Water, Electrolyte and Acid-Base Balance

Chapter 21

• Balance – a state of equilibrium – substances are maintained in the right amounts and in the right place in the body

Water Balance

• 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.

• 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%

Fluid compartments

• 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

Composition of 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

Movement of water

• Hydrostatic pressure – pressure of fluids

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

pressure (COP)

Water intake = Water loss• 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

Regulation of water intake

• Main regulator is thirst.

• Dehydration (output>intake) as little as 1% decrease in body water causes:– Decreased production of saliva– Increased blood osmotic pressure –

stimulates osmoreceptors in the hypothalamus

– Decreased blood volume – renin is produced

• The 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.

Sources of water loss

• Through kidneys in urine – 1500 ml (60%)

• Through intestines - 150 ml (6%)– Can be significant in vomiting and diarhhea

• From skin (sweat) - 150 ml (6%)

• From lungs and skin 700 ml (28%)

• Last is called insensible loss– (menstruation)

Regulation of Water Output

• 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

• 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

Water toxicity• 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!

Overhydration

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

• Extra fluid puts strain on heart

• 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.

Edema

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

Electrolyte Balance

• 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

Functions of electrolytes• Certain ions control the osmosis of water

between body compartments

• Ions help maintain the 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 the optimal activity of enzymes

Electrolyte intake

• Food and water

• Produced by metabolism

• Salt craving

Electrolyte loss

• Sweat

• Feces

• Urine

Osmolarity

• 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

Sodium (Na+)

• 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+

Potassium (K+)

• Most numerous intracellular cation

• Membrane potential and repolarization

• Controlled by aldosterone – causes loss of K+ in urine

Calcium (Ca++)• 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

Chloride (Cl-)

• 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+

Bicarbonate (HCO3-)

• 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.

Phosphate (HPO42-)

• Like calcium, most of the phosphate is found in the 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.

Acid-Base Balance

• pH – negative log of H+ concentration

• Affects functioning of proteins (enzymes)

• Can affect concentrations of other ions

• Modify hormone actions (proteins)

Acid intake

• Foods

• Produced by cellular metabolism

Strengths of Acids and Bases

• 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)

• 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+.

Control of Acid-Base Balance

1. Buffer systems

2. Exhalation of carbon dioxide

3. Kidney excretion

Buffers

• 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.

Bicarbonate Buffer System• 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.

• 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-

Phosphate Buffer System

• 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 + HPO4

2-

Protein Buffer System

• The most abundant in body cells and plasma.

• Carboxyl group -COOH ↔ -COO- + H+

• Amino group –NH2 ↔ -NH3+

Respiratory Mechanisms – Exhalation of CO2

• 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.

Kidney excretion of H+

• 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

The regulators work at different rates

• 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

pH imbalances

• 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.

Respiratory problems

• 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

Metabolic 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.