A+P II Chp 25 (pt 2): Urinary System

What are 5 kidney functions?

1) removal of toxins, metabolic wastes, excess ions from blood

2) regulation of BV, chemicals, and pH

3) gluconeogenesis during fasting

4) endocrine functions: renin and erythropoietin

5) activation of vit D

How does each contribute to the endocrine function of the kidney?

a) renin

b) erythropoeitin

a) regulates BP

b) regulates RBC production

What does high resistance in afferent and efferent arterioles cause?

BP to decline from ~95 mmHg to ~8mmHg in kidneys

What is filtrate made up of?

blood plasma minus proteins

For urine,

a) how much of the total filtrate does it make up

b) what does it contain

a) <1%

b) metabolic wastes + unneeded substances

What are the 3 mechanisms of urine formation?

1) glomerular filtration: dumping everything

2) tubular reabsorption: removes stuff from urine (reclaiming in blood)

3) tubular secretion: adds stuff to urine

Glomerular filtration is a passive mechanical process driven by ____________________.

hydrostatic pressure

Why is the glomerulus a very efficient filter? (2 reasons)

1) its membrane is very permeable with a large SA

2) glomerular BP is higher than other capillaries

Molecules >5nm, like ______________, are not filtered and function to maintain ____________________ of the blood.

plasma proteins; colloid osmotic pressure

What is the net filtration pressure (NFP)?

the pressure responsible for filtrate formation (10 mmHg)

Which 3 forces determine NFP?

1) chief force: glomerular hydrostatic pressure (HPg)

2) opposing force: colloid osmotic pressure (OPg)

3) opposing force: capsular hydrostatic pressure (HPc)

What is the pressure of each force?

a) HPg

b) OPg

c) HPc

a) 55mmHg

b) 30mmHg

c) 15mmHg

What is the equation to determine NFP (and solve it)?

1) NFP = HPg - (OPg + HPc)

2) NFP = 55 - (30 + 15)

3) NFP = 10mmHg

What would happen if the glomerular hydrostatic pressure (HPg) were less than 55 mmHG?

the glomerulus would have trouble pushing blood/fluids (or can't push fluids at all if less than 45mmHg)

What is the glomerular filtration rate (GFR)?

volume of filtrate formed per minute by the kidneys

What 3 things is the GFR governed by (and directly proportional to - meaning if it's greater, then GFR is greater)?

1) total SA available for filtration

2) filtration membrane permeability

3) NFP

GFR is tightly controlled by which 2 types of mechanisms?

1) intrinsic controls (renal autoregulation): within the kidney

2) extrinsic controls: nervous and endocrine mechanisms

Intrinsic controls maintain a nearly constant GFR. What are the 2 types of renal autoregulation?

1) myogenic mechanism: regulated by muscles in blood vessels

2) tubuloglomerular feedback mechanism: senses changes in the JGA

Regarding the myogenic mechanism, what happens when...

a) BP increases

b) BP decreases

a) afferent arterioles constrict --> less blood flow --> lower GFR/NFP to normal

b) afferent arteriole dilate --> more blood flow --> raise GFR/NFP to normal

What happens in tubuloglomerular feedback mechanism?

macula densa senses the NaCl concentration of the filtrate + regulates the vasodilation/constriction of afferent arteriole

Regarding the tubuloglomerular feedback mechanism, what happens when...

a) NaCl conc is high

b) NaCl conc is low

a) more salt --> high GFR --> macula densa constricts afferent arteriole

b) less salt --> low GFR --> macula densa dilates afferent arteriole

What are the 2 mechanisms of extrinsic control?

1) neural mechanism: sympathetic control

2) hormonal mechanism: renin-angiotensin mechanism

Which mechanism (intrinsic or extrinsic) is GFR controlled by in each situation?

a) sympathetic NS is at rest

b) sympathetic NS is active/under stress

a) intrinsic controls: blood vessels dilated

b) extrinsic controls: blood vessels constrict

Under stress,

a) which 2 hormones are released

b) what do afferent arterioles do

a) norepinephrine + epinephrine (from adrenal medulla)

b) constrict and inhibit filtration

What happens if someone is under too much stress (such as having a surgery)?

too much constriction --> kidneys shut down --> dialysis needed to keep person alive until they're stable

What are the steps of the renin-angiotensin mechanism?

1) JG cells release renin

2) renin: angiotensinogen to angiotensin I

3) angiotensin converting enzyme (ACE): angiotensin I to angiotensin II

What 2 things does angiotensin II do?

1) causes systemic and glomerular hydrostatic pressure to rise

2) releases aldosterone

What are 3 methods of tubular reabsorption?

1) ATP: primary and secondary

2) passive transport: diffusion, facilitated diffusion, osmosis

3) water + ion reabsorption: controlled by hormones

What are the 2 routes of tubular reabsorption? (from lumen into blood)

1) transcellular: across cell

2) paracellular: between cells

What is the transcellular route? (4 steps)

1) apical membrane of tubule cells

2) cytosol of tubule cells

3) basolateral membrane of tubule cells

4) endothelium of peritubular capillaries

What is the paracellular route?

What are some molecules that take this route?

- between cells

- limited by tight junctions, but leaky in proximal nephron

- water and some ions get reabsorbed in the PCT

What are 3 things that get reabsorbed?

1) all nutrients: glucose + amino acids

2) ions

3) water

_____ is the most abundant cation in filtrate.

Na+

How is Na+ transported across the...

a) apical membrane

b) basolateral membrane

a) secondary active transport

b) primary transport via Na+-K+ ATPase pump: maintains sodium gradient

How are glucose, amino acids, ions, and vitamins reabsorbed by the kidneys? (4 steps)

1) sodium gradient created

2) low intracellular Na+

3) nutrients cotransported with Na+ into cell (secondary active transport)

4) facilitated diffusion out of cell to peritubular capillaries

How is water reabsorbed by the kidneys? (3 steps)

1) movement of Na+/other solutes creates osmotic gradient

2) goes through aquaporins: water-filled pores

3) water is reabsorped passively to peritubular capillaries

What are the 2 types of passive tubular water reabsorption and where does each occur?

1) obligatory water reabsorption: PCT

2) facultative water reabsorption: collecting ducts

Are aquaporins present in the...

a) PCT

b) collecting duct

a) always present

b) absent; inserted only when ADH is present (ex: during dehydration)

How are solutes reabsorbed by kidneys? (3 steps)

1) as water is reabsorbed, solute conc in filtrate increases

2) a concentration gradient is created for solutes

3) osmotic/solute drive: passive reabsorption to peritubular capillaries

How are other substances - such as fat-soluble substances, urea, lipid-soluble drugs - reabsorbed by the kidneys?

- passively

- they go down concentration gradients + follow water into peritubular capillaries

What is the transport maximum (Tm)? What is this due to?

- when no more of a particular substance can be reabsorbed

- transcellular proteins are specific and limited

When carriers (transport proteins) are saturated...

a) where does the excess go

b) give an example

a) urine

b) hyperglycemia: too much glucose in blood, so glucose remains in urine (no more can be reabsorbed)

The ______ is the site of most absorption in the nephron.

PCT

What are 4 things the PCT reabsorbs?

1) all nutrients: glucose and amino acids

2) 65% of Na+ and H2O

3) many ions

4) uric acid and urea

What are the reabsorptive capabilities of the...

a) descending limb

b) ascending limb

a) H2O leaves (reabsorbed), solutes stay

b) H2O stays, solutes leave (reabsorbed)

What does each segment of the ascending limb have?

a) thin segment

b) thick segment

a) passive Na+ movement

b) Na+-K+-Cl- symporter and Na+-H+ antiporter

The reabsorption of the DCT and collecting duct are ____________ regulated.

hormonally

Which 3 hormones regulate the DCT and collecting duct and what does each regulate?

1) antidiuretic hormone (ADH): water

2) aldosterone: sodium (therefore water)

3) PTH: calcium

Regarding the antidiuretic hormone (ADH),

a) what gland is it released by

b) what does it cause

c) what happens as ADH levels rise

a) posterior pituitary gland

b) principal cells of collecting ducts insert aquaporins in apical membranes: facultative water reabsorption

c) increased water reabsorption

Regarding aldosterone,

a) which parts of the nephron does it target

b) what does it promote

c) what are some of its functions

a) principal cells of collecting ducts + DCT

b) synthesis of apical sodium channels + basolateral Na+/K+ ATPases: Na+ and water reabsorption

c) increase Na+ and water, decrease K+

Regarding the atrial natriuretic peptide (ANP),

a) what releases it

b) what does it cause

a) cardiac atrial cells due to high BV/BP

b) less blood Na+: decreased BV/BP

The parathyroid hormone acts on _________ to increase __________ reabsorption.

DCT; Ca2+

Almost all tubular secretion occurs in the ___________.

PCT

What are some selected substances that are secreted?

K+, H+, NH4+, creatine, organic acids and bases

What is the pathway of tubular secretion? (3 steps)

1) peritubular capillaries

2) tubule cells: can synthesize substances like HCO3-

3) filtrate

What are 4 important functions of tubular secretion?

1) disposing drugs and metabolites

2) eliminating urea and uric acid

3) getting rid of excess K+

4) controlling blood pH

How does the nephron get rid of excess K+? (2 steps)

1) reabsorbed in PCT

2) secreted by aldosterone in DCT and collecting duct

What does the nephron do if...

a) pH decreases

b) pH increases

a) secrete H+ and reabsorb HCO3-

b) reabsorb H+ and secrete HCO3-