18 - Excretion System

7/27/2019 18 - Excretion System

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Excretory SystemKK Chapter 14, Hildebrand Chapter 15


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Excretory System Functions

1) Maintain salt/water balance by releasing controlledamounts of both under the influence of the endocrinesystem (adrenal and pituitary glands).

2) Eliminate toxic wastes, especially of protein metabolism.De-amination of amino acids leads to NH4

+

(ammonium). This is readily excreted from the gills inmost fish as NH4,

+ but otherwise the liver may turn NH4+

into less toxic urea. In birds and reptiles, the kidneysturn NH4

+ into uric acid.


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The Nephron - the basic unit of the kidney

Renal corpuscle = renal (Bowmans) capsule and glomerulus

Nephron = renal corpuscle + tubule

KK 14.2, H&G 15.5

The renal corpuscle is

the filter of the kidney,

where filtrate passesfrom the blood-vascular

system to the capsule,

while the tubule

reclaims solutes from

the filtrate and conductsurine towards the

outside of the body.


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Development of the Kidney

The kidney tissue develops from thethe mesomere, which swells to create

a nephric ridge on the dorsal wall of

the coelom. It develops from front to

back, changing morphology as it does

so. The first segments of themesomere produce one pair of

glomeruli and nephrons per segment,

and this anterior first kidney is called

the pronephros. A branch of the

dorsal aorta in each segment leads to aglomerulus, and each is associated

with a capsule and tubule.

KK 14.3a, H&G 15.1


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Development of the Kidney 2KK 14.3b&c, H&G 15.1

The pronephros may have originally emptied

into the coelom, as it does in hagfishes, but

otherwise collects in a nephric duct that

transports the urine towards the cloaca. The

pronephros is a functional kidney in larval

cyclostomes, many fishes and Amphibia. Inamniotes, even in their embryonic stages, the

pronephros appears but is never a functional

kidney.

In teleosts, the pronephros

may persist and have a

glandular role. Physiologists

refer to it as head kidney.KK 14.3


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Vertebrate kidneys develop from from the

mesomere, front to back, beginning with

the pronephros. Behind the pronephros, a

second kind of kidney, the mesonephros,

develops. It has many corpuscles per

segment. It becomes the adult kidney innon-amniotes, and is then called the

opisthonephros(tail kidney).

In amniotes, a third kidney or

metanephros develops behind theembryonic mesonephros. It does not

share the archinephric duct but rather a

new duct emerges from the cloaca to meet

it. In the adult, this duct is called the

ureter rather than a metanephric duct.

Development

of the Kidney 3KK 14.4, H&G 15.2

KK 14.4


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Amniote embryo illustrating all 3 kidney types

KK 14.5, H&G 15.2

Note that the same

duct can be called the

pronephric duct, the

mesonephric duct,

opisthonephric duct,the archinephric duct,

or simply the nephric

duct. And, as weshall see, thats not all!


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Opisthonephric Kidney of a (Male) Salamander

KK 14.8

There are many renalcorpuscles per segment.

The anterior portion of

the male kidney is

intimately associated

with the testis, and acts

as a sperm storage

organ. They share the

same duct leading to

the cloaca.

KK 14.8


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Mammalian Kidney FunctionThe blood supply to the

glomerulus is from the renalartery. After leaving the

glomerulus, the blood travels

to more capillaries around the

nephric tubule that re-gain

needed water and solutes lost

to the capsule. If a renal

portal vein is present, it

contributes to these

capillaries.

Na+ ions are activelyremoved, and Cl- water

follows passively.

KK 14.14, H&G 15.8


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Mammalian Kidney Function 2The intermediate part of the tubule is a

site of active tranport of Na+

into thesurronding tissue, resulting in a very high

Na+ concentration in that tissue. Cl-

follows Na+ to maintain charge balance.

The high salt concentration in the tissue

causes much of the water still in the urine

to be reclaimed as it passes down the

collecting tubule.


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Human Metanephric KidneyThe human kidneys contain 2 x106 nephrons, whose capsules have 0.76 m2 of filter

area, or about 1/2 the surface area of the body. There is about 120 km of tubule.

They receive about 20% of the cardiac output, so about 1700 L d-1. About 10% or180 L d-1 passes into the glomerulus, or about 125 mL per minute.

The kidney re-adsorbs all

but 1.5 L d-1, achieving a

100x concentration of thefiltrate.

The corpuscles are in the

cortex of the kidney,

while the loops of Henle

are lined up parallel in the

medulla.

KK 14.1, H&G


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Bird KidneysBirds and mammals are the only vertebrates that can produce urine that has a salt

concentration higher than their blood. Birds also have loops in their nephric

tubules, but are thought to have evolved this independently.

The organization of the bird

kidney is very different from

that of mammals. The loops

are short peripherally, andlong centrally, in each

lobule.

KK 14.9


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Osmoregulation and Excretion in Vertebrates 1

Land animals always face a

problem of water loss, and mostdrink to replace that water.

Fish in freshwater tend to gain

water and must conserve salt.Fish

in salt water generally, sharksexcepted, tend to lose water also

as they are hyposmotic to sea

water. Some saltwater fish are

osmoconformers and allow their

salt content to vary so that they

remain isosmotic.

Relatively few fish move back

and forth between fresh and salt

water.

KK 14.12


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Osmoregulation and

excretion in different

vertebrates 1KK 14.10, H&G. 15.7Saltwater teleosts are hyposmotic,

and tend to lose water. The renal

corpuscle is small or absent, and a

minimum of water is lost as urine.

NH4+ or urea is lost through gills. Salt

gained in feeding is excreted via thegills.

Freshwater fish arehyperosmotic

to their environment, so tend to gain

water through their gills and in

feeding. They actively transport saltin through their gills. The renal

corpuscle is large, to produce copious

dilute urine. NH4+ is lost through

gills.

Sharks are slightly hyperosmotic to

seawater, which they achieve by

accumulating urea. The renal corpuscle is

large to eliminate water gained through gills

and feeding. Excess salt is excreted through

their rectal gland.


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Osmoregulation and excretion in

different vertebrates 2KK 14.10, H&G. 15.7

Birds and reptiles are water conservers. They have small

renal corpuscles, and reduce the need to produce urine by

converting NH4+ to uric acid. Birds, like mammals, have a

loop in the intermediate segment of the tubule.

Marine reptiles and birds are extreme water conservers.

When they drink water they get salt as well. They secretetheir excess salt through specialized glands of the eye,

nasal area or mouth.

Mammals are generally less effective at conserving water

than reptiles and birds. They secrete NH4+ as urea, which

is soluble, so they must produce urine. Loops of Henlehelp to minimize water loss. Marine mammals are unique

in being able to produce urine that is hyperosmotic to sea

water, so they can drink sea water without accumulating

salt.


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Summary of Pathways of Ammonia Elimination

in Vertebrates

KK 14.11

Fish and aquaticreptiles

Birds and reptiles

Sharks,

Sarcopterygii,

Amphibia,

mammals

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18 - Excretion System