Osmoregulation. Ionic and osmotic balance in multicellular organisms the interstitial fluid is the internal environment – composition resembles that of.

Osmoregulation

Ionic and osmotic balance• in multicellular organisms the interstitial fluid

is the internal environment – composition resembles that of the ancient sea: high Na+, low K+, low Ca++ and Mg++

• osmoregulatory organs maintain this environment isovolumic, isotonic, isoionic, isohydric, etc.

• further task: removal of poisonous end products of metabolism (mostly NH3 from proteins)

• marine invertebrates: osmotic pressure and ionic constitution in equilibrium with seawater

• marine vertebrates: ion concentration is one third of seawater except in hagfish (Eptatretus - Cyclostomata), MgSO4, Cl- much lower; in sharks-rays ion concentration is lower, but osmotic pressure is maintained by urea

• fresh water, terrestrial: ion concentration is one third; hyperosmotic to freshwater, hyposmotic to seawater

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Osmotic exchange• plasma membrane separates fluids with

different ionic composition, but equal osmotic pressure

• epithelium separates fluids that are different in both respects

• animals cannot isolate themselves from the environment: exchange of gases, absorption of nutritients; exception: Artemia salina

• obligate and regulated osmotic exchange• obligate exchange depends on physical

factors, that animals cannot readily regulate

• regulated exchange compensates for changes caused by the obligate

• only some parts of the epithelium participate in osmoregulation: gill, kidney, salt gland, enteric system

3/21

Obligate osmotic exchange• occurs through the skin, respiratory

epithelium and other epithelia in contact with environment

• influencing factors:– gradient: determines direction of exchange; a frog

sitting in a pond takes up water through the skin; a marine fish is loosing water in the sea, but gains NaCl; a freshwater fish is taking up water, but loosing salt through the gill

– surface: small animal – relatively larger surface, faster exchange, e.g. dehydration

– permeability:• transcellular and paracellular exchange (but: tight

junction);• skin of amphibians, gill of fish have high permeability• skin of reptiles, desert amphibians, birds, mammals are

much less permeable (leather containers), but mammals loose water through sweating

– eating, metabolism, excretes: metabolic water is very important for desert animals, but also for marine mammals (seals put on weight eating fish, but loose weight burning fat when is feeding on invertebrates)

– respiration: function of nose – condense water during exhalation – dropping nose in winter

4/21

Osmotic regulation I.

• most vertebrates are strict osmoregulators - exception: shark, ray and hagfish

• marine invertebrates are in equilibrium, other invertebrates, similar to vertebrates, are hyperosmotic in freshwater, hyposmotic in seawater

• some of the invertebrates is conformer, others are osmoregulators

• freshwater animals breathing from water– they are hyperosmotic: 200-300 mOsm;

freshwater in general below 50 mOsm – water inflow, salt outflow

– to compensate, they produce dilute urine, take up salt with food and through active transport (fish, frog), decrease the permeability of their skin

– they do not drink freshwater

5/21

• marine animals breathing from water– invertebrates are in equilibrium

– hagfish: Ca++, Mg++, SO42- regulation only

– shark-ray osmotic equilibrium due to urea accumulation, excess salt removed by rectal glands

– fish are loosing water, drinking seawater; excess salt removed through the gill (chloride cells)

• marine animals breathing air– loosing water through respiratory epithelium

and other epithelia– marine reptiles and birds are drinking seawater –

cannot produce strongly hyperosmotic urine (just as fish) – excess salt removed through salt glands

– marine mammals do not drink seawater, water is taken in with food and produced metabolically, urine is hyperosmotic

– lion seal males spend 3 months on the beach without eating or drinking; seal pups do similarly for 8-10 weeks while mother is on the sea

Osmotic regulation II.6/21

Osmotic regulation III.

• terrestrial animals breathing air– if freshwater is available then water lost

through breathing can be replenished by drinking, salt loss (urine, faces, sweating) compensated from food – sparing is important

– problem of shipwrecked sailors: kidney can remove 6 g Na+/l urine, seawater contains 12 g Na +/l – drinking of seawater leads to salt gain

– desert animals have two problems: heat and lack of water

– kangaroo rat: remains in cool burrow during daytime, active only during the night; gaining water metabolically, water condensation in nose

– camel: cannot hide in burrow; when dehydrated do not sweat, body temperature raises changing between 35-41° C; hyperosmotic or no urine, urea stored in tissues; faces dry

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Overview

body fluids compared to seawater

breathing environment group problem, solution

isosmotic seawater seawater invertebrates, hagfish, shark, ray

no problem

seawater seawater marine fish water loss, drinking seawater, salt excretion through gill

fresh water

fresh water freshwater fish water inflow, active salt accumulation, diluted urine, no drinking

marine reptile water loss, drinking seawater, salt gland

seawater

marine reptile salt intake (food), hyperosmotic urine, metabolic water, no drinking

amphibian water inflow, active salt accumulation through skin, diluted urine, no drinking

fresh water

reptile, mammal skin impermeable, water loss through breathing, drinking freshwater as needed

all groups water loss through breathing and skin, drinking freshwater as needed

desert animal metabolic water, hyperosmotic urine, dry faeces, behavior

1/3 osmotic pressure

air

air

marine bird drinking seawater, salt gland

8/21

Water compartments

• human body contains 60% water on average, differences between male-female, young-old

• distributed in different compartments• intracellularly 2/3, extracellularly 1/3• of the extracellular water: 3/4 interstitially,

1/4 in blood plasma• barriers and, transport rules are already

known• measurement of volumes applying dilution

principle: Evans-blue, inulin, tritiated water• homeostasis is very important . cholera,

diarrhea - dehydration, working by a furnace in tropical areas – water poisoning, severe burns – dehydration due to loss of skin

• the most important regulator in humans is the kidney, behavioral regulation is also important – metabolic water is limited

9/21

Human kidney• osmoregulatory organs always contain

transport epithelium (skin, gill, kidney, gut) : polarized - apical (luminal, mucosal) and basal (serosal) surfaces are different

• capacity of the transport epithelium is increased by its special structure: tubular organization

• functioning of the mammalian kidney is well known – though it does not represent all types of vertebrate kidneys

• 0,5% of body weight, 20-25% of cardiac output

• cortex, medulla, renal pyramid, renal pelvis, ureter, urinary bladder, urethra

• volume of urine is 1 l daily, slightly acidic (pH 6), composition, volume changes with the food and the requirements of the water homeostasis - beer, Amidazophen, etc.

10/21

The nephron

• functional unit of human kidney is the nephron

• afferent and efferent arterioles, in between glomerulus; Bowman capsule, proximal tubule, loop of Henle, distal tubule, collecting duct

• most of the nephrons (85%) are cortical, the rest juxtamedullary (15%) nephron

• steps in the formation of urine:– ultrafiltration– reabsorption– secretion

• the kidney is very important in pH regulation

• the kidney removes ammonia formed during the decomposition of proteins

11/21

Ultrafiltration• in the kidney 15-25% of water and solutes is

filtrated, 180 l daily – proteins and blood cells remain

• filtration depends on:– the hydrostatic pressure between the capillaries

and the lumen of the Bowman capsule: 55-15 = 40 mmHg

– the colloid osmotic pressure of the blood: 30 mmHg – effective filtration pressure 40-30 = 10 mmHg

– the permeability of the filter: fenestrated capillaries, basal membrane (collagen + negative glycoproteins), podocytes (filtration slits between pedicels)

• voluminous blood supply due to the relatively low resistance – afferent arteriole is thick and short – high pressure in the glomerulus

• regulation of the blood flow: basal miogenic tone, paracrine effect of juxtaglomerular apparatus, sympathetic effect (afferent arteriole, glomerulus, podocyte)

12/21

Clearance• clearance of a substance is the volume of

plasma that is completely cleaned from the given substance in the kidney in every minute

VU CP = VU that is C = ------ P

C - clearance, P – concentration in plasma,V – volume of urine in 1 minute, U –

concentration in urine

• clearance of a substance that is neither reabsorbed nor secreted (e.g. inulin) equals the glomerulus filtration rate : GFR

• clearance of a substance that is not only filtrated, but completely secreted as well (e.g. PAH) equals the renal plasma flow : RPF

• knowing the hematocrit, renal blood flow (RBF) can be calculated

13/21

Tubular reabsorption I.• 180 l primary filtrate is produced every

day, but only 1 l is excreted, of 1800 g filtrated NaCl only 10 g remains in the urine

• the process of reabsorption has been successfully examined since the 1920’s using the method of micropuncture

• role played by the subsequent sections of the tubules:– proximal tubule

• 70% of Na+ is reabsorbed by active transport, Cl- and water follow passively, obligate reabsorption

• filtrate is isosmotic, but concentration of substances that are not reabsorbed increases 4-fold

• on the apical membrane of epithelial cells microvilli• virtually all filtrated glucose and amino acids are

reabsorbed using Na+ dependent symporter• tubular maximum for glucose: below 1.8 mg/ml

complete reabsorption (normal value: 1.0 mg/ml), above 3.0 mg/ml linear increase – sugar in urine in diabetes

• Ca++, phosphate and other electrolytes are reabsorbed as needed – see later

14/21

Tubular reabsorption II.

– descending part of Henle’s loop• no microvilli, few mitochondria – no active

transport• low permeability for NaCl and urea, high for water

– thin ascending part of Henle’s loop• no microvilli, few mitochondria – no active

transport• low permeability for water, high for NaCl

– thick ascending part of Henle’s loop• active reabsorption of Na+ • low water permeability

– distal tubule• active reabsorption of Na+, and passive

reabsorption of water• K+, H+ and NH3 transport as needed – see later (pH

regulation)• transport is regulated by hormones – facultative

reabsorption

– collecting duct• active reabsorption of Na+ at the cortical part,

high urea permeability in the internal medullary part

• regulated water permeability (ADH)

15/21

Tubular secretion

• several substances are secreted from the plasma to the tubule in the nephron – best examined: different electrolytes (K+, H+, NH3) organic acids and bases

• active transport – recognizes substances conjugated with glucuronic acid in the liver

• K+ is reabsorbed in the proximal tubule and Henle’s loop (Na/2Cl/K transporter)

• if K+ concentration is too high, secretion in the distal tubule depending on aldosterone and coupled to Na+ reabsorption - K+ acts directly on aldosterone, Na+ through renin-angiotensin

• conflicting demands: insulin secretion is induced by high K+ - excess K+ is taken up by adipose tissue

• secretion of H+ and NH3 serves pH regulation

16/21

pH regulation I.• normal pH 7.4 – 7.35 acidosis, 7.45 alkalosis• normal functioning is possible between 7.0-

7.8• regulation: buffer systems, respiration, kidney• Henderson-Hasselbalch equation:

[A-]pH = pK + log ------

[HA]• for CO2 two equations; following

rearrangement:•

[HCO3-]

pH = pK’ + log ------ [αCO2]

- solubility of CO2

• pK’ = 6.08, i.e. not good buffer at normal pH – as CO2 and HCO3

- can be easily modified (respiration, kidney)

• plasma proteins (14-15% Hgb, 6-8% other) pK is same as blood pH – good buffers

• phosphate concentration low, negligible effect

17/21

pH regulation II.• respiratory alkalosis and acidosis: caused

by hyper-, or hypoventilation• metabolic alkalosis – e.g. Cl- loss because

of vomiting• metabolic acidosis – anaerobic energy

production, ketosis in diabetes mellitus• in the first case: kidney compensates, in

the second: breathing (short-term) and the kidney (long-term)

• proximal tubule, Henle’s loop: Na+/H+ exchanger, distal tubule, collecting duct: HCO3

- uptake through A-cells

• in distal tubule and collecting duct: HCO3-

secretion through B-cells • in acidosis HCO3

- level is low in the filtrate: NH3 secretion – binds to H+, NH4

+ cannot go back, H+ secretion increases

18/21

Hyperosmotic urine• birds and mammals can produce

hyperosmotic urine - water reabsorption in the collecting duct due to osmotic pressure differences

• common characteristic: Henle’s loop, the longer the loop, the more concentrated the urine – very long in kangaroo rat

• pressure difference is achieved through the counter-current principle

• Na+ transport in the ascending part of the Henle’s loop – do not enter the descending part, but attracts water leading to the same result

• in addition, urea present in high concentration because of the reabsorption of water, can only leave the tubule in the internal medulla

• osmotic pressure increases from the cortex to the medulla

• blood supply to the tubules (vasa recta) is running in parallel to the Henle’s loop , does not decrease the osmotic gradient

19/21

Regulation of the kidney• granular cells in the juxtaglomerular

apparatus produce renin in response to a decrease in blood pressure or NaCl delivery to the distal tubule

• renin cuts off angiotensin I (10 amino acids) from angiotensinogen (glycoprotein)

• converting enzyme (mostly in the lung) cuts off 2 amino acids from angiotensin I – angiotensin II

• angiotensin II enhances aldosterone secretion in the adrenal gland, increases blood pressure through vasoconstriction and increases ADH production

• aldosterone increase Na+ reabsorption through 3 different ways: facilitation of the pump, ATP production, increased apical Na+ permeability

• ADH producing cells detect blood pressure and osmolality and are sensitive to alcohol

• atrial natriuretic peptide (ANP) – released in the atria when venous pressure increases - inhibits renin, aldosterone, ADH production

20/21

Nitrogen removal• part of the digested amino acids are reused,

amino groups from the others have to be removed as NH3 and NH4

+ are poisonous• three forms: ammonia, urea, uric acid • ammonia

– poisonous – huge volume is needed to provide low concentration in the cell and high outward gradient

– 0.5 l water/1 g nitrogen– fish, aquatic invertebrates, mammals in low

amount– transport in the form of glutamine from the liver to

the kidney• urea

– less poisonous, 0.05 l water/1 g nitrogen– synthesis requires ATP– vertebrates, except fish, synthesize urea in the

ornithine-urea cycle, fish and invertebrates from uric acid

– hominoids cannot metabolize uric acid (from nucleic acids) – can accumulate and lead to gout

• uric acid– low solubility: 0.001 l water/1 g nitrogen– white precipitate - guano in birds (uric acid,

guanine)– fish, reptiles, terrestrial arthropods

21/21

Extracellular ion concentrations

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Tab. 14-1.

Osmoregulation in animals

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-8.

Structure of mammalian kidney


Structure of a nephron


Glomerular filtration

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-18

Podocytes of the capsule


Podocytes of the capsule - EM

Berne and Levy, Mosby Year Book Inc, 1993, Fig. 41-7

Juxtaglomerular apparatus


Na+ reabsorption


Processes of reabsorption


Mechanism of K+ secretion


pH regulation


Release of ammonia


Counter-current principle

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, SL. 14-2.

Mechanism of urine concentration


Osmotic pressure in the kidney


Renin-angiotensin system


Actions of aldosterone


Regulation of ADH secretion


Methods of nitrogen release

Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-49, 50.

Osmoregulation. Ionic and osmotic balance in multicellular organisms the interstitial fluid is the internal environment – composition resembles that of.

Documents

water invertebrates

water condensation

metabolic water

mammals loose water

air loosing water

marine fish

obligate osmotic exchange

urea fresh water