5 excretion

8/13/2019 5 Excretion

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Human Biology 1 Excretion

A. Excretion Notes

1. Definitions

Excretion - release of metabolic wastes and excess

water

Defecation (or elimination) - the release of unabsorbed

wastes (e.g., feces) from the digestive tract

2. Components

a. Liver

-excretes bile pigments-bile pigments derived from heme portion of

hemoglobin, and incorporated into the bile here

b. Skin

-perspiration (solution of water, salt, and urea)- helps to maintain body temperature, by cooling

(evaporation)

- may also rid body of excess ureac. Large Intestines

-excretes Calcium and Iron salts-defecation

d. Kidney

- ammonia, a toxic by-product of metabolism ofproteins is converted to urea in the liver, which isexcreted here

-hypothalamus regulates function with hormones- ADH raises the osmotic concentration within

kidney, and so promotes the reabsorption ofwater

- Aldosterone stimulates sodium ion reabsorptione. Lungs

- CO2, and water3. Key Definitions

a. Kidney

- the organ the filters the blood (600 L/day) toremove nitrogenous wastes and regulates the

balance of water and solutes in the blood plasma.

- Cortex

Text Diagram:Kidney

Generally speaking,o Hormones aresecreted.o Wastes are excreted.

and

o Bile issecretedand/orexcreteddepending on yourpoint of view.

Intake of water

o Drink 1500ml/dayo Food 1000ml/dayo

Metabolism 200-350ml/day

Excretion of water

o Lungs 500ml/dayo Skin 400ml/dayo Feces 200ml/dayo Urine 1600ml/day

ErythropoietinThe kidneysmake this hormone, whichstimulates the bone (red) marrow tomake blood cells, in response to lowlevels of O2in the blood.


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Excretion


- Outer Medulla- Inner Medulla- Renal Pelvis

- Renal Circulation (abridged pathway)

Aorta !Renal artery !Afferent arteriole!

Glomerulus (capillaries)!Efferent arteriole !

Peritubular capillaries (and vasa recta)!Renal vein!Inferior vena cava

b. Ureters

- the tubes (2) carrying urine from the kidneys to thebladder

c. Bladder

- stores urine, prior to eliminationd. Urethra

- the tube carrying urine from the bladder to the exteriorof the body


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Excretion


4. The Nephron

- the functional unit of the kidney; one of numeroustubules (about a million) involved in filtration andselective reabsorption of blood. Each one consists of aBowmans capsule, an enclosed glomerulus, and arenal tubule

Arterioles

- Afferent arteriole

Text Diagram:Nephron

Adrenal Gland

- Adrenalin medulla- Aldosterone - cortex


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Excretion


- Juxtaglomerular apparatus- leads into Glomerulus

- Efferent arteriole- exits glomerulus- Tubular excretion (secretion)- Peritubular capillaries!venule!vein

Peritubular Capillaries

- takes up reabsorbed material, and drains into the Renalvein

- helps maintain concentration gradient in the medullafor reabsorption

Glomerulus (L. little ball)- a cluster of capillaries enclosed by the Bowmans

capsule.

Bowmans Capsule- the bulbous unit of the nephron, which surrounds the

glomerulus. The kidney works by forced filtration,

blood pressure driving blood plasma from theglomerular capillaries into the Bowmans capsule,

after which it passes through thenephron, where most

water and ions are reabsorbed into the bloodstream

and the residue is excreted as urine- filtrate

Glomerular (Pressure) Filtration of Blood

- blood plasma is forced, under pressure, out of theglomerular capillaries into the Bowmans capsule,

through which it enters the renal tubule; the filtrate

contains water and ions (which are recovered) andmetabolic wastes (which are eliminated as urine), but

not red blood cells or large proteins, which are too

large to pass through the glomerular capillary wall.

Proximal Convoluted Tubule (PCT)

- Tubular (Selective) Reabsorptiono Glucose (100%), a.a. (100%), bicarbonate

(80-90%), water (65%), Na+

(65%)

Distal Convoluted Tubule (DCT)- Tubular Secretion (Excretion)

o H+, K+, creatine, ammonia, uric acid,penicillin, estrogen/progesterone

o Many organic acids and bases (either end-products of metabolism or exogenous)

Juxtaglomerular apparatus

- is located between the DCTand the afferent ar teriole

- note the variation in thisdiagramit is actually a morerealistic image than thesplayed open image generally

used to label the parts on theprovincial exam, and seen onthe previous page.


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Excretion


o Anions:" Endogenous: Urate, Oxalate" Exogenous: Penicillin, Salicylates

o Cations:" Creatinine Quinine, Dopamine, Adrenaline /

Epinephrine Isoproterenol

-Aldosterone, increased reabsorbtion of Na+(excretionof K+) and water along the ascending limb and arch)

Collecting Duct (CD, CT)

- ADH, increased permeability to water, results inincreased water reabsorption

- Aldosterone, increased permeability to salt (Na+),results in increased salt (Na+) reabsorption

o Side affect increased reabsorption of watero Increased excretion of K+in urineo Co-transport of Cl-, HCO3-, H+.

- drains into the Renal Pelvis!ureter!bladder!urethra

5. How The Kidney Works- The human kidney achieves a high degree of water

reabsorption by using the salts and urea in the

glomerular filtrate to increase osmotic concentration of

the kidney tissue. This facilitates the movement ofwater from the filtrate out into surrounding tissue,

where it is collected by blood vessels impermeable tothe high urea concentration but permeable to water.

- as such human urine may be as much as 4.2 times asconcentrated as blood plasma, dessert animals such as

the gerbil are even higher, 14 times.

- the kidney uses the hairpin loop of Henle to set up acountercurrent flow. The longer the loop the greater

the water reabsorption. The countercurrent processesinvolves the passage of two solutes across the

membrane of the loop: salt (NaCl) and urea.

- basically then the kidney is divided into two zones:1.The outer portion, cortex, contains the upper

portion of the loop including the upper ascending

arm where reabsorption of salt from the filtrate byactive transport occurs

Tubular SecretionThis is an essential mechanism forremoving unwanted substances from theplasma. Substances such as H+, K+,

creatine, NH4+, and certain organic acids

move from the blood of the peritubularcapillaries through the tubule cells, or

from the tubule cells themselves into thefiltrate. Tubular secretion is essential for:

o Removal of substances notalready in the filtrate, such ascertain drugs;

o Removing unwanted substancesor metabolic waste productssuch as urea and uric acid,which have been reabsorbed by

passive processes;

o Removal of excessive K+; ando Controlling blood pH.

If blood becomes acidic, the renal tubule

cells actively secrete H+(at DCT) intothe filtrate and retain more HCO3

-(at

PCT), and K+. By contrast, if bloodbecomes alkaline, Cl-is reabsorbed andmore HCO3

-leaves the body in the urine.


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Excretion


2.The inner portion, medulla, contains both the lower

portion of the loop and the bottom of the collecting

duct, which is permeable to urea

- the active reabsorption of salt in the cortex drives theprocess. Salt reabsorption from the filtrate of one arm

of the loop establishes a gradient of salt concentration,

with concentration higher in the medulla at the bottomof the loop. It is the high salt concentration that raises

the total osmotic concentration so high that water

passes by osmosis out the collecting duct.

6. Nitrogenous Wastes

- Deamination occurs in the liver- enzymes break down amino acids by removing

the amino group(NH2), and combine with H+

ions to form ammonia(NH3); (the remainder of

the amino acid is converted to sugar or lipid.)

- Ammoniais toxic to all cells, therefore it must beremoved from the body

- but even low concentrations can kill cells,therefore it is necessary to transport it in very low

concentrations

-

Humans and most mammals detoxify ammoniabyconverting it to urea in the liverwhich is far lesstoxic, and can be transported at far higherconcentrations

- the urea is carried by the blood stream to thekidneys, where it is excretedas the principalcomponent of urine

7. The Kidney As Regulator Organ- The Kidney helps regulate the composition of the

blood

-and therefore also internal body chemistry

-by selectively removing substances from the blood,it can control concentrations of ions and other

chemicals

- while most amino acids are retained in the kidneys,almost half of the urea entering the blood is


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Excretion


eliminated- glucose not normally eliminated, is lost by

diabetics

- concentrations of H+, Na+, K+, Cl-, Mg++, Ca++,and HCO3-are maintained

-this then serves to maintain a constant blood pH

8. Hormones and Homeostasis- The kidney is concerned with homeostasis

-regulation is controlled by the central nervoussystem

- voluntary, autonomic, and hormonal controls- the body requires that the osmotic concentrationof

blood be maintained within a narrow margin-for this reason it is not always desirable for your

body to retain the same amount of water

- affects ion concentration- e.g., if there is too much salt, the kidney

can,i. dilute it by reabsorbing more water by

producing more ADH

ii. reabsorb less salt by decreasing theproduction of Aldosterone

a. Water Balance (Osmoregulation)

- Antidiuretic hormone (ADH) aka vasopressinregulates the volume of water excreted by the

kidneys.- ADH increases the permeability of the distal

convoluted tubuleand the collecting ductto

water.

- ADH is produced in hypothalamusanddescends along nerve fibres to the posterior

pituitary, where it is stored for subsequent- release

- Its secretion is regulated by the osmolality of thebody fluids and the blood volumeandpressure.

- Changes in body fluid osmolality of a fewpercent are sufficient to significantly alter

ADH secretion.

Review: What is Albumin?

- made in the liver, a key plasmaproteinresponsible formaintaining the osmotic pressure

in the blood.- cf. Digestive system notes

Hormones:

- Antidiuretic Hormone (ADH)which stimulates passive waterreabsorption

- Aldosteronewhich stimulates activesodium reabsorption


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- Decreases in blood volumeandpressureof 10% to 15% or more are

needed to effect ADH secretion.

- The blood volume and pressure sensors are found inthe large pulmonary vessels, the carotid sinus, and the

aortic arch. These baroreceptors (stretchreceptors) respond to stretch of the vessel wall,

which in turn is dependent on blood volume andpressure.

If Too Much Water:

- osmoreceptors in the hypothalamus detect thedecrease in blood solute concentration (osmolality)

and lessen the hypothalamus output of ADH

- baroreceptors/stretch receptorsin the pulmonary vessels, carotid and aortic arch

detect expansion of the vessel with increasedreabsorption of watersignaling a decrease inADH production

If Too Little Water:

- ADH release is increased- by high blood osmolalityaffecting

hypothalamic osmoreceptors and

- by low blood volumeaffecting thebaroreceptors/stretch receptors;

- low osmolality and high blood volumeinhibit ADH release

- ADH causes walls of collecting ductstobecome more permeable to water and thus

permits osmolar equilibration and absorption

of water into the hypertonicmedulla;- a small volume of highly concentrated

(hypertonic) urine is excretedADHmakes the walls of the DCT andcollecting tubule more permeable towaterso that more water will bereabsorbed and less will be excretedwith urine.

Alcoholinhibits ADH, decreasing the

reabsorption of water, and filling thebladder faster, resulting in

Diuresis, the increased production of

urine by the kidneyleading to

increased micturition/ voiding/urination aka #1 or peeing.

Osmolality:- measure of the solute

concentration.

High osmolality means more solute, andless solvent (water).

Low osmolality means less solute, andmore solvent (water).

So why is the medulla hypertonic?

of aldosterone (see notes below)

- DCT and CD are madepermeable to Na+ which floodthe medulla


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Source: VPL Science Librarian

b. Salt Balance

- for various reasons thesalt levelsin your bloodmust not vary widely; although intake may vary

- when sodium ion levels drop in association withlow blood volume(low blood pressure), the

hormone Renin stimulates the adrenal cortex

increases production of aldosterone

- a steroid hormone that stimulates activesodium (and water) reabsorption

- via the:- DCT (ascending limb and arch)and CD

- Juxtaglomerular apparatus contact between theafferent arteriole and DCT

- when blood volume (and blood P) drops,- detected bystretch receptors

Aldosteronecauses Na+to be

reabsorbed at the DCT (~4%) slightly

more than at the CD (~3%).- Drawing water, HCO3-(of

neg. charge, as with Cl-)- And excreting K+

- Na+is actively reabsorbed atthe PCT, DCT and CD

- Na+ is passivley reabsorbed atthe ascending loop of Henle


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(osmorecepotors) in the Juxtaglomerularapparatus i.e., reduced stretch ofJuxtaglomerular cells

- and glomerular (pressure) filtration decreases theJuxtaglomerular apparatus secretes Renin

-

Renin- (changes angiotensinogen [a plasma protein fromthe liver] into angiotensin Iwhich then becomesangiotensin II, a vasocontrictor that also)

- stimulates the adrenal cortex to secretealdosterone

- The reabsorption of Na+is followed by thereabsorption of water. Therefore, blood volume andblood pressure increase.

- Byproduct is the co-transport of Cl-, HCO3-, H+, andK+(although, generally K+are more likely to flowingin the opposite direction, excreted with the urinebecause of aldosterone).

- Na+have a positive charge, attracting negativelycharged Cl-. Thus, the Cl-follow the Na+out of theLoop and into surrounding tissue.

- In the transport of Na+/K+ (pump), 3 Na+move outof the tubule (are reabsorbed) for every 2 K+thatgets pulled into the tubule (are excreted).

- Because of the (hypertonic) concentration gradient setup by aldosterone, water, will also be reabsorbed (cf.

ADH)

Source: VPL Science Libarian

- Aside:- ANH (Atrial Natriuretic Hormone)

- akaANP(Atrial Natriuretic Peptide)- Source:Atria (of heart)- Stimulus:cardiac cells stretched due to

increased blood volume

Filtered LoadFactors ThatStimulate

Factors ThatInhibit

Reabsorbed(%)

Reabsorption Reabsorption

Proximaltubule

67Angiotensin IISympathetic nerves Dopamine

Loop of Henle 25 Sympathetic nerves

Distal tubule ~4 Aldosterone

Collecting

duct

~3 AldosteroneAtrial natriuretic

peptide (ANP)

The kidney is also responsible for

activating vitamin D3.

A critical anti-cancer vitamin.

Vitamin D (cholecalciferol) can eitherbe ingested with food or made from 7-

dehdrocholesterol by the action ofultraviolet light (sit out in the sun andmake vitamin D). The liver convertscholecalciferol into 25-hydroxycholecalciferol and the proximaltubule cells of the kidney convert 25-hydroxycholecalciferol into 1,25-dihydroxycholecalciferol. This is the

active form of vitamin D3. Vitamin D isvery important to dentistsbecause(along with parathyroid hormone, PTHand calcitonin) it controls many aspectsof calcium metabolism in the wholebody. (Do not confuse regulation ofcalcium metabolism by vitamin D etc.

with the role of calcium as anintracellular second messenger. Thereare indeed overlaps between theseprocesses, but tread carefully.) VitaminD is involved in stimulating theabsorption of calcium from the diet, inlaying down of calcium into the bone

and, most importantly (at least from thepoint of view of a dentist) in themineralisation processes involved indentinogenesis and amelogenesis. If youdon't have enough vitamin D as a child

you get rickets (bendy bones) and badteeth. The traditional illustration of this

is in Victorian child mineworkers whorarely saw the sun (and probably had apoor diet as well).

- source: Pete Smith


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Excretion


Source: VPL Science Librarian

- Fcn: inhibits Renin, therefore promotes theexcretion of Na+ (natriuresis),

- which in turn promotes the excretion of water(and blood P and volume decrease).

Renin-Angiotensin-AldosteroneSystem


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Excretion


+ efferent arterioles !peritubular capillaries!(venule) !ultimately to the Renal Vein

+ below the Inner medulla feeding the Ureter is theRenal Pelvis

Glomerular Filtration Tubular Reabsorption Tubular Secretion (Excretion)

Ammonia, uric

acid, H+, penicillin,

creatine


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Key Hormones (Simplified)

Hormone Summary

Source - Hypothalamus, but stored and released from the posteriorpituitary gland

Stimulus - Secretion in response to reduced plasma volumeis activated bypressure receptors in the veins, atria, and carotids. (too little water)

- Secretion in response to increases in plasma osmotic pressureismediated by osmoreceptors in the hypothalamus (too many

solutes)

also

- Angiotensin II (cf. Renin) may stimulate the secretion of ADH

Antidiuretic

Hormone (ADH)

aka Vasopressin*

Function - Constricts arterioles (increased BP)- Lowers heart rate (associated drop in body temp.)- Increased reabsorption of water

Source - Adrenal cortexStimulus - Presence of Renin(due to sodium deficiency);

- Decreased BP(as detected by stretch receptorsin the atria of theheart)

Aldosterone

Function - reabsorbs Na+,(and indirectly water)-

excretes K

+

,- raises blood pressureSource - juxtaglomerular apparatus(between glomerulus and DCT)Stimulus - Low blood volume

- Decreased salt (NaCl)Renin

Function Renin activates the renin-angiotensin system by cleavingangiotensinogen, produced by the liver, to yield angiotensin I, which is

further converted into angiotensin II by ACE, the angiotensin-

converting enzyme primarily within the capillaries of the lungs.

Angiotensin II then constricts blood vessels,

- increases the secretion of ADH and aldosterone, and- stimulates the hypothalamus to activate the thirst reflex, leading to

increased blood pressure.

* This is much more complex hormone, potentially having many more functions in the body not directlyrelated to the excretory system.

What gets Reabsorbed & WhereTubule Segment Substance Reabsorbed Mechanism

Na+ Active transport

Virtually all nutrients (glucose,,a.a. , vitamins)

Active transport (cotransportwith Na+)

Anions (Cl-,HCO3-) Passive transport(cotransportwith Na+ for HCO3- by activetransport)

Cations (K+, Mg2+, Ca2+) Passive transport

Water OsmosisUrea and lipid-soluble solutes Passive diffusion (side effect

of gradient created bymovement of water)

PCT

- Tubular Reabsorption- Selective Reabsorption

Small proteins Endocytosis (digested to a.a.within tubule cells)


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Loop of HenleDescending Loop Water Osmosis

Salt (Na+, Cl-) Passive transport

Salt (Na+, K+, Cl-) Active transportAscending Loop

Ca2+, Mg2+ Passive transportNa+ Active transport (Aldosterone)Ca2+ Active transport (PTH)

Cl- Diffusion,some co-transportw/ Na+ (active)

DCT

Water Osmosis (ADH)Na+, Active transport (Aldosterone)Cl-, HCO3- Passive transport (side effect of

gradient created byaldosterone)

K+, Cl-, H+, HCO3- Co-tansport w/ Na+ (active)Water Osmosis (ADH)

Collecting Duct

Urea Facilitated diffusion (mostremains in the medulla)

Source: Human Anatomy and Physiology 6ed., E. N. Marieb

Some Percentages of Reabsorption

PCT Loop DCT CD

Water 65% 10-20% ADH & Aldosterone impact

HCO3- 80-90% * *

Glucose 100% - - -

Affected by hormone levels

o Aldosterone = ~ rest of 7-9%(but more K+is secreted)o ANH = ~0Na+ 65% 25%(ascending)4% (greater) 3% (lesser)

Cl- 50% 35%~14%(both active & passive; as

NaCl)

K+ 55% 30% * (~8%)

a.a. 100% - - -

H+ (secreted) - (secreted) *

NH4+ (secreted) - (secreted)

Some Drugs (secreted) - (secreted) -Urea ? of 53% - (secreted) ? of 53%

* Can be reabsorbed or secreted depending on what is required to maintain blood pH


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Summary of Nephron Function

Source: Human Anatomy and Physiology 6ed., E. N. Marieb

SummarySource: Pete Smith (http://www.liv.ac.uk/~petesmif/teaching/notetoc.htm)

In order to be an efficient blood scrubber, the kidney has got to come into contact with a lot ofblood. It does. The kidney receives 25% of cardiac output, which is about 1200 ml/min or over1700 L/day. Every day the kidney filters 180 L of plasma into the urine and, on an average day,reabsorbs 99.4% of it, leaving a daily urine production of about 1L

Overall, the kidney is fantastically good at both

Substance

(mM)filtered secreted excreted reabsorbed %reabsorbed

Sodium 26,000 150 25,850 99.4Potassium 600 50 90 560 93.3

Chloride 18,000 150 17,850 99.2Bicarbonate 4,900 0 4,900 100Urea 870 410 460 53

Glucose 800 0 800 100Total solute 54,000 100 700 53,400 87


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(mOsm)Water (L) 180 1 179 99.4

Each day the kidney filters and reabsorbs an incredible 1.5kg of salt dissolved in 180 L of water.

The final stage of urine production is storage in the bladder and micturition.

Summary of ProcessesSource: http://www.bioeng.auckland.ac.nz/physiome/ontologies/urinary/tissues.php

Glomerular FiltrationIs a passive, non-selective process, where fluids and solutes are driven through a membraneunder hydrostatic pressure. All molecules, with the exception of high molecular mass proteins,are filtered out of the blood flowing through the glomerulus, into the glomerular capsule of therenal tubule.

Tubular Reabsorption

Of the ~125 ml of plasma filtered by the glomeruli, 124 ml is reabsorbed during passage throughthe renal tubules. Tubular reabsorption is a transepithelial process carried out in the PT, loop ofHenle, DCT and the collecting ducts. Water, ions and nutrients are reabsorbed either passivelyalong their electrochemical gradients, by simple diffusion, facilitated diffusion, and osmosis, orthey are actively reabsorbed via cotransporters. Cotransporters often couple the free energyreleased by the energetically favourable movement of Na+ along its electrochemical gradient tothe transport of substances such as amino acids and glucose against their electrochemicalgradients.

To some degree, the entire length of the renal tubule is involved in reabsorption, however, thecells of different regions of the renal tubule are adapted to perform specific transport functions,and consequently, the absorptive capacities of the different regions of the renal tubule differ. Theproximal tubule is the site of most reabsorption.

Tubular SecretionThis is an essential mechanism for removing unwanted substances from the plasma. Substancessuch as H

+, K

+, creatine, NH4

+, and certain organic acids move from the blood of the peritubular

capillaries through the tubule cells, or from the tubule cells themselves into the filtrate. With theexception of K+(which is mainly secreted from the distal tubule and collecting duct), theproximal tubule is the main site of secretion. Tubular secretion is essential for:

o Removal of substances not already in the filtrate, such as certain drugs;o Removing unwanted substances or metabolic waste products such as urea and uric acid,

which have been reabsorbed by passive processes;

o Removal of excessive K+; and


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o Controlling blood pH.If blood becomes acidic, the renal tubule cells actively secrete H

+into the filtrate and retain more

HCO3-, and K

+. By contrast, if blood becomes alkaline, Cl

-is reabsorbed and more HCO3

-leaves

the body in the urine.

5 excretion

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