define & differentiate between osmolarity ecf

8/7/2019 Define & Differentiate Between Osmolarity ECF

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DEFINE & DIFFERENTIATE BETWEEN

OSMOLARITY, ECF/ICF, TONICITY,

PHYSIOLOGY OF FLUID & ELECTROLYTEBALANCE

R U I


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DAILY INTAKE OF WATER2 major sources:

2100ml/day

200ml/day

Varies on individual, climate, habits & level of physical activity


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DAILY LOSS OF BODY WATER

Insensible water loss

Water evaporation from the respiratory tract

300-400ml/day

In respiratory tract, air vapour pressure- 47 mm Hg

Inspired air vapour pressure less than 47 mm Hg

Cold weather nearly 0

Diffusion through the skin (independently of sweating) 300-400ml/day

Minimized by the cholesterol-filled cornified layer of the skin

Layer denuded (extensive burns) 3-5 L/day

Sweat 100ml/day2L/hour

Water loss in faeces 100ml/day

Water loss by the kidneys 0.5L 20L/day


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REVISION:

Osmosis: the net diffusion of water across a selectively

permeable membrane from a region of high water

concentration to one that has a lower water concentration.

* Plasma membrane is selectively permeable


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OSMOLES

the total number of particles in a solution

1 osmole = 1 mole (6.02 X 1023)

Refers to the number of osmotically active particles in a solution rather than to

the molar concentration

NaCl Na+ & Cl-

1 mole/L = 2 osm/L


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OSMOLARITY

the osmolal concentration of a solution when express as osmoles per liter

Osmolality- osmoles per kg

Total osmolarity of 3 compartments = 300mOsm/L


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ISOSMOTIC, HYPEROSMOTIC, HYPO-OSMOTIC

Isosmotic solutions with the same Osm

Hyperosmotic- solution with higher Osm than

another

Hypo-osmotic- solution with lower Osm than theanother

Osm [water]

Osm [water]


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OSMOTIC EQUILIBRIUM:

Large osmotic pressure can develop across the cell membrane with relatively small

changes in the concentration of solutes in the ECF

For each mOsm concentration gradient of an impermeant solute, about 19.3mm Hg

osmotic pressure is exerted across the cell membranre


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OSMOLARITY & OSMOTIC PRESSURE

Osmolarity (Osm): sum of all solutes in a given volume (moles per liter)

Osmotic pressure (Posm): force generated by osmosis

Measure of the tendency to take on water by osmosis

For an isosmotic solution to be isotonic, the membrane must be equally permeable or

equally impermeable to all solutes

All isotonic solutions are isosmotic

Not all isosmotic solutions are isotonic


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TONICITY

Isotonic

Water concentration in the ICF & ECF is equal

Neither shrinks nor swells

0.9% solution ofNaCl (9g/L)

5% solution of glucose (50g/L)

Hypotonic:

Water concentration in the ECF is higher than ICF

Water diffuse into cells dilutingICF Cells swell

Hypertonic:

Water concentration in the ICF is higher than ECF

Water diffuse out of the cells concentratingICF

Cells shrink


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PHYSIOLOGY


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Extracellular fluidvolume

Cardiac output

Effective arterial bloodvolume

Arterial underfilling

Unloading of high-pressure volume receptors

Stimulation of sympatheticnervous system

Nonosmotic ADH release Activation ofRAAS

peripheral and renal arterial vascular resistance and Na+ & H2O retention


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Everything

- protein

65% of

filtered load

of H20, Na &

> Cl

20% of the filtered water

25% of

the

filtered

loads

ofNa,

Cl & K

5% of the

filtered

NaCl

Principle &intercalated

cells

>10%

of

filtered

water

& Na


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PROXIMAL TUBULAR REABSORPTION

65% of the filtered load of sodium & water and a slightly lower percentage of filtered chloride

are reabsorbed

Due to the highly metabolic epithelial cells with large number ofmitochondria and brush

border on the luminal side of the membrane which is also loaded with protein carrier

molecules (co-transport of sodium & glucose/amino acid, counter transport of sodium &

hydrogen), as well as an extensive labyrinth of intercellular and basal channels (increase

surface area)

Proximal tubule is also important for secretion of organic acids and bases such as bile salts,oxalate, urate and catecholamines

Filtration + secretion absorption

Para-aminohippuric acid (PAH)

Secreted so rapid that the average person can clear 90% of PAH from the plasma

PAH clearance can be used to estimate the renal plasma flow


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LOOP OF HENLE: THIN DESCENDING SEGMENT

no brush borders, few mitochondria, minimal levels of metabolic activity

Highly permeable to water

Moderately permeable to most solutes including urea & sodium

Function: to allow simple diffusion of substances through its wall

Almost 20% of the filtered water is reabsorbed here


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LOOP OF HENLE: THIN ASCENDING SEGMENTno brush borders, few mitochondria, minimal levels of metabolic activity

Impermeable to water

Lower reabsorptive capacity


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LOOP OF HENLE: THICK ASCENDING SEGMENT

Thick epithelial cells with high metabolic activity and are capable ofactive reabsorption ofsodium, chloride, and potassium

Almost 25% of the filtered loads of sodium, chloride and potassium are reabsorbed here

Sodium-potassium pump in the basolateral membraneimportant component

The reabsorption of other solutes is closely linked with the reabsorptive capability of the

sodium potassium pump, which maintains a low intracellular sodium concentration which

provide a concentration gradient for movement of sodium from the tubular fluid into the cell Also has sodium-hydrogen counter transport mechanism in its luminal membrane

Referred as the diluting segments


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DISTAL TUBULE1st portion forms the macula densa, a group of closely packed

epithelial cells that is part ofthe juxtaglomerular complex &

provides feedback control of GFR and blood flow in this same

nephron.

The next portion of the distal tubule is highly convoluted

avidly reabsorbs most of the ions but is impermeable to water &

urea)

5% of the filtered load of sodium chloride is reabsorbed in the

early distal tubule

sodium-chloride co-transporter moves sodium chloride from the

tubular lumen into the cellsodium-potassium pump transport sodium out of the cell across

the basolateral membrane

chloride diffuses out of the cell into the renal interstitial fluid

through chloride channels in the basolateral membrane


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Late distal tubule and cortical collecting tubule

Second half of the distal tubule and the subsequent cortical collecting tubule have similar

functional characteristics

They are composed of 2 distinct cell type:

Principle cells

Reabsorb sodium and water from the lumen and secrete potassium ion into the lumen

(sodium potassium pump in basolateral membrane which lowers down sodium

concentration in the cell, hence diffusion of sodium ions across the luminal membrane)

Intercalated cells Reabsorbed potassium ions and secrete hydrogen ions into the tubular lumen

Hydrogen ATPase transporter

Hydrogen is generated by the action of carbonic anhydrase on water and carbon dioxide

to form carbonic acid, which then dissociates into hydrogen ions and bicarbonate ions

For each hydrogen ion secreted into the tubular lumen, a bicarbonate ion becomes

available for reabsorption across the basolateral membranePermeability is controlled by the concentration ofADH


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MEDULLARY COLLECTING DUCT

Reabsorb less than 10% of the filtered water and sodium

Final site for processing urine

Epithelial cells are nearly cuboidal in shape with smooth surfaces andrelatively few mitochondria

Permeability is controlled by the concentration ofADH High level ofADH, water is avidly reabsorb into the medullary

interstitium, thereby reducing the urine volume and concentratingmost of the solutes in the urine

Medullary collecting duct is permeable to urea and there are specialurea transporters that facilitate urea diffusion across the luminaland basolateral membranes.

Some urea is reabsorbed into the medullary interstitium, helping to

raise the osmolarityMedullary collecting duct is capable of secreting hydrogen ions against

a large concentration gradient, as also occur in cortical collectingtubule. Thus regulating the acid-base balance


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GLOMERULOTUBULAR BALANCE

intrinsic ability of the tubules to increase their reabsorption rate in response to increased

tubular load

can occur independently of hormones and can be demonstrated in completely isolated

kidneys or even in completely isolated proximal tubular segments

helps to prevent overloading of the distal tubular segments when GFR increases


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REGULATION OF PERITUBULAR CAPILLARY

PHYSICAL FORCES2 determinants of peritubular capillary reabsorption that are directly influenced by

renal hemodynamic changes are the hydrostatic and colloid osmotic pressures of

the peritubular capillaries.


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PERITUBULAR CAPILLARY HYDROSTATIC

PRESSURE

influenced by the arterial pressure and resistance of the afferent and efferent

arterioles.

increase in arterial pressure tend to raise peritubular capillary hydrostatic pressure

and decrease reabsorption rate

increase in resistance of either the afferent or the efferent arterioles reduces

peritubular capillary hydrostatic pressure and tends to increase reabsorption rate


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COLLOID OSMOTIC PRESSURE OF

PERITUBULAR CAPILLARY IS DETERMINED BY:

the systemic plasma colloid osmotic pressure

increasing the plasma protein concentration of systemic blood tends to raise peritubular

capillary colloid osmotic pressure, thereby increasing reabsorption

the filtration fraction

the higher the filtration fraction, the greater the fraction of plasma filtered through the

glomerulus and more concentrated the protein becomes in the plasma.


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Changes in peritubular capillary physical forces influence tubular reabsorption by changing

the physical forces in the renal interstitium surrounding the tubules.


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URINE CONCENTRATION

Established by LOH, CD and

vasa rectap reabsorption

of varying amounts of H2Oand Na+

Key player: ADH (= Vasopressin)


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URINE CONCENTRATION, CONTD

Often expressed in osmolaritymM/L or osmolality mM/kg

Blood: 300 mOsm

Filtrate in Bowmans Capsule: 300

mOsm Bottom of LOH: 1200 mOsm

Urine: 50-1200 mOsm

Regulated by ADH (vasopressin)

Osmoreceptors in hypothalamus

BP and blood volume, too

Fig. 20-4


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EFFECT OF ADH

Controls Urine concentration via

regulation of water reabsorption

from the filtrate in the collecting

ductOsmoreceptors in hypothalamus

ADH caused by:

Na+ and/or osmolality in the ECF

H2O deprivation

renal blood flow

Hi [ADH] Lo [ADH]

Fig 20-5


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EFFECT OF ADH, CONTD

ADHReceptors in CD cells

Luminal CM is generally

impermeable toH2O

Aquaporins (rememberCh. 5) oncellmembranesofCD are variably

active, dependenton ADH

Membrane Recycling via

exocytosisofAQP2

AllowsosmosisofH2O intovasa recta


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TROUBLES WITH ADH?

Diabetes insipidus

Central

Nephrogenic

Nocturnal enuresis

ADHdeficiency:

ADHExcess:

AKA Inappropriate ADHsecretion

XSH2O retention


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In presence of ADH: Insertionof H2O pores into tubularluminal CM

At maximal H2O permeability:Net H2O movement stopsat equilibrium

Maximum osmolarity of urineup to 1200 mOsm

No ADH:

DCT & CD

impermeable to H2O

Osmolarity can plunge

to ~ 50 mOsm

CONCENTRATED VS. DILUTE

URINE

Review:


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LOH:COUNTERCURRENT

MULTIPLIER

leads to

Hyperosmotic IF inmedulla

Hyposmotic fluid

leaving LOH


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REGULATION OF BP:

NA+ BALANCE AND ECF VOLUME

[Na+] affects plasma & ECF osmolarity

(Normal [Na+]ECF ~ 140 Mosm)

[Na+] affects blood pressure & ECF volume

[] Gradients

Aldosterone stimulatesNa+ reabsorption and K+ excretion in last 1/3 of DCTand CD

Type of hormone? Where produced? Type of mechanism?

o Aldosterone secretiono Na+ absorption from DCT

Secretion of aldosterone by two mechanisms o K+ in ECF

BP

The signal to release aldosterone is via angiotensin II

Opposite ofAldosterone? ANP (from the atria) causes loss ofNa+


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ALDOSTERONE MECHANISMALDOSTERONE MECHANISM

Na+/K+ATPase activityo

K+ secretion o

Fig 20-13

Here (unlike normally) H2O does

not necessarilyfollowNa+

absorption. Thisonly happens in

presence of. . .


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REGULATION OF BP:

RAAS PATHWAYS

RAAS = renin-angiotensin-aldosterone system

JG cells release renin in responseto BP

Renin converts Angiotensinogen toAngiotensin I

ANG I converted to ANG II by ACE


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RAAS PATHWAYS, CONTD

ANG II causes BP via ADH Secretion

Thirst

VasoconstrictionSympathetic stimulation of heart

HR and CO

ACE inhibitors will BP


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KEY PROCESSES

Countercurrent Multiplication: Segments of Loop of Henle

Countercurrent multiplication: Urea recycling

Countercurrent Exchange: Vasa recta


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ADH

Produce by hypothalamus

Stored in posterior pituitary gland

Blood osmolarity

Release ofADH intodistal convoluted tubule

& collecting duct

Act on aquaporin

Aquaporin move tosurface

Allow water to movefrom tubule to interstitial

and then into capillary

Act on medullary

collecting duct

Permeability

to urea

osmolarity


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ALDOSTERONE

Mineralocorticoid

Produced in adrenal gland in kidney

BP

Low Na inMedulla densa

RAAS

Aldosterone

sodiumreabsorption

H20reabsorption

Angiotensinogen

Angiotensin I II

vasoconstriction

Blood volume BP

define & differentiate between osmolarity ecf

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