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TUMS

Azin Nowrouzi (PhD)Tehran University of Medical sciences

Azin Nowrouzi (PhD)Tehran University of Medical sciences

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Principal functions of waterPrincipal functions of water

• Universal solvent and suspending medium• Helps to regulate body temperature• Participates in hydrolysis reactions

– It is a medium where most cell’s metabolic reactions take place

• Lubricates organs• Provides cellular turgidity• Helps to maintain body homeostasis

– Ionization of water and its acid-base reactions important for the functions of proteins and nucleic acids

• The Shapes of proteins and nucleic acids and structure of biological membranes are consequence of their interaction with water.

DiffusionDiffusion• Diffusion progresses until there is no net movement

any longer called diffusional equilibrium.

DiffusionDiffusion

• Diffusion is the tendency of molecules, and ions in solution to move from areas of higher concentration to areas of lower concentration.

• The difference in concentration is the concentration gradient.

• Diffusion occurs because particles are in constant motion attempting to equilibrate across a membrane or inside an organelle.

Diffusion in Living SystemsDiffusion in Living Systems

• Diffusional equilibrium is more correctly referred to as seeking of a physiological steady state.

• Diffusion of substances occurs if the membrane is permeable to that substance and if a concentration gradient exists

• Examples: oxygen, carbon dioxide, cholesterol

Facilitated DiffusionFacilitated Diffusion

• Substances that are insoluble in lipids and too large to move through pores move by facilitated diffusion.

• Facilitated diffusion includes protein channels and protein carriers.

• Molecules fit into the carrier and are transported across the membrane.

• The number of carriers limits the rate of movement.

Figure 3.24

FiltrationFiltration• Molecules can be filtered by force (such as pressure

differences) through membranes as is done in the kidney nephrons.

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Permeability propertiesPermeability properties

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Solution EquilibriumSolution Equilibrium

11Molar distributions

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Donnan’s EquilibriumDonnan’s Equilibrium

a. According to Donnan’s equilibrium, the product of diffusible electrolytes in both compartments will be equal.[K+]L x [Cl-]L = [K+]R x [Cl-]R

9 x 4 = 6 x 6

b. The electrical neutrality in each compartment is maintained (the number of anions should equal the number of cations)In left: Na+ = R- + Cl- 9 = 5 + 4In right: Na+ = Cl- 6 = 6

• Total number of each type of ion is the same before and after equilibriumNa+ = 9 + 6 = 15Cl- = 4 + 6 = 10

• When there is nondiffusible anion in one side of a semipermeable membrane, the diffusible cations are more and diffusible anions are less, in that side.

Left Right

(5) K+

(5) Pr -

(10) K+

(10) Cl-

Before equilibrium

After equilibrium

Left Right

(9) K+

(5) Pr -

(4) Cl-

(6) K+

(6) Cl-

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Osmosis = diffusion of water across a semipermeable membrane (like a cell membrane) from an area of low solute concentration to an area of high solute concentration.

Osmosis &Osmosis &Osmotic pressureOsmotic pressure

OsmosisOsmosis• Osmosis is the diffusion of water molecules from a

region of higher water concentration to a region of lower water concentration across a selectively permeable membrane.

OsmosisOsmosis

• One might also say: Osmosis is the diffusion of water molecules from a region of low solute concentration to a region of high solute concentration across a selectively permeable membrane.

• Red Blood Cells (RBC) exhibit how osmosis works nicely: when in isotonic or plasma solution, they appear donut shaped.

• When they are placed into a hypotonic solution, they take up water and swell and might even burst.

• When they are placed into a hypertonic solution, they loose water and shrink in size.

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TonicityTonicity

Tonicity is a unitless concept that can be expressed only in reference to a physiologic system:

• A hypertonic solution is one that would shrink cells.

• A hypotonic one would cause them to swell.• In an isotonic solution cells are not affected.

• Loss of free water in the extracellular compartment → The fluid becomes too concentrated (increased osmolarity) → Hypertonic

• Gain in free water → The fluid becomes too dilute → It is called Hypotonic

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Cell membrane is a selectively Cell membrane is a selectively permeable membrane permeable membrane

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Osmotically effective solutesOsmotically effective solutes

• The major extracellular solute is Na and its associated anions.

• The major intracellular solute is K and its associated anions.

• These solutes are relatively restricted to their compartments

Solutes that are relatively restricted to one particular body fluid compartment are able to exert an osmotic force for water movement from other compartments.

Such solutes are called osmotically effective, or simply effective solutes.

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Noneffective solutesNoneffective solutes

• Some solutes, notably urea pass freely across cell membranes and do not exert a force for water movement between the two major body fluid compartments.

• Ethanol, methanol and ethylene glycol are also noneffective solutes.

• Such noneffective solutes contribute to body osmolality but not to tonicity.

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Osmolarity, OsmolalityOsmolarity, Osmolality

Osmolarity: Number of particles dissolved in 1 L water = osm/L H2O

– 1 osmol = 6x1023 solute molecules per liter – example: 150 mM NaCl = 300 mosmol solute (150

mMNa+ + 150 mM Cl-)– It is not an SI unit

Volume depends on factors like temperature, thus osmolality is used: Number of particle in 1kg of water = osm/kg H2OFor Glucose: molality = osmolalityFor NaCl: osmolality = molality x 2

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Osmolality & osmolarity measurementOsmolality & osmolarity measurement

• Osmolality can be measured using an osmometer. Osmometers are useful for determining the concentration of dissolved salts or sugars in blood or urine samples.

• Osmolarity = 2 x [Na+] + [urea] + [glucose]

(Concentrations are in mmol/L)• Difference between the two is osmolar gap. It

occurs when abnormal osmotically active species are present in plasma (such as poisons)

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In medicineIn medicine

Relative to blood plasma, a solution can be:

• Isosmolal: Equal osmolality• Hyposmolal: Lower osmolality• Hyperosmolal: Higher osmolality

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Mean concentrations of the more Mean concentrations of the more important solutes in cell compartmentsimportant solutes in cell compartments

Fluid Na+ K+ Ca2+ Mg2+ Cl- Amino

acids

Glucose

mg%

Extracellular 142 4 5 3 103 5 90

Intracellular 10 140 1 58 4 40 0-20

All concentrations except those of glucose are in milliequivalents per liter.

• The osmolality of ECF is in the range of 282-290 mOsm/kg of water (282-295 mmmol/Kg of water).

• Intracellular fluid osmolality is the same (about 282-290 mOsm/kg water).– Water freely permeates across cell membranes.– Major extracellular and intracellular effective solutes do not. – Any loss or gain of water in the ECF will affect the water

concentration in the ICF.

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ECF osmolalityECF osmolality

Major contributors to ECF osmolality:• Sodium • Chloride • Bicarbonate• Glucose• Urea

Osmotically noneffective solutes, such as urea, contribute to body osmolality but not to tonicity.

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• Liquid inside cells (Intracellular fluid) - 40 % • Extracellular fluids - 15 % • Liquid making the plasma of blood - 5 %

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Total body water

50%-60% BW

Intracellular fluid

35%-40% BW

Extracellular fluid

15%-20% BW

Blood plasma (intravascular fluid)

4%-5% BW

Interstitial fluid

(extravascular)

11%-15%BW

Lymph

Transcellular fluid:

Cerebrospinal fluid

Intraocular fluid

Synovial fluid

Pericardial fluid

Pleural fluid

Peritoneal fluid

BW = Body Weight

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Measurement of volumes of the fluid Measurement of volumes of the fluid compartments by compartments by

an indirect dilution techniquean indirect dilution technique

Compartment Substance used

Total 3H2O (radioactive water), antipyrine

Extracellular Thiosulfate, inulin

Blood plasma Evans blue

Compartment V (ml) = Substance concentration in compartment (mg/ml)Quantity of substance introduced (mg)

Interstitial fluid = Extracellular fluid – Plasma volume

Intracellular fluid = Total body water – Extracellular fluid volume

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How does total body water vary?How does total body water vary?

Age Infants: Body water ~75%-80% of BW.

Elderly people: Water only 40%-50% of BW.

The percentage decreases with age.

Sex Women usually have less body water than men because the greater proportion of adipose tissue in women contains lesser amounts of water than other tissue types.

Weight Obese people have less body water because of and abundance of adipose tissue. Total body water content is inversely related to body fat content.

Environmental temperature, Physiological state, Food quality and quantity

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How much water can we lose?How much water can we lose?Infants compared to adultsInfants compared to adults

• In adults: Percent body water is high compared to the skin surface – Adults are not very prone to

dehydration.

• Infants are at high risk of dehydration when febrile or lose fluids due to vomiting or diarrhea.– It is critical to administer

fluids to a febrile infant to maintain body homeostasis.

• Body can lose nearly all fat and over half of its protein and live.

But• 10 -15% loss of water will

result in death.• Loss of water through

skin is increased 13% for each degree rise in centigrade in body temperature.

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Sources of body waterSources of body water

1. Drinking water2. In food3. Metabolic water from nutrient oxidaton

– Glucose + 6O2 6CO2 + 6H2O– 1g carbohydreate = 0.6ml water– Alanine + 3O2 2.5 CO2 + CO(NH2)2 + 2.5

H2O– 1g protein releases 0.4ml water– Palmitic acid + 23O2 16CO2 + 16H2O– 1g fat generates 1.1ml water

– About 1000 kcal is equivalent to intake of 125ml of water

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Water lossesWater losses• Urine

– Ambient temperature– Digestible dry matter

intake– N intake, metabolism and

excretion

• Feces– Water intake– Dry matter intake– Fiber content of food

• Insensible loss through evaporation– Ambient temperature

• Physiological state– Lactation– Pregnancy

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Water balanceWater balance

Fluid intake = fluid outputHydration Positive water balance

When body water intake exceeds water input

Dehydration Negative water balance

When water output exceeds intake

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Water and salt disturbancesWater and salt disturbances1. Depletion

– Water depletion (Dehydration or hypovolemia)• Inadequate water intake • Excessive loss

– Sodium depletion (Hyponatremia)• Inadequate oral intake• Inadequate parenteral input• Excessive sodium loss (isotonic loss for example in plasma,

hypotonic loss (in sweat or urine)

2. Excess– Water excess (overhydration or hypervolemia)

• Impaired excretion• Excessive intake (psychiatric disorders, organic brain disease for

example trauma and following surgery)

– Sodium excess (Hypernatremia)• Increased intake• Decreased excretion (renal disease, primary adrenal disease,

secondary hyperaldosteronism).

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Dehydration or hypovolemiaDehydration or hypovolemia(Water loss)(Water loss)

• Causes: – Decreased intake (lack of water, psychogenic refusal to drink) – Increased output (vomiting, diarrhea, loss of blood, drainage from

burns, diabetes mellitus, diuretic use, lack of ADH due to diabetes insipidus.

• Symptoms: loss of weight, rise in body temperature, increase in heart rate and cardiac output, decrease in blood pressure, sunken eyeballs.

• Response: – Decrease in salivary secretion and drying of the mouth and pharynx

thirst– Increased osmolality and low blood volume and pressure release

of ADH from posterior pituitory, increased reabsorption of water in kidney tubules

– Aldosterone secretion from adrenal gland

39Water intoxication=consumption of too much water too quickly

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Overhydration or hypervolemiaOverhydration or hypervolemia(Water gain)(Water gain)

• Causes: – Excessive IV administration of fluids, psychogenic

drinking episodes, decreased urinary output because of renal failure, congestive heart failure

• Symptoms: Decrease in body temperature, increased blood pressure, edema, weight gain.

• Response: – Decreased osmolarity of fluids in the hypothalamus

inhibition of thirst, decreased release of ADH and decreased aldosterone secretion increased urinary output

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Hyponatremia (sodium deficit)Hyponatremia (sodium deficit) Hypernatremia (Sodium excess) Hypernatremia (Sodium excess)

• Normal [Na+] in blood plasma = 150mEq/L• Excess of water [Na+] below 120 mEq/L

lethargy, coma, or death.Sodium loss: Decreased ECF volume

Aldosterone secretion Renal sodium reabsorption decreased Na+ excretion.

• [Na+] affects plasma & ECF osmolarity• [Na+] affects blood pressure & ECF

volume

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Too little aldosterone:

Addison’s disease

Hyperaldosteronism: 1- Primary (due to tumors of adrenal cortex)

2- Secondary: liver disease, heart failure, pregnancy, nephrosis…

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The hormones interact when blood loss The hormones interact when blood loss or dehydration occurs to maintain or dehydration occurs to maintain

intravascular volume intravascular volume

Factors that stimulate renin release:

a. Decreased blood pressure

b. Salt depletionc. Prostaglandinsd. Beta-adrenergic drugs

Inhibitors of renin release:a. Increased blood pressureb. Salt intakec. Prostaglandin inhibitorsd. Beta-adrenergic

antagonistse. Angiotensin II

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Salt & water Disturbance

ECS ICS Osm Examples

1 Isosmotic loss

vomiting, diarrhea, diuretic therapy, blood loss, burn, drainage of ascites

2 Water deficit ICSECS sweating, hyperventilation, osmotic diuresis, chronic renal disease, diabetes insipidus

3 Salt deficit ECSICS

vomiting, diarrhea, sweating, adrenal insufficiency, hypokalemia, CNS lesion, salt-losing nephritis.

4 Isosmotic excess

Heart failure, nephrosis, acute glomerulonephritis, decompensated cirrhosis

5 Water excess

ECSICS

Water drinking, excessive ADH secretion, intensive gastric lavage, infusion of glucose solution.

6 Salt excess ICSECS

Infusion of hypertonic saline, adrenal hyperactivity, steroid therapy, drinking sea water, CNS lesions

1, 2, and 3 result in hypovolemia

3 and 5 lead to intracellular edema (including cerebral swelling)

4, 5, and 6 result in extracellular edema (for example, pulmonary edema)

Disturbances in salt and water balance:

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Role of the KidneyRole of the Kidney

• GFR = Volume of filtrate formed by all the nephrons of both kidneys each minute.– In adult female GFR = 110 ml/min– In male, GFR = 125 ml/min

• Thus a volume of 7.5 L/h, or 180 L/day is formed.– ~99% of the filtrate is reabsorbed from renal tubules and

returned to the bloodstream.– ~1% is excreted in urine

• Urine volume is regulated according to the needs of the body.

• Most solutes are reabsorbed completely or almost completely according to the needs of the body.

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Renal handling of different substancesRenal handling of different substances

Substance Kg/day

filtered

Kg/day

excreted

Percentage reabsorbed

Water 180.00 1.8 99%

Glucose 0.180 0.180 100%

Sodium 0.630 0.0032 99.5%

Urea 0.056 0.028 50%

• Substances that are actively transported from peritubular capillaries to the tubule lumen:Hydrogen, potassium, penicillin, poisons, drugs, metabolic toxins, chemicals that are not normally present in the body.

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Nephron segmentNephron segment

• Proximal tubule– Glucose & Na+

• Loop of Henle– H2O, Na+, K+ & Cl-

• Distal tubule– Na+ & H2O

• Collecting duct– H2O, Na+ & urea– Hormone regulated

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Regulation of Urine Concentration by the kidneysRegulation of Urine Concentration by the kidneys

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Countercurrent multiplier exchangeCountercurrent multiplier exchange

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6- Diabetic Hyperosmolar state6- Diabetic Hyperosmolar state

• In diabetics with serum glucose ~ 700 mg/dL, the hyperosmolar state caused efflux of cellular water osmotic dilution of serum sodium. (For each 100 mg/dL increase in serum glucose, there is 1.6 mEq/L decrease in the serum [Na+]).

• Transport of glucose into cells happens along with concurrent potassium transport into cells insulin resistance causes high serum potassium.

• Pattern:• Low serum sodium and high serum potassium

(same as hypoaldosternism), • High glucose levels.

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Drugs affecting water absorption in kidney Drugs affecting water absorption in kidney tubules: Diuretics & Antidiureticstubules: Diuretics & Antidiuretics

• Diuretic drugs:• Act on nephronic

tubules • Increase the excretion

of salt (NaCl, NaHCO3) and water.

used in heart failure management

BP control reduction of edema correction of base-acid

balance (pH)

• Antidiuretic drugs: • Decrease the

secretion of salt and water.

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Free Water Passing Through Membrane PoresFree Water Passing Through Membrane Pores

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Cellular Mechanisms of Water TransportCellular Mechanisms of Water Transport

• The precise pathways whereby water crosses membranes has only recently been discovered.

• A family of membrane proteins has now been described, termed aquaporins.

• To date three members of this family have been described in mammals and are termed CHIP28, WCH-CD and MIP26 and display about 45% homology.

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Water channel moleculesWater channel molecules• CHIP28 (channel forming integral protein) was the first

known water channel molecule.

– A 28 kDa protein, and within the kidney is found exclusively in the proximal tubule.

– CHIP28 protein is present in many tissues including lung, small intestine, and red blood cells, and therefore plays an important role in water transport in different organs

• WCH-CD (water channel of the collecting duct) – Found exclusively in the collecting duct, on the luminal surface

of the tubule, and is sensitive to ADH (whereas CHIP28 is not).

• MIP26 (major intrinsic protein of the mammalian lens) has not been detected in renal tissue.

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Formation of Water Pores: Formation of Water Pores: Mechanism of Vasopressin ActionMechanism of Vasopressin Action

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Potassium inside the kidneyPotassium inside the kidney

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MetalloenzymesMetalloenzymes• Metal is firmly bound• Metal to protein ratio is constant• Metal to enzyme activity ratio is constant• Metal is unique• No enzyme activity without metal

Examples of metalloenzymes:– superoxide dismutase (Zn and Cu)– carboxypeptidase A (Zn)– carbonic anhydrase (Zn)– cytochrome oxidase (Fe and Cu)– xanthine oxidase (Co and Fe)

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Metal-activated enzymesMetal-activated enzymes

• Metal is reversibly bound• Metal to protein ratio is variable• Metal to enzyme activity ratio is variable• Metal is not necessarily unique• Enzyme activity may stop without metal

• Examples of metal-activated enzymes– creatine kinase (Mg, Mn, Ca or Co)– glycogen phosphorylase kinase (Ca)– salivary and pancreatic alpha-amylases (Ca)

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Requirement Requirement Recommendation Recommendation

• The minimum amount of a nutrient necessary to prevent occurrence of specific diseases or disease symptoms.

 • Is usually determined for an

individual person by extensive scientific investigations.

 • Varies from individual to

individual and is affected by body size, age, sex, and health status.

• The average daily dietary intake level that is sufficient to meet the nutrient needs of nearly all (97-98 percent) healthy individuals in a group.”

• Does not include intake for those with any disease, (which can be upwards of 50% of all Americans at any given time).

• Must come from consumption of foods (i.e., Dietary), does not include intake from supplements.

Requirement Recommended

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Clinical conditions regarding NaClinical conditions regarding Na++

• Hyponatremia– Plasma [Na+] may be normal (if isotonic

loss), high (if hypotonic loss), low (vasopressin secretion secondary to hypovolemia causes water retention).

– Cause:• Depletion of sodium (hypovolemic hyponatremia)

• Water excess (euvolemic hyponatremia)

• Combined water and sodium excess (hypervolemic hyponatremia)

• Hypernatremia

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Water and sodium regulationWater and sodium regulation1. Through the action of osmoreceptors in the hypothalamus,

and baroreceptors (stretch receptors) in atria.– Atrial Natriuretic Peptide (ANP)

• Released when atrial pressure is increased e.g. in heart failure or fluid overload. It promotes loss of sodium and chloride ions and water chiefly by increasing GFR.

– Antidiuretic Hormone (ADH) (vasopressin) • Increases the water permeability of the distal tubule and collecting duct,

thus increasing the concentration of urine.

2. NaCl content of body determines the size of extracellular fluid.– Renin

• Increases the production of angiotensin II which is released when there is a fall in intravascular volume e.g. haemorrhage and dehydration

– Aldosterone • Promotes sodium ion and water reabsorption in the distal tubule and

collecting duct where Na+ is exchanged for potassium (K+) and hydrogen ions by a specific cellular pump