water, electrolyte, and acid-base balance. function of water: most of cellular activities are...

Water, Electrolyte, and Acid-Base Balance

Function of Water: Most of cellular activities are performed in

water solutions.

16% TBW

40% TBW4% TBW

- makes up ~60% of total body weight (TBW)

- distributed in three fluid compartments.

Body Fluid

16% TBW

40% TBW4% TBW

Fluid is continually exchanged between the three compartments.

16% TBW

40% TBW4% TBWExchange between Blood & Tissue Fluid

- determined by four factors:

capillary blood pressure

plasma colloid osmotic pressure

interstitium Hydrostatic Pressure

Interstitium colloid osmotic pressure

16% TBW

40% TBW4% TBW

- not affected by electrolyte concentrations

- Edema = water accumulation in tissue fluid

Exchange between Blood & Tissue Fluid

16% TBW

40% TBW4% TBWExchange between Tissue Fluid &

Intracellular Fluid

- determined by two:

1) intracellular osmotic pressure

electrolytes

2) interstitial osmotic pressure

electrolytes

Water Gain

Water is gained from three sources.

1) food (~700 ml/day)

2) drink – voluntarily controlled

3) metabolic water (200 ml/day) --- produced as a byproduct of aerobic respiration

Routes of water loss

1) Urine – obligatory (unavoidable) and physiologically regulated, minimum 400 ml/day

2) Feces -- obligatory water loss, ~200 ml/day

3) Breath – obligatory water loss, ~300 ml/day

4) Cutaneous evaporation -- obligatory water loss, ~400 ml/day

5) Sweat – for releasing heat, varies significantly

Regulation of Water Intake

- governed by thirst.

blood volume and osmolarity

peripheral volume sensors central osmoreceptors

hypothalamus

thirst felt

Regulation of Water Output

- The only physiological control is through variations in urine volume.

- urine volume regulated by hormones

1) ADH dehydration

blood volume and/or osmolarity

hypothalamic receptors / peripheral volume sensors

posterior pituitary to release ADH

H2O reabsorption

Water retention

2) Atrial Natriuretic Factor

blood volume

atrial volume sensors

atria to release ANF

inhibits Na+ and H2O reabsorption

water output

Dehydration

- decrease in body fluid

- Causes

1) the lack of drinking water

2) excessive loss of body fluid due to:

overheat

diabetes

overuse of diuretics

diarrhea

Edema

- the accumulation of fluid in the interstitial spaces

caused by:

1) increased capillary filtration, or

2) reduced capillary reabsorption, or

3) obstructed lymphatic drainage

ELECTROLYTE BALANCE

Electrolytes = small ions that carry charges

Major cations

Na+

K+

Ca++

H+

Major anions

Cl-

HCO3-

PO4---

Na+

K+Ca++

Cl-PO4

---

Distribution of Electrolytes

Cell

Extracellularspace

Sodium Na+

Functions

- involved in generating action membrane potential of cells

- make a major contribution to extracellular osmolarity.

Na+

K+Ca++

Cl- PO4---

Cell

Regulation of plasma Na+

1) Aldosterone

plasma Na+

aldosterone

renal Na + excretion

plasma Na +

Na+

plasma

2) Renin-angiotensin-II

renin

angiotensin-II

aldosterone

renal Na+ excretion

plasma Na+

Na+

plasma

3) ADH

increases water reabsorption in

kidneys

water retention

dilute plasma Na+

plasma

Na+

H2O

4) Atrial Natriuretic Factor

inhibits renal reabsorption of Na+ and H2O and

the excretion of renin and ADH

eliminate more sodium

and water

plasma Na +

Na+

plasma Na+

Sodium imbalance

hypernatremia plasma sodium > 145 mEq/L,

hyponatremia plasma sodium < 130 mEq/L

Potassium

Functions

- the greatest contributor to intracellular osmosis and cell volume

- determines the resting membrane potentials

- an essential cofactor for protein synthesis and some other metabolic processes.

K+Na+

K+Ca++

Cl- PO4---

Cell

Regulation of Potassium

- by aldosterone

Aldosterone

stimulates K+

secretion by the kidneys

Plasma K+

K+

plasma K+

Potassium Imbalance

hyperkalemia (> 5.5 mEq/L)

hypokalemia (< 3.5 mEq/L)

Chloride

- makes a major contribution to extracellular osmolarity

- required for the formation of stomach acid (HCl)

Na+

K+Ca++

Cl- PO4---

Cell

Regulation of Cl–

- No direct regulation

- indirectly regulated as an effect of Na+ homeostasis. As sodium is retained or excreted, Cl– passively follows.

Chloride Imbalance

hyperchloremia (> 105 mEq/L)

hypochloremia (< 95 mEq/L).

Calcium

Na+

K+Ca++

Cl- PO4---

Cell

Functions of Ca++

- lends strength to the skeleton

Functions of Ca++


- activates muscle contraction

Excitation Contraction[ Ca++ ]i

(Action Potentials) (shortening)

Functions of Ca++



- serves as a second messenger for some hormones and neurotransmitters

Functions of Ca++




- activates exocytosis

of neurotransmitters and

other cellular secretions

Functions of Ca++




- activates exocytosis of neurotransmitters and other cellular secretions

- essential factor

in blood clotting.

Functions of Ca++




- activates exocytosis of neurotransmitters and other cellular secretions

- essential factor in blood clotting.

- activates many cellular

enzymes

Dynamics of Calcium

Ca++

plasma

Ca++

Ca++

Ca++

Regulation of calcium

1) parathyroid hormone (PTH):

Regulation of calcium

1) parathyroid hormone (PTH):

- dissolving Ca++ in bones

- reducing renal excretion of Ca++

plasma

Ca++

Ca++

2) calcitonin (secreted by C cells in thyroid gland):

2) calcitonin (secreted by C cells in thyroid gland):

depositing Ca++ in bones

plasma

Ca++

Ca++

3) calcitrol (derivative of vitamin D):

- enhancing intestinal absorption of Ca++ from food

plasma

Ca++

Ca++

Ca++

Calcium imbalances

hypocalcemia (< 4.5 mEq/L)

hypercalcemia (> 5.8 mEq/L).

Phosphates

- needed for the synthesis of:

ATP, GTP

DNA, RNA

phospholipids

Regulation of Phosphate

- by parathyroid hormone

PTH

increases renal excretion of phosphate

decrease plasma

phosphate

- no real phosphate

imbalances

PO4---

plasma PO4---

ACID-BASE BALANCE

Acid An acid is any chemical that releases H+ in

solution.

Base A base is any chemical that accepts H+.

pH

is the negative logarithm of H+ concentration, and an indicator of acidity.

pH = - log [H+ ]

Example: [H+ ] = 0.1 M = 10 –7 M

pH


pH = - log [10 –7 ]

Example: [H+ ] = 0.1 M = 10 –7 M

= 7 log 10 = 7

pH


pH = - log [10 –8 ]

Example: [H+ ] = 0.01 M = 10 –8 M

= 8 log 10 = 8

[ H+ ] = pH

[ H+ ] = pH 0.01 M [ H+ ] = pH 8

0.1 M [ H+ ] = pH 7

Normal functions of proteins (especially

enzymes) heavily depend on an optimal pH.

pH7.35-pH7.45

Regulation of acid-base balance

1) Chemical Buffers

2) Respiratory Control of pH

3) Renal Control of pH

Buffer

is any mechanism that resists changes in

pH.

H2OpH 7.0

BufferpH 7.0

acid

pH 3.0 pH 6.8

acid

H2OpH 7.0

BufferpH 7.0

base

pH 11.0 pH 7.2

base

3) The Protein Buffer

There are three major buffers in body fluid.

1) The Bicarbonate (HCO3-) Buffer

2) The Phosphate Buffer

Chemical Buffers

1) The Bicarbonate (HCO3-) Buffer System

H + HCO3- H2CO3 H2O + CO2

- reversible depending on the equilibrium between the substrates and products.

- The lungs constantly remove CO2.

2) The Phosphate Buffer System

H + HPO42– H2PO4

– + H H3PO4

3) The Protein Buffer System

- more concentrated than either bicarbonate or phosphate buffers

- accounts for about three-quarters of all chemical buffering ability of the body fluids.

- The carboxyl groups release H+ when pH rises and amino groups bind H+ when pH falls.

NH2-CH2-CH2 CH2-CH2-COOH

H+ H+

Properties of Chemical Buffers

- respond to pH changes within a fraction of a second.

- Bind to H but can not remove H out of the body

- Limited ability to correct pH changes

H2CO3 H2O + CO2

10 20 20


10 10 10 10 10

1)

20 10 10 10 10

2) H + HCO3- H2CO3 H2O + CO2

10 0 20 10 10

3) H + HCO3- H2CO3 H2O + CO2

Respiratory Control of pH

H + HCO- H2CO3 CO2 + H2O

H + HCO- H2CO3 CO2 + H2O

pH

stimulate peripheral/central chemoreceptors

pulmonary ventilation

removal of CO2 and pH

H2CO3

H + HCO3- H2O + CO2

Limit to respiratory control of pH

The respiratory regulatory mechanism cannot remove H+ out of the body. Its efficiency depends on the availability of HCO3

- .


Renal Control of pH

1. The kidneys can neutralize more acid or base than both the respiratory system and chemical buffers.

a. Renal tubules secrete hydrogen ions into the tubular fluid, where most of it combines with bicarbonate, ammonia, and phosphate buffers.

b. Bound and free H+ are then excreted in urine.

2. The kidneys are the only organs that actually expel H+ from the body. Other buffering systems only reduce its concentration by binding it to another chemical.

3. Tubular secretion of H+ continues as long as a sufficient concentration gradient exists between the tubule cells and the tubular fluid.

Disorders of Acid-Base Balance

Acidosis: < pH 7.35 , Alkalosis: > pH 7.45

- Mild acidosisdepresses CNS, causing

confusion, disorientation, and coma.

- Mild alkalosis CNS becomes hyperexcitable. Nerves fire spontaneously and overstimulate skeletal muscles.

- Severe acidosis or alkalosis is lethal.

Respiratory vs Metabolic Cause

Respiratory acidosis / alkalosis - caused by hypoventilation or hyperventilation

H + HCO- H2CO3 H2O + CO2

Initial change

Emphysema

Respiratory acidosis / alkalosis - caused by hypoventilation or hyperventilation

Metabolic acidosis or alkalosis

- result from any causes but respiratory problems

Diabetes

production of organic acids

metabolic acidosis

Chronic vomiting

loss of stomach acid

metabolic alkalosis

water, electrolyte, and acid-base balance. function of water: most of cellular activities are...

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