Water, Electrolyte and Acid- Base Balance Chapter 21.
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Transcript of Water, Electrolyte and Acid- Base Balance Chapter 21.
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.