BIOL 320 Exam 4

3 functions of the urinary system

1. filtration

2. reabsorption

3. secretion

4 components of urine

1. water

2. ions

3. urea (NH₃)

4. toxic substances

What volume of blood do the kidneys filter per day?

150-200 L/day

What produces renin?

kidneys

What does renin do?

released when blood pressure, blood volume, or sodium levels are low

initiates the renin-angiotensin-aldosterone mechanism

Describe the renin-angiotensin-aldosterone mechanism

1. renin converts angiotensinogen (liver) to angiotensin I

2. ACE converts angiotensin I to angiotensin II

3. angiotensin II causes:

   1. vasoconstriction

   2. aldosterone release

   3. increase in BP

What are the 6 major ions that the kidneys regulate?

1. sodium (Na⁺)

2. potassium (K⁺)

3. chlorine (Cl^-)

4. calcium (Ca²⁺)

5. bicarbonate (HCO₃^-)

6. hydrogen phosphate (HPO₄^2-)

What secretes erythropoietin (EPO) and what does this hormone do?

kindneys; signals bone marrow to make more RBCs

What is the kidneys’ role in vitamin D synthesis?

the kidneys convert inactive vitamin D (calcidiol) from the liver into active vitamin D (calcitriol)

What is the function of calcitriol?

active form of vitamin D

increases Ca²⁺ reabsorption

Where are the kidneys located?

retroperitoneal space

What is the renal capsule?

fibrous connective tissue that surrounds each kidney and prevents infection

Purpose of adipose tissue around the kidneys?

engulfs renal capsule and cushions

What is the renal fascia?

outer layer around capsule of collagenous connective tissue

• collagen to anchor kidneys to abdomen wall

What is the renal hilus?

where the renal artery and nerves enter, and renal vein and ureters exit

opens into renal sinus filled with fat and loose connective tissue

What are renal columns?

part of cortical tissue between renal pyramids that extends into medulla

What are renal pyramids?

part of medulla

bases project into cortex as medullary rays

apex of pyramid is renal papilla (points toward sinus)

What are medullary rays?

bundles of straight kidney tubules in the cortex that help move filtrate toward the medulla

What is a minor calyx?

papillae extend into funnel of minor calyx

What is a major calyx?

converge to form renal pelvis

What is the renal pelvis?

enlarged chamber connected to ureter

What does the ureter connect?

exits at the hilus of the kidney and connects to urinary bladder

What is the path of urine?

1. DCT

2. collecting duct

3. renal papilla

4. minor calyx

5. major calyx

6. pelvis

7. ureter

8. bladder

9. urethra

What are the 2 types of nephrons?

1. juxtamedullary nephrons (15%)

2. cortical nephrons (85%)

Describe juxtamedullary nephrons

located in the renal corpuscle near cortical-medullary border

nephron loop extends deep into medulla

Describe cortical nephrons

located in the renal corpuscle nearer to the periphery of cortex

nephron loops are more shallow

What is the glomerular (Bowman’s) capsule?

outer parietal and visceral layers housing the glomerulus

Describe the parietal layer of the glomerular capsule

outer layer

simple squamous epithelium

Describe the visceral layer of the glomerular capsule

inner layer wrapping capillaries

composed of podocytes

What is the glomerulus?

network of capillaries

blood enters through afferent (enter) arterioles and leaves through efferent arterioles (away)

What are fenestrae?

window-like openings in endothelial cells of glomerular capillaries

What are filtration slits?

gaps between cell processes of podocytes (visceral layer of glomerular capsule)

What is the filtration membrane?

capillary endothelium (fenestrae), basement membrane, and podocytes (filtration slits)

1st stage of urine formation occurs here

fluid from blood in capillaries moves across filtration membrane into lumen of Bowman’s capsule

3 characteristics promoting filtration in the renal corpuscle?

1. porous capillaries — fenestrae

   1. large proteins and blood cells cannot pass

2. porous visceral layer — filtration slits between podocytes

3. high pressure — capillaries have high pressure because afferent (enter) arterioles are wider than efferent (away) arterioles

What is the juxtaglomerular apparatus?

specialized structure near glomerulus and site of renin production

What are juxtaglomerular (JG) cells?

smooth muscle cells in wall of afferent (enter) arteriole

serve as baroreceptors and measure stretch of afferent arteriole

store and release renin

Function of renin?

secreted by JG cells in afferent arterioles of nephrons in the kidneys in response to low blood pressure

increases blood pressure

What are macula densa (MD) cells?

specialized cells of the DCT positioned between afferent and efferent arterioles

sense [NaCl]

• low NaCl → promotes renin secretion

• high NaCl → inhibits renin secretion

What are mesangial cells?

pass signals between macula densa (MD) cells and juxtaglomerular (JG) cells

also play a role in clearing debris & responding to injury

What is the proximal convoluted tubule (PCT)?

simple cuboidal epithelium with microvilli

bulk reabsorption (water, nutrients, ions)

What is the nephron loop (loop of henle)?

descending and ascending limbs

creates concentration gradient

What is the distal convoluted tubule (DCT)?

shorter than PCT

simple cuboidal epithelium with few microvilli

fine-tunes ions (hormone control)

secretion into urine

What are the collecting ducts (CDs)?

form where many DCTs come together

large diameter, simple cuboidal epithelium

form medullary rays (CDs central axis)

Flow of blood into kidney?

1. aorta

2. renal artery

3. segmental artery

4. interlobar artery

5. arcuate artery

6. cortical radiate artery

7. afferent arteriole

8. glomerulus

Flow of blood out of kidney?

1. glomerulus

2. efferent arteriole

3. vasa recta (peritubular capillaries)

4. cortical radiate vein

5. arcuate vein

6. interlobar vein

7. segmental vein

8. renal vein

9. IVC

Blood pressure in renal circulation?

declines as blood moves through circulation

relatively high in glomerulus so that blood enters

What plasma component does filtrate NOT contain?

protein

3 steps of urine production?

1. removal of filtrate from blood (glomerular filtration)

2. removal of keepers from filtrate (reabsorption)

3. secretion of waste into filtrate (secretion)

What is the renal fraction?

part of total cardiac output (CO) that passes through the kidneys

• avg. 21%

What is the glomerular filtration rate (GFR)?

amount of filtrate produced per min (~180 L/day)

avg. urine produced in a day is 1-2 L

Role of filtration membrane?

prevents blood cells and proteins from entering lumen of glomerular capsule

more permeable than a typical capillary

some albumin and small protein hormones may leak through, but they’re reabsorbed

What is net filtration pressure?

pressure gradient that forces fluid from glomerular capillary across membrane into lumen of glomerular capsule

NFP = HPgc - (OPgc + HPcs)

What is glomerular hydrostatic pressure (HPgc)?

pressure inside capillary is high, pushing fluid into Bowman’s capsule

What is capsule hydrostatic pressure (HPcs)?

pressure of filtrate in lumen pushing out

What is glomerular blood colloid osmostic pressure (OPgc)?

osmotic pressure caused by proteins, pushing fluid out of lumen and into capillary

3 reasons glomerular filtration is efficient

1. permeability

2. high blood pressure

3. high NFP

What happens if GFR is too high/low?

Too high: needed substances cannot be reabsorbed

Too low: everything is reabsorbed

3 GFR regulatory mechanisms?

1. renal autoregulation (intrinsic)

2. neural controls (extrinsic)

3. hormonal mechanism (extrinsic)

What is renal autoregulation?

changes in degree of constriction in afferent (enter) arterioles

What is the myogenic mechanism?

• afferent (enter) arterioles constrict as systemic BP increases and prevent increase in renal blood flow

  • protects against glomerular damage

• afferent (enter) arterioles dilate as systemic BP decreases

  • BP low, so blood can safely flow into glomerulus

What is tubuloglomerular feedback?

High GFR → fast flow → less time for reabsorption → more NaCl reaches macula densa → afferent arteriole constricts to lower GFR

TGF acts to prevent excessive NaCl loss

Sympathetic nervous system effect on GFR

during severe conditions, SNS prevails

• NE & E → vasoconstriction to decrease blood flow and GFR

• Renin secreted from JG cells → angiotensin II

  • Aldosterone

  • ADH

Under resting conditions, what controls GFR?

intrinsic controls (renal autoregulation)

What is tubular reabsorption?

selective transport of water and solutes from filtrate into blood to reclaim desirable substances

primarily occurs in the PCT

once carriers are saturated excess is not reabsorbed

What is not reabsorbed (urine)?

urea, uric acid, creatinine, potassium

3 structures that substances pass through for reabsorption in the PCT

1. apical membrane (outer) — contains microvilli

2. basolateral membrane (inner)

3. capillary endothelial cells

What controls Na+ and water reabsorption?

hormones

How does Na+ reabsorb?

active transport

drives reabsorption of other solutes and water

What part of the kidney is high in solutes?

medulla

Descending thin segment of the loop of henle reabsorption

highly permeable to water

moderately permeable to solutes

water: tubule → vasa recta

solutes: vasa recta → tubule

volume of filtrate reduced ~15%

Ascending thin segment of the loop of henle reabsorption

only permeable to solutes (not water); NaCl diffusion

solutes: tubule → vasa recta

What is the osmolality at the end of the nephron loop?

100 mOSm/kg in tubule

300 mOSm/kg in interstitial fluid

Ascending thick segment of the loop of henle reabsorption

impermeable to water and solutes, so cells have ATP-powered pumps and carrier proteins for transport

• Na+ uses active transport and establishes a concentration gradient

• K+ and Cl- are symported with Na+ (facilitated diffusion)

result: dilute filtrate

DCT and Collecting Duct reabsorption

ADH dictates osmosis of water into and out of DCT and collecting duct

K+ and H+ reabsorbed

Ca2+ (PTH)

Na+ (aldosterone and ANP)

Function of parathyroid hormone (PTH)?

increases Ca²⁺ reabsorption in the DCT

Function of aldosterone?

increases Na⁺ and water (via aquaporins) reabsorption (↑BP, ↓[solute])

• osmoreceptors in hypothalamus

• baroreceptors in atria

Function of atrial natriuretic peptide (ANP)?

decreases Na⁺ reabsorption, leading to more urine formation

Nephron permeability to urea?

poorly permeable

only 40-60% passively reabsorbed

What is tubular secretion?

kidneys do a “second sweep” of the blood to dump extra solutes into the urine that weren’t filtered the first time

detoxifies blood, controls pH, and regulates K⁺ levels

can be active (H⁺, K⁺, penicillin) or passive (ammonia)

Why is urine dilute by default?

collecting ducts are impermeable to water, so it’s not reabsorbed from blood → tubule

3 urine control mechanisms

1. countercurrent mechanisms

2. medullary concentration gradient

3. hormonal mechanisms

What are countercurrent mechanisms?

fluid in separate structures flow in opposite directions & materials may be exchanges between fluids as they pass

What is the countercurrent multiplier?

in nephron loop; responsible for high solute concentration in medulla

• descending limb: water leaves and concentrates filtrate

• ascending limb: solutes leaves and concentrate IF

What is the countercurrent exchanger?

in vasa recta; maintains high solute concentration in IF of medulla

blood flows down into medulla then back up to cortex

• flow is slow and pressure is low → vasa recta equilibrium w/ IF

What is the medullary concentration gradient?

IF of medulla has higher solute concentration than that of the cortex

Compare the molality of cortex vs. medulla

cortex = 300 mOsm/kg

medulla (tip of pyramid) = 1200 mOsm/kg

What 2 things maintain the medullary concentration gradient?

1. countercurrent mechanisms

2. urea recycling

What is urea recycling?

passive diffusion of urea (NH3) from collecting ducts into IF of medulla

the ascending limb is impermeable to urea (and water)

What might result from insufficient ADH secretion?

diabetes insipidus: increased dilute urine production → dehydration + thirst

What is the trigone?

triangular interior of urinary bladder between entry of ureters and exit of urethra

How much can the bladder hold when it streches?

~1 L

What is the micturition reflex?

activated when urinary bladder is stretched

parasympathetic APs cause detrusor to contract

What is insensible perspiration?

water lost through the skin; regulates heat loss

What is sensible perspiration?

water lost as sweat through the sweat glands; increases with heat and exercise

What is the blood buffer system?

H₂CO₃ = HCO₃^- + H⁺

When does H⁺ secretion increase?

↓pH

↑aldosterone

Scrotum

2-chambered sac that contains testes

What muscles help regulate temperature of the testes?

dartos and cremaster

Testes

exocrine — sperm cell secretion

endocrine — testosterone

Tunica vaginalis

serous membrane