacteria
• Infection
• Clearance tests
– Determine how much of substance can be cleared from
blood by kidney per unit time
• Indirect measure of GFR, tubular reabs’n/secr’n, renal blood flow
– GFR
• Best estimate of overall kidney health
• Decreases w/ lost or damaged nephrons
• Use creatinine clinically
– Biochem prod’d by muscle, released to blood at constant rate
– Freely filtered at the glomerulus
– Neither reabs=d nor secr=d as it progresses through the tubule, so:
• Amount creatinine excreted = amount creatinine filtered
– Can be calculated:
» Amount creatinine in urine over time = (vol. urine/time) x
(urine concentration of creatinine)
• Blood tests
– Plasma creatinine concentration (Pcr)
• Normal levels = 0.7-1.5 mg/dL
• Use same biochem described for clearance test, but measured
in blood instead of in urine
• Again, creatinine filtered at glomerulus; neither reabs’d nor
secr’d; and amount creatinine filtered = amount excreted
• Get increased Pcr if chronic disease affecting GFR
– When GFR decr’d, get decr’d filtration of creatinine out of blood
(and into urine), so
– Blood (plasma) creatinine increases
• Useful:
– To monitor changes in chronic renal function
• BUT Pcr increases with trauma, muscle tissue breakdown
– Blood Urea Nitrogen (BUN)
• Urea prod’d constantly as cells metabolize proteins
• Measurement of BUN reflects both GFR and urine
concentrating ability
• Normal levels 10-20 mg/dL
• Urea filtered at glomerulus
• Urea reabsorbed back into blood at tubules
– If decr’d GFR, get incr’d BUN
– OR if decr’d blood flow to kidney, get incr’d BUN
– Incr’d BUN or Pcr = increase in nitrogenous
substances in blood = azotemia