27-1

Chapter 27

Water, Electrolytes, and

Acid-Base Balance

27-2

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

27-4

Hormonal Regulation of Blood Osmolality

27-5

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 leads to increased pressure in glomerulus leading to increased filtration and increased urine output.

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

27-6

Increase in Blood Volume

27-7

Decrease in Blood Volume

27-8

Regulation of ECF Volume

27-9

Regulation of Electrolytes in ECF• Electrolytes

– Molecules or ions with an electrical charge

• Ingestion adds electrolytes to body

• Kidneys, liver, skin, 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+

27-10

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.

27-11

Potassium Ion Regulation in ECF

27-12

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

27-13

Regulation of Calcium Ions

27-14

Regulation of Blood Magnesium

27-15

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

27-16

Comparison of Strong and Weak Acids

27-17

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; if pH falls, distal tubule increases H+ secretion into the urine

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Increase in Blood pH

27-19

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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Kidney Regulation of Acid-Base Balance

27-23

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.