Chapter 24: The Urinary System

function of kidneys

1. main excretory organ

2. maintains body’s internal environment

   1. total water volume

   2. total solute concentration in water

   3. ion concentration in ECF

3. long-term acid-base balance

4. excreting metabolic wastes, toxins, drugs

5. produces EPO and renin

6. activates vitamin D

ureters

1. transports urine

2. kidneys to bladder

urinary bladder

temporary storage

urethra

transports urine out

renal cortex

granular-appearing superficial region

renal medulla

deep to cortex, composed of cone-shaped medullary (renal) pyramids

renal pelvis

1. funnel-shaped tube continuous with ureter

2. minor calyces

3. major calyces

minor calyces

1. cup-shaped areas

2. collects urine from pyramidal papillae

major calcyes

1. collect urine from minor calyces

2. empty into renal pelvis

urine flow

1. renal pyramid

2. minor calyx

3. major calyx

4. renal pelvis

5. ureter

blood

1. kidneys cleanse blood

2. adjust its composition

3. rich renal blood supply

renal arteries

deliver 25% of CO/min to kidneys

nephrons

1. structural and functional units that form urine

2. less than 1 million per kidney

2 main parts of nephrons

1. renal corpuscle

2. renal tubule

glomerulus

1. capillaries that have fenestrated endothelium

2. allows for efficient filtrate formation

   1. plasma-derived fluid that tubules process to form urine

glomerular capsule (Bowman’s)

1. surrounds glomerulus

2. consists of 2 layers

   1. parietal layer

   2. visceral layer

histology of parietal layer of glomerular capsule

simple squamous

branching endothelium podocytes

1. extensions terminate in foot processes that cling to basement membrane

2. filtration slits between foot processes allow filtrate to pass into capsular space

renal tubule and collecting duct

1. consists of single layer of epithelial cells

2. 3 major parts

   1. proximal convoluted tubule

   2. nephron loop

   3. distal convoluted tubule drains into collecting duct

proximal convoluted tubule (PCT)

1. cuboidal cells with dense microvilli

2. functions in reabsorption and secretion

3. confined to cortex

nephron loop (loop of Henle)

1. U-shaped structure consists of two limbs

2. descending limb

3. ascending limb

descending limb

1. distal portion also called descending thin limb

2. simple squamous

ascending limb

1. thick ascending limb

2. cuboidal or columnar cells

distal convoluted tubule (DCT)

1. cuboidal cells with very few microvilli

2. functions more in secretion than reabsorption

3. confined to cortex

2 cell types of collecting ducts

1. principal cells

2. intercalated cells

principal cells

1. sparse with short microvilli

2. maintain water and Na+ balance

intercalated cells

1. cuboidal cells with abundant microvilli

2. help maintain acid-base balance of blood

collecting duct

1. receive filtrate from many nephrons

2. run through medullary pyramids

   1. striped appearance

3. ducts fuse, deliver urine through papillae into minor calyces

classes of nephrons

1. cortical nephrons

2. juxtamedullary nephrons

cortical nephrons

1. make up 85% of nephrons

2. almost entirely in cortex

3. peritubular capillaries

juxtamedullary nephrons

1. long nephron loops invade medulla

2. important in production of concentrated urine

3. vasa recta

glomerulus vasculature

1. capillaries are specialized for filtration

   1. fed and drained by arteriole

2. afferent arteriole enters glomerulus

3. efferent arteriole leaves and feeds into either the peritubular capillaries or vasa recta

BP in glomerulus

1. high

2. afferent arterioles are larger in diameter than efferent arteriole

peritubular capillaries

1. low-pressure, porous capillaries

   1. adapted for absorption of water and solutes

2. cling to adj. renal tubules in cortex that empty into venules

vasa recta

1. vessels parallel to long nephron loops of juxtamedullary nephrons

2. function in formation of concentrated urine

juxtaglomerular complex (JGC)

1. in each nephron

2. important in regulating

   1. rate of filtrate formation

   2. BP

components of JGC

1. macula densa

2. granular cells

3. extraglomerular mesangial cells

macula densa

1. ascending limb

2. chemoreceptors

   1. sense NaCl content of filtrate

granular cells

1. smooth muscle

2. mechanoreceptors

3. sense BP in afferent arteriole

4. secretory granules that contain renin enzyme

extraglomerular mesangial cells

1. located between arteriole and tubule cells

   1. gap junctions

2. pass signals between macula densa and granular cells

physiology of kidney

1. kidneys filter body’s entire plasma volume 60x a day

2. consume 20-25% of O2 used by body at rest

3. filtrate = blood plasma - proteins

4. urine is produced from filtrate

urine

1. less than 1% of og filtrate

2. metabolic wastes and unneeded substances

3 processes involved in urine formation and adjustment of blood composition

1. glomerular filtration

2. tubular reabsorption

3. tubular secretion

glomerular filtration

produces cell and protein-free filtrate

tubular reabsorption

selectively returns 99% to bloof

tubular secretion

selectively moves substances from blood to filtrate

glomerular filtration

1. passive process

2. hydrostatic pressure forces fluids/solutes through filtration mem into glomerular capsule

   1. no reabsorption into capillaries of glomerulus occurs

3 components of filtration membrane

1. fenestrated endothelium

2. basement membrane

3. foot processes of podocytes

fenestrated endothelium

glomerular capillaries

basement membrane

fused basal laminae of other 2 layers

foot processes of podocytes

filtration slits

filtration membrane function

separates the blood in the glomerular capillaries from the filtrate in the glomerular capsule

outward pressures that promote filtration

1. hydrostatic pressure in glomerular capillaries (HP_gc)

   1. glomerular BP

   2. force pushing water, solutes out of blood

   3. 55 mm Hg—high

tubular reabsorption by transcellular route

1. transport across the apical mem

2. diffusion through the cytosol

3. transport across basolateral meme

4. movement through interstitial fluid and into capillary

tubular reabsorption by paracellular route

1. movement through TJs

2. movement through interstitial fluid and into capillary

sodium transport across basolateral mem

1. Na+ most abundant cation in filtrate

2. transport of Na+ across basolateral mem of tubule cell via primary active transport

passive tubular reabsorption of water

Water is reabsorbed by osmosis by aquaporins

obligatory water reabsorption

aquaporins are always present in PCT

faculative water reabsorption

aquaporins are inserted in collecting ducts if ADH is present

Tubular Reabsorption of Water and
Nutrients Uses Active and Passive
Transport

1. at basolateral mem, Na+ is pumped into interstitial space by Na-K ATPase. active Na+ transport creates CGs that drive

2. downhill Na+ entry at the apical mem

3. reabsorption of organic nutrients and certain ions by cotransport at the apical mem

4. reabsorption of water by osmosis through aquaporins. water reabsorption increases the concentration of the solutes that are left behind. these solutes can be reabsorbed as they move down their gradients

   1. lipid-soluble substances diffuse by the transcellular route

   2. various ions and urea diffuse by the paracellular route

proximal convoluted tubule

1. site of most reabsorption

2. all nutrients such as glucose and AA are reabsorbed

3. 65% of Na+ and water reabsorbed

4. many ions

nephron loop components

1. descending limb

2. ascending limb

descending limb

water can leave, solutes cannot

ascending limb

water cannot leave, solutes can

distal convoluted tubule and collecting duct

reabsorption is hormonally regulated in these areas

ADH

1. release by posterior pit

2. causes principal cells of collecting ducts to insert aquaporins in apical mems

3. increases water reabsorption

aldosterone

1. targets collecting ducts (principal cells) and distal DCT

2. synthesis of apical Na+ and K+ channels

3. synthesis of basolateral Na-K-ATPase for Na+ reabsorption (water follows)

4. little Na+ leaves body

5. increases BP

ANP

1. reduces blood Na+

2. decreased blood volume and blood pressure

3. released by cardiac atrial cells if blood volume or pressure elevated

parathryoid hormone

acts on DCT to increase Ca2+ reabsorption

urine composition

1. 95% water and 5% solutes

2. urea

3. uric acid

4. creatinine

urea

1. largest solute component

2. AA breakdown

urea cycle

1. ammonia into urea

2. liver

uric acid

1. nucleic acids

2. liver

creatinine

1. metabolite of creatine phosphate

2. transported to skeletal muscles

3. quick energy source

4. breakdown product

5. filtered in kidneys

6. used to measure kidney function

ions

1. Na+, K+, PO4, SO4, Ca2+, Mg2+, HCO3-

2. abnormally high concentrations of any constituent, or blood proteins, WBCs indicates pathology

tubular secretion function

1. reverse reabsorption

2. PCT

3. moved from peritubular capillaries through tubule cells into filtrate

tubular secretion importance

1. disposing drugs

2. eliminating urea and uric acid

3. ridding body of excess K+ (aldosterone effect)

4. control blood pH by altering amounts of H+ or HCO3- in urine

regulation of urine concentration and volume

1. countercurrent multipler creates gradient

2. countercurrent exchanger preserves gradient

3. collecting ducts use gradient to vary [urine]

what creates the medullary osmotic gradient

long nephron loops of juxtamedullary nephrons

what preserves the medullary osmotic gradient

vasa recta—countercurrent exchangers

what adjusts urine osmolarity

collecting ducts of all nephrons

3 properties of the countercurrent multiplier of the nephron loop to establish the osmotic gradient

1. filtrate flows in the opposite direction (countercurrent) through 2 adj parallel sections of a nephron loop

2. descending limb

3. ascending limb

positive feedback cycle

1. salt is pumped out of the ascending limb

2. increase in intestinal fluid osmolarity

3. water leaves the descending limb

4. increase in osmolality of filtrate in descending limb

5. increase of filtrate entering the ascending limb

medullary osmotic gradient

1. filtrate entering the nephron loop is isosmotic to both blood plasma and cortical interstitial fluid

2. water moves out of the filtrate in the descending limb down its osmotic gradient. concentrates the filtrate

3. filtrate reaches its highest concentration at the bend of the loop

4. Na+ and Cl- are pumped out of the filtrate. this increases the IF osmolality

5. filtrate is at its most dilute as it leaves the nephron loop

vasa recta permeability

permeable to water and solutes

countercurrent exchange

occur between each section of the vasa recta and its surrounding fluid

diuretics

1. chemicals that enhance urinary output

2. ADH inhibitors, such as alcohol

Na+ reabsorption inhibitors

1. caffeine

2. drugs for hypertension or edema

urinary bladder anatomy

1. muscular sac for temporary storage of urine

2. muscular layer: thick smooth detrusor muscle

3. collapses when empty, rugae appear

4. expands and rises superiorly during filling

urethra anatomy

muscular tube that drains urinary bladder

internal sphincter of urethra

1. involuntary smooth muscle

2. bladder-urethra junction

3. contracts to open

external sphincter

1. voluntary skeletal muscle

2. surrounds urethra

micturition

urination

distension of bladder activates…

1. stretch receptors

2. contraction of detrusor by ANS

3. opening of internal urethral sphincter by ANS

4. opening of exteral urethral sphincter by somatic ANS

how does the vasa recta act as a countercurrent exchanger

reabsorbs water and solutes into general circulation without undoing the osmotic gradient created by the countercurrent multiplier

overhydration produces

large volume of dilute urine

dehydration produces

small volume of concentrated urine

ADH and urea

ADH increases urea recycling