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Rod R. SeeleyIdaho State University

Trent D. StephensIdaho State University

Philip TatePhoenix College

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Chapter 27Chapter 27

Lecture OutlineLecture Outline**

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Chapter 27

Water, Electrolytes, and

Acid-Base Balance

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Body Fluids

• Intracellular fluid compartment– All fluids inside cells of body– About 40% of total body weight

• Extracellular fluid compartment– All fluids outside cells– About 20% of total body weight– Subcompartments

• Interstitial fluid and plasma; lymph, CSF, synovial fluid• Primary intracellular ions, interstitial fluid ions, and plasma ions

– Intracellular cation = K+

– Interstitial fluid cation = Na+

– Plasma cation = Na+

– Intracellular anion = Phosphate– Interstitial fluid = Cl-

– Plasma anion = Cl-

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Body Fluid Compartments

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Regulation of Water Content

• Content regulated so total volume of water in body remains constant

• Kidneys are primary regulators of water excretion

• Regulation processes– Osmosis– Osmolality– Baroreceptors– Learned behavior

• Sources of water– Ingestion

– Cellular metabolism

• Routes of water loss– Urine

– Evaporation• Perspiration

• Respiratory passages

– Feces

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Extracellular Fluid Osmolality• Osmolality

– Measure of water vs. solute concentration; the higher the solute concentration, the higher the osmolality

– Adding or removing water from a solution changes osmolality

• Increased osmolality: triggers thirst and ADH secretion

• Decreased osmolality: inhibits thirst and ADH secretion

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Hormonal Regulation of Blood Osmolality

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Regulation of ECF Volume• ECF can increase or decrease even if osmolality of

extracellular fluid is maintained• Carotid sinus and aortic arch baroreceptors monitor blood

pressure, juxtaglomerular apparatuses monitor pressure changes, receptors in walls of atria and large vessels respond to small changes in BP

• These receptors activate these mechanisms– Neural: increase in BP recognized by baroreceptors. Decreased

sympathetic stimulation of afferent arteriole leads to increased pressure in glomerulus leading to increased filtration and increased urine output.

– Renin-angiotensin-aldosterone– Atrial natriuretic hormone (ANH)– Antidiuretic hormone (ADH)

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Increase in Blood Volume

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Decrease in Blood Volume

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Regulation of ECF Volume

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Regulation of Electrolytes in ECF• Electrolytes

– Molecules or ions with an electrical charge

• Ingestion adds electrolytes to body

• Kidneys, liver, skin, lungs remove from body

– Concentration changes only when growing, gaining or losing weight

• Na+ Ions– Dominant ECF cations– Responsible for 90-95% of

osmotic pressure

• Regulation of Na+ ions– Kidneys major route of

excretion– Small quantities lost in sweat

{sweat = (in decreasing amounts) water, Na+, urea, Cl-

K+, NH3}. Insensible perspiration is water evaporating from skin. Sensible perspiration is secreted by the sweat glands. Contains solutes

• Terms– Hypernatremia: elevated

plasma Na+ – Hyponatremia: decreased

Na+

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Regulation of ICF and ECF

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Regulation of Chloride, Potassium, Magnesium Ions

• Chloride ions– Predominant anions in ECF

• Magnesium ions– Capacity of kidney to

reabsorb is limited

– Excess lost in urine

– Decreased extracellular magnesium results in greater degree of reabsorption

• Potassium ions– Maintained in narrow range

– Affect resting membrane potentials

– Aldosterone increases amount secreted

• Terms– Hyperkalemia: abnormally

high levels of potassium in extracellular fluid

– Hypokalemia: abnormally low levels of potassium in extracellular fluid.

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Potassium Ion Regulation in ECF

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Regulation of Calcium Ions

• Regulated within narrow range– Elevated extracellular

levels prevent membrane depolarization

– Decreased levels lead to spontaneous action potential generation

• Terms– Hypocalcemia– Hypercalcemia

• PTH increases Ca2+ extracellular levels and decreases extracellular phosphate levels

• Vitamin D stimulates Ca2+ uptake in intestines

• Calcitonin decreases extracellular Ca2+ levels

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Regulation of Calcium Ions

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Regulation of Blood Magnesium

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Regulation of Phosphate Ions

• Under normal conditions, reabsorption of phosphate occurs at maximum rate in the nephron

• An increase in plasma phosphate increases amount of phosphate in nephron beyond that which can be reabsorbed; excess is lost in urine

– Hypophosphatemia: reduced absorption from intestine due to vitamin D deficiency or alcohol abuse.

– Hyperphosphatemia: renal failure, chemotherapy, hyperparathyroidism (secondary to elevated plasma calcium levels)

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Regulation of Blood Phosphate

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Acids and Basesand Buffers

• Acids– Release H+ into

solution

• Bases– Remove H+ from

solution

• Acids and bases– Grouped as strong or

weak

• Buffers: Resist changes in pH– When H+ added, buffer

removes it– When H+ removed, buffer

replaces it

• Types of buffer systems– Carbonic acid/bicarbonate– Protein– Phosphate

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Comparison of Strong and Weak Acids

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Regulation of Acid/Base Balance• Buffers: if pH rises, buffers bind H+; if pH falls,

buffers release H+

– Protein buffer: Intracellular and plasma proteins absorb H+. Provide ¾ of buffering in body. E.g., hemoglobin.

– Bicarbonate buffering system: Important in plasma– Phosphate buffer system: important as an intracellular

buffer• Respiratory center: if pH rises, respiratory rate

decreases; if pH falls, respiratory rate increases• Kidneys: if pH rises, distal tubule decreases H+

secretion into the urine and decreases HCO3-

absorption into the blood (more H2CO3 will dissociate into H+ and HCO3

-); if pH falls, distal tubule increases H+ secretion into the urine and increases HCO3

- absorption into the blood

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Increase in Blood pH

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Decrease in Blood pH

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Respiratory Regulation ofAcid-Base Balance

• Achieved through carbonic acid/bicarbonate buffer system– As carbon dioxide levels increase, pH decreases

– As carbon dioxide levels decrease, pH increases

– Carbon dioxide levels and pH affect respiratory centers

• Hypoventilation increases blood carbon dioxide levels

• Hyperventilation decreases blood carbon dioxide levels

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Respiratory Regulation ofAcid-Base Balance

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Renal Regulation of Acid-Base Balance

• Secretion of H+ into filtrate and reabsorption of HCO3

- into ECF cause extracellular pH to increase

• HCO3- in filtrate reabsorbed

• Rate of H+ secretion increases as body fluid pH decreases or as aldosterone levels increase

• Secretion of H+ inhibited when urine pH falls below 4.5

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Kidney Regulation of Acid-Base Balance

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Hydrogen Ion Buffering

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Acidosis and Alkalosis• Acidosis: pH body fluids below 7.35

– Respiratory: Caused by inadequate ventilation- reduced elimination of CO2, asthma, damage to respiratory center in brain, emphysema.

– Metabolic: Results from all conditions other than respiratory that decrease pH- diarrhea, vomiting, ingesting overdose of aspirin, untreated diabetes mellitus, anaerobic respiration

• Alkalosis: pH body fluids above 7.45– Respiratory: Caused by hyperventilation, high altitude (reduced

partial pressure of O2

– Metabolic: Results from all conditions other than respiratory that increase pH- severe vomiting, too much aldosterone, ingestion of substances like bicarbonate of soda.

• Compensatory mechanisms

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