Water, Electrolyte and Acid-Base Balance

Homeostasis—Balance a state of equilibrium – substances

are maintained in the right amounts and in the right place in the body

Osmosis is the primary method of water movement into and out of body fluid compartments.

Osmosis is the net movement of water molecules through a selectively permeable membrane from an area of high water concentration to an area of lower water concentration.

Water Balance

The concentration of solutes determines the direction of water movement.

Most solutes in the body are electrolytes – inorganic compounds which dissociate into ions in solution.

“Where sodium goes, water follows.”

About 40 Liters (10.56 gallons) of body water Babies – 75% water Men – 63 % Women – 52%

Separated by selectively permeable membranes Intracellular – 2/3 (63%) of total body water Extracellular – 1/3 (37%)

Interstitial fluid – 80 % of extracellular water Blood plasma – 20 % of extracellular water

Fluid compartments

Extracellular fluids: High in Na+, Cl-, Ca++, HCO3-

Blood plasma has more protein than interstitial fluid and lymph

Intracellular fluids: High in K+, phosphate, Mg++, and more protein

than plasma

Composition of compartments

Hydrostatic pressure – pressure of fluids

Osmotic pressure – solute concentration (often Na+) In blood referred to as colloid osmotic pressure

(COP)

Movement of water

Average adult takes in about 2,500 ml/day Sources of water:

Preformed water: 2,300 ml Drinking water: 1,500 ml (60%) Moist food : 750 ml (30%)

Water of metabolism: 250 ml (10%) Cellular respiration Dehydration synthesis

Water intake = Water loss

Thirst center in hypothalamus is stimulated ( or mistakenly, the hunger center) and person feels thirsty

Wetting of the mouth and stretching of stomach or intestines decrease thirst before we take in too much water.

Water is absorbed, and blood osmotic pressure decreases.

Regulation of water intake:Process

Through kidneys in urine – 1500 ml (60%) Through intestines - 150 ml (6%)

Can be significant in vomiting and diarrhea From skin (sweat) - 150 ml (6%) From lungs and skin - 700 ml (28%) Last is called insensible loss

(menstruation)

Sources of water loss

Through regulating urine formation ADH – production stimulated by ↑ blood

tonicity of decrease in volume. Acts on distal convoluted tubules and

collecting ducts of kidney – permits reabsorption of water

Regulation of Water Output

Aldosterone – production is stimulated by angiotensin II through renin production Causes sodium ( and water) to be reabsorbed

ANP – causes sodium (and water) loss when pressure in right atrium is too high

Dehydration is the imbalance seen most often. Prolonged diarrhea or vomiting Excessive sweating

Water imbalances

If lose water by sweating, we also lose sodium. Rapidly drinking large quantities of water

decreases plasma sodium concentration initially, then see decrease in ISF as well.

Water is drawn into cells This increases ISF tonicity, and water is drawn

from blood Add salt when replacing fluids like this!

Water toxicity

Can occur if I.V. fluids are given too rapidly or in too large amounts.

Extra fluid puts strain on heart

Overhydration

Water that moves back into capillaries depends on concentration of plasma proteins.

Decrease in blood proteins caused by: Dietary deficiency in proteins Liver failure Blockage of lymphatic system Increased capillary permeability

Burns, infection

Fluid moves from the blood to the interstitial fluid. Get large amounts of fluid in the intercellular

spaces – Edema

Of the three main compartments (IVF, ICF and ISF) the interstitial fluid varies the most.

Can be caused by: Decrease in plasma proteins Retention of electrolytes, esp. Na+ Increase in capillary blood pressure

Edema

Cations – positively charged ions Anions – negatively charged ions Body fluids also contain charged organic

molecules Only a small percentage of molecules in

fluids are non-electrolytes: glucose, urea, creatinine

Electrolyte Balance

Certain ions control osmosis of water between body compartments

Ions help maintain acid-base balance necessary for cellular activity

Ions carry electric current, which allows for action potentials and secretion of neurotransmitters

Several ions are cofactors needed for optimal activity of enzymes

Functions of electrolytes

Food and water Produced by metabolism Salt craving

Electrolyte intake

Sweat Feces Urine

Electrolyte loss

The total concentration of dissolved particles determines osmolarity.

Glucose – one dissolved particle NaCl – dissolves into two particles One mole of NaCl = 2 osmoles Osmoles/L = osmolarity of solution

Osmolarity

90 % of extracellular cations and half the osmolarity of extracellular solutions

Necessary for action potentials in nerve & muscle cells

Aldosterone increases reabsorption from DCT and collecting ducts ↓ blood volume, ↓ extracellular Na+ ,↑

extracellular K+ ANP causes loss of Na+

Sodium (Na+)

Most numerous intracellular cation Membrane potential and repolarization Controlled by aldosterone – causes loss of

K+ in urine

Potassium (K+)

Part of bone, most abundant mineral in body. 98% of Ca is in bone

Extracellular cation Needed for blood clotting, nerve and muscle

function PTH causes reabsorption of bone and increases

reabsorption from G.I tract and glomerular filtrate Calcitonin inhibits osteoclasts and stimulates

osteoblast, so calcium is removed from blood

Calcium (Ca++)

Most common extracellular anions

Cl- diffuses easily between compartments – can help balance charges (RBC’s)

Parietal cells in stomach secrete Cl- & H+

Aldosterone indirectly adjusts Cl- when it

increases the reabsorption of Na+ - Cl- follows the

Na+

Chloride (Cl-)

Part of the body’s chief buffer and transports CO2 in blood stream.

CO2 + H2O ↔H2CO3 ↔ H+ + HCO3-

The kidneys are the main regulators of bicarbonate: they form bicarb when levels are low and excrete it when levels are high.

Bicarbonate (HCO3-)

Like calcium, most of the phosphate is found in bones. 15% is ionized

Found in combination with lipids, proteins, carbohydrates, nucleic acids and ATP.

Three different forms Part of the phosphate buffer system PTH causes phosphate to be released from bones and to

be excreted by the kidneys. Calcitonin removes phosphate by encouraging bone formation.

Phosphate (HPO42-)

pH – negative log of H+ concentration Affects functioning of proteins (enzymes) Can affect concentrations of other ions Modify hormone actions (proteins)

Acid-Base Balance

Foods Produced by cellular metabolism

Acid intake

Acids and bases that ionize (break apart) completely are strong acids and bases. (HCl; NaOH)

Acids and bases that do not completely dissociate in solution are weak acids and bases. (lactic acid, carbonic acid)

Strengths of Acids and Bases

Remember, blood needs to stay between 7.35 and 7.45 for the body to function properly.

Since more acids than bases are formed, pH balance is mainly a matter of controlling excess H+.

1. Buffer systems2. Exhalation of carbon

dioxide3. Kidney excretion

Control of Acid-Base Balance

Are pairs of chemical substances that prevent a sharp change in the pH of a solution.

Buffers exchange strong acids for weaker acids that do not release as much H+ and thus change the pH less.

Buffers

NaHCO3 + H2CO3

sodium bicarbonate carbonic acid

Addition of a strong acid: HCl + NaHCO3 → H2CO3 + NaCl

Carbonic acid does not dissociate completely, and pH is changed much less.

Bicarbonate Buffer System

Addition of a strong base: NaOH + H2CO3 → NaHCO3 + H2O

Water dissociates very little, and pH remains nearly the same.

Usually the body is called upon to buffer weaker organic acids, such as lactic acid.

Carbonic acid is formed, and amount of bicarbonate ion decreases.

Blood needs to maintain a 20:1 ratio of bicarbonate ion : carbonic acid.

H+ concentration increases slightly pH drops slightly

Carbonic acid is the most abundant acid in the body because it is constantly being formed by buffering fixed acids and by:

H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-

Is present in extracellular and intracellular fluids, most important in intracellular fluids and renal tubules.

H+ + HPO42- → H2PO4

-

monohydrogen dihydrogen phosphate phosphate OH- + H2PO4

- → H2O + HPO42-

Phosphate Buffer System

The most abundant in body cells and plasma.

Carboxyl group -COOH ↔ -COO- + H+

Amino group –NH2 ↔ -NH3+

Protein Buffer System

Because carbonic acid can be eliminated by breathing out CO2 it is called a volatile acid.

Body pH can be adjusted this way in about 1-3 minutes

pH also affects breathing rate Powerful eliminator of acid, but can only deal

with carbonic acid.

Respiratory Mechanisms – Exhalation of CO2

Metabolic reactions produce large amounts of fixed acids. Kidneys can eliminate larger amounts of acids than the

lungs Can also excrete bases Can excrete acids while conserving bicarbonate ion Can produce more bicarbonate ion Kidneys are the most effective regulators of pH; if kidneys

fail, pH balance fails

Kidney excretion of H+

Buffers are the first line of defense because they work almost instantaneously.

Secondary defenses take longer to work: Respiratory mechanisms take several minutes to

hours Renal mechanisms may take several days

The regulators work at different rates

The normal blood pH range is 7.35 – 7.45 Any pH below this range is considered to be a

condition of acidosis Any pH above this range is considered to be a

condition of alkalosis The body response to acid-base imbalance is

called compensation: Compensation may be complete if the blood pH is brought back to normal, or partial if it is still outside the norms.

pH imbalances

Respiratory acidosis is a carbonic acid excess (blood CO2 is too high)

Respiratory alkalosis is a carbonic acid deficit (blood CO2 is too low)

Compensation would occur through the kidneys

Respiratory problems

Metabolic acidosis is a bicarbonate deficit Metabolic alkalosis is a bicarbonate excess Compensation would occur through changes in

the depth and rate of respiration.

Metabolic problems