Lecture 4

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3 forms of nitrogenous waste

ammonia, uric acid, urea

advantages of ammonia excretion

deamination of aa's & requires little energy to produce

disadvantages of ammonia excretion

highly toxic, requires lots of energy to store and excrete

uric acid excretion advantages and disadvantages

adv - low toxicity, excreted w/ small amt of water; energetically costly though

urea excretion adv and disadv

adv - only slightly toxic, relatively inexpensive to produce, but perturbing solute (high conc affect the cell via protein denaturation)

how much water is needed to eliminate 1 g N2 as ammonia? How much energy used?

0.5 L; less than 1 ATP/mol ammonia

how much water is needed to eliminate 1g N2 as urea? How much energy?

0.05L; 2.5 ATP/mol of N excretion as urea

how much water is needed to eliminate 1 g N2 as uric acid? How much energy?

0.001 L ; 1.75 ATP/mol of N as uric acid, even though it is less than urea, the uric acid pellets include many proteins, so those rep an indirect cost of uricotelism

what are some environmental and ecological constraints that determine animal excretory strategy?

water availability, dietary needs, metabolic costs

describe the ornithine-urea cycle

used for ureoteles to produce urea; CPSI and CPSIII for ureotelic animals; 1 usess NH4+, 3 uses glutamin, once carbamoyl phosphate is produce, it enters the ornithine-urea cycle

which hormones stimulate the OU cycle enzymes?

glucagon and glucocorticoids

which hormones inhibit the OU cycle enzymes?

insulin

T/F urea syntehsis is regulated by enzyme quantity and allosteric regulation

T; allosteric regulator N-acetyl glutamate; when aa levels are high, elevation in glutamate levels increases the activity of enzyme N-acetyl glutatmate synthetase

T/F sharks use uric acid to increase tissue osmolarity

F - they use urea- helps them prevent water loss in marine env and urea's perturbing effects are counteracted by TMAO; by relying on counteracting solutes, sharks can maintain the conc of inorganic ions (perturbing solutes) at low levels

why are sharks able to accumulate large amounts of urea?

b/c they have solutes that counteract the perturbing effects of urea (TMAO)

T/F type of nitrogen compound excreted is unrelated to animal's environment

F - of course it's related! Aquatic animals - ammonia; terrestrial - either urea of uric acid

Name some examples of animals that can change their mode of nitrogen excretion in response to water availability

lake magadi tilapia, gulf toadfish, african lungfish and some early developmental stages of several ammoniotelic species

lak magadi tilapia

live in water w/ pH so high that gill cannot excrete nh3, so must excrete as urea instead

gulf toadfish

can convert to ureotelism when it moves to crowded conditions; urea stored in blood and is released once/twice daily in short pulses across gill following the insertion of urea transporters into gill epithelia; probably done to reduce the risk of local fouling of water; also, many animals use nh3 as a cue to detect prey, so urea production can be used to confuse predators;

african lungfish

water levels decrease, so they have to burrow into the mud and form a mucus cocoon; cannot excrete ammonia, so must induce the expression of urea cycle enzymes and glutamine sytnhase to convert to ureotelism

how are some early developmental stages of several ammoniotelic species ureotelic?

most amphibians excrete ammonia as larva but urea as adults; other transitions are b/c of env conditions - dehydration causes some lunfish to convert from ammoniotelism -> ureotelism

T/F most animals maintain ion and water balance using some form of internal organ

T

what are the roles of vertebrate kidneys in homeostasis?

ion balance, osmotic balance, bp, pH balance, excretion of metabolic wastes and toxins, hormone production

what are the layers of the kidney?

outer cortex, inner medulla

what is the functional unit of the kidney composed of?

(nephron) - composed of renal tubule (where urine is adapted and changed), and vasculature which includes glomerulus and capillary beds surrounding renal tubule

what is the glomerulus composed of?

its a ball of capillaries surrounded by bowman's capsule

primary urine

aqueous solution is introduced into kidney's tubules (filtrate)

definitive urine

solution is modified as it move through kidney tubules before it is eliminated; ruled by epithelial cells lining renal tubule

four processes of urine production

filtration, reabsorption, secretion, excretion

filtration

filtrate of blood formed at glomerulus

reabsorption

specific molecules in filtrate is removed - ions & specific molecules, as well as H2O

secretion

specific molecules added to filtrate

excretion

urine is excreted from body

how is primary urine introduced into the kidney tubules?

by ultrafiltration - pressure driven mass flow

when the liquid components are filtered into bowman's capsule, what is filtered and what isnt"

water and small solutes cross the glomerular wall and blood cells and large macromolecules unfiltered

T/F glomerular capillaries are not very leaky

F - they are aka fenestrated capillaries; formed by podocytes w/ foot processes

what do mesangial cells control?

bp and filtration w/i glomerulus - they widen/tighten the capillaries to affect flow rate

GFR

glomerular filtration rate - amt of filtrate produced/min

what determines GFR

pressure across glomerular wall; controlled primarily by factors that affect the net glomerular filtration pressure - the balance of forces acting on fluids on either side of filter

3 main forces of GFR

glomerular capillary hydrostatic p, bowman's capsule hydrostatic p & oncotic p (osmotic p due to protein conc in blood)

3 intrinsic pathways for regulating GFR

myogenic regulation, tubuloglomerular feedback, mesangial control

myogenic regulation

used to intrinsically regulate GFR - constrict/dilation of afferent arteriole - widens/lowers space that's letting stuff out of capillary bed

tubouloglomerular feedback

juxtaglomerular apparatus - macula densa cells in distal tubule contact specialized juxtaglomerular cells in walls of aff arteriole. When high flow in distal tubule, tubule cells signal aff arteriole, causing vasoconstrciton, a decrease in hdrostatic P causing vasoconstriction - decrease in hydrostatic p, decreasing GFR

mesangial control

alter the permeability of glomerulus by increasing GFR

oncotic pressure

1 force of GFR - due to proteins that remain in the plasma, opposes movement into the lumen

t/F the oncotic pressure gradient opposes movement inot the lumen

T

how do regulate GFR extrinsically?

hormones - vasopressin, renin-angiotensin-aldosteron pathway & atrial natriuretic peptide

the initial filtrate filtered in bowman's capsule is hypo/hyper/isosmotic to blood?

isosmotic

T/F most water and salt in primary urine reabsorbed using transport proteins and energy

T

what is the rate of reabsorption limited by?

#A of transporters

renal threshold

conc of a specific solute that will overwhelm reabsorptive cap

where does most reabsorption of solutes and water take place in kidney?

prox tubule; many solutes reabsorbed by Na+ cotransport, water follows thru osmosis

how is glc reabsorbed?

secondary active transport, coupled w/ Na+

what kind of molecules is secreted in the kidnyes? And where?

K+, Nh3+, H+, pharmaceuticals, water soluble vitamins in the distal tubule; requires transport proteins and energy

what is the function of the prox tubule?

site of most solute and water reabsorption

loop of henle functions

making urine concentrated; descending limb - water removal ; ascending limb - "single effect" of solute pumping; only solutes leave, but it drives the removal of H2O in descending limb

distal tubule fucntion

reabsorption completed for most solutes; reabsorbs most of the remaining Na+ and Cl-; capable of being affected by hormones; improtant for recovery of watervasopressin, secretion of K+

collecting duct fucntion

malleable region that drains multiple nephrons and carries urine to renal pelvis

how is distal tubule regulated?

by hormones - mediates regulation of uptake of solutes and water

which hormones are used for hormone mediated regulation of uptake of solutes and water in the distal tubule?

mineralocorticoids (aldosterone), vasopressin & PTH

where are principal cells located and what do they secrete and reabsorb?

collecting duct- secrete K+, reabsorb Na+; responsive to aldosterone

intercalated cells located ? What do they secrete?

collecting duct - secrete H+ or HCO3 depending on acid base balance of animal; convert H2o-> HCO3-; makes interstitial fluid and blood more alkaline, urine more acidic

T/F the descending limb is impermeable to water

F has increased aquaporins for water reabsorption and decreases the vol of the primary urine and also concentrates it

T/F the ascending limb is impermeable to water

T - only ions reabsorbed; primary urine becomes dilute

where do the reabsorbed ions from the loop of henle go?

to the interstitial fluid caused by an osmotic gradient created in the medulla

the single effect

even though the descending and ascending limbs have separate duties, the SINGLE effect is to remove water; in the desc limb, there is increasing osmolarity and aquaporins, so much water is able to be reabsorbed much more; there is a very high solute conc; ion pumps pull solutes to the int. fluid

how is water transported from hte lumen to the peritubular fluid in the loop of henle?

thru aquaporins

in the ascending limb, what kind of transporters are used?

NKCC, K chs on apical side to move it back to lumen; Na/K Atpase on basolateral, Cl/K electroneutral cotrasnporter on basolat side; whole thing driven by Na/K atpase

how does the loop of henle act as a countercurrent multiplier?

creates an osmotic gradient that facilitates reabsorption of water; low osmolarity near cortex, high osmolarity deep in medulla

what maintains the osmotic gradient by countercurrent exchanger?

vasa recta

what kind of hormones affect kidney function?

steroid - aldosterone (Na uptake) and peptide - vasopressin (water reuptake and instertion of aquaporins @ c duct and distal tubules)

what kind of dietary factors affect urine output?

diuretics - stimulate excretion of water and antidiuretics - reduce excretion of water

ADH

controls water excretion by increasing aquaporin concentration

mineralocorticoids (aldosterone)

conserve Na+ to increase water reuptake

natriuretic hormones (ANP; heart)

promotes Na+excretion to increase H2O loss; antag to mineralocoricoids

T/F vertebrates have both ADH and DH

F only ADH; some species of insects have DH (bloodsuckers)

what does the osmotic conc of final urine depend on?

the permeability of the collecting duct which can be regulated by vasopressin (AQUAPORINS) ; if impermeable, water won't be reabsorbed from CD, dilute urine will be excreted ~50 mosm vs. if permeable, water reabsorbed; conc urine excreted ~1200mosm

what kind of hormone is ADH/vasopressin?

peptide hormone; produced in hypothalamus, released by neurohypophysis

when is ADH released? Inhibited?

release is stimulated by increasing plasma osmolarity detected by osmoreceptors in the hypothalamus; inhibited by increasing bp detected by stretch receptors in atria and barorecptors in carotid and aortic bodies

mechanism of vasopressin action

vasopressin binds to G-protein-linked receptor, which activates adenylate cyclase, increasing cAMP, activating PKA; phosphorylation of cytoskeletal and vesicle proteins occurs, triggering translocation of vesicle to the cell membrane, w/ insertion of aquaporins

what kind of hormones control ion excretion

mineralocorticoids

in tetrapods, where are mineralocorticoids produced? And which type of mineralocorticoid is it?

produced in adrenal cortex; aldosteroid (steroid hormone) targets cells in distal tubule and collecting duct to stimulate Na reabsortption from urine, and enhances K+ excretion;

in fish, where are mineralocorticoids produce? What type are they?

interrenal tissue in fish; cortisol

what is the aldosterone mechanism?

aldosterone diffuses into cell, binds to receptor (t-factor); activated tfactor stimulatestranscription of genes for transporters; new transporter proteins are made in the ER and exported in vesicles ; vescles containing proteins are sent to the plasma membrane

which is faster, aldosterone or vasopressin?

vasopressin, (peptide hormone); aldosterone - more permanent

how is renin secretion controlled?

juxtaglomerular cells secrete it; controlled by baroreceptors in juxtaglomerular cells release it in response to low bp; sympathetic neurons in cardiovascular control center of medulla oblongatat trigger renin secretion in response to low bp; macula densa cells in distal tubule respond to decreases in flow by releasing a paracrine signal that induces juxtaglomerular cells to release renin

when is renin secreted?

secreted when bp or gfr is lower than normal

what does renin do to angiotensinogen?

cleaves it to angiotensin I; then ACE (angiotensin converting enzyme) on epithelia of bv's converst I to II

what is angiootensinogen?

an inactive protein in plasma

what does angiotensin II do?

vasoconstrictor (raises bp by increasing resistance) causes syntehsis and release of aldosterone from adrenal cortex; it is a steroid hormone that targets cells in distal tubule & CD; stimulates Na+ and water reabsorption from urine, enhances K excretion

what does the RAA pathway do?

helps regulate bp

T/F aldosterone increases Na+ and water retention; does it raise bp? if so, how?

T - raises bp by increasing bv

ANP is produced where?

in specialized cells w/i atria

when is ANP secreted?

in response to stretch associated w/ increase in bv

Does ANP decrease/increase urine output?

increases, so lowers bv and bp; antagonistic to RAA pathway; increases Na excretion in urine

does ANP increase/decrease GFR? how does it do this?

increases - by relaxing contractile (mesangial cells) that control size fo filtration slits of glomerulus

what detects and controls thirst?

hypothalamus; osmoreceptors monitor conc of body fluids; increased levels of angiotensin II = body requesting aldosteron /therefore thirsty

how do sponges expel cellular waste?

simple contractile vacuoles

what is the kidney equivalent for simple animals like worm taxa?

protonephridia - similar to tubule; fluids are taken from int space; mostly for freshwater orgs

what is the kidney equivalent for molluscs and annelids?

metanephridia - fluid is taken from blood or coelom

what do protonephridia have?

flame cells and solenocytes

what is a malphigian tubule? What types of orgs use it?

insect equivalent to vertebrate kidney that empties to hindgut

insects urine production 2 process

secretion-absorption system; no ultrafiltration; primary urine formed by secretion, reabsorption in hindgut mods primary urine

what kind of diruetic hormones are found in insect urine formation?

CRF related diuretic hormones (pep hormones), insect kinins and cardioacceleratory peptides

do decapod crustaceans produce dilute/conc urine?

very dilute

what kind of urine production systems do decapon crustaceans use?

filtration-reabsorptoion - high energy cost

do fish nephons have a loop of henle?

no

T/F freshwater fish - ions are reabsorbed from primary urine and excrete very dilute urine

T

T/F seawater fish - produce lots of urine, most ion, water and nitrogen excretion responsibilities met by gills and skin

F - produce small amts of urine; don't need distal convoluted tubules

is the GFR high/low in freshwater teleosts?

high w/ lots of nephrons and well developed glomeruli

T/F uric acid is actively secreted into urine (in reptiles & birds)

T

how does uric acid move to the lower intestine in reptiles & birds

reverse peristalsis

in birds & reptiles, supersaturated colloidal suspensions (uric acid production) is stabilized by _____-

proteins, then precipitated before excretion

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