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