Water, Electrolyte and Acid-Base Balance
Jan 12, 2016
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