Lecture 12 fluid, electrolyte and acid base balance

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Chapter 22

Fluid, Electrolyte and Acid-Base Balance


End of Chapter 22

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Fluid Compartments Total body water = 55-60% of lean body

mass Remainder: solid parts of bone, muscles, tendons

Major compartments (3): ICF, IF, plasma Intracellular fluid (ICF): inside cells= 2/3 Extracellular Fluid (ECF): outside cells = 1/3

Interstitial fluid (IF): 80% of ECF Includes: lymph; cerebrospinal, synovial, pericardial,

pleural and peritoneal fluids; fluid in eyes and ears Blood plasma: 20% ECF


Fluid Compartments


Barriers Between Compartments Plasma membrane: between ECF and ICF

Blood vessel walls: between plasma and interstitial fluid

Fluid balance correct distribution of water & solutes

Water redistributes rapidly by osmosis Thus fluid balance depends on solute

(electrolyte) balance


Fluid Balance Fluid balance requires

Appropriate total volume of body fluid Appropriate distribution of water and solutes

Fluid balance depends on solute (electrolyte and nonelectrolyte) balance Fluids and electrolytes are closely linked

Water redistributes rapidly by osmosis


Fluid BalanceInteractions Animations

Water and Fluid Flow

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Water Gain and Loss Gain: ingestion + metabolic reactions

Ingestion (food and drink): 2300 mL/day Metabolism: 200 mL/day

Gain should = loss Daily intake = daily output. Both 2500 mL/day

Loss: skin, lungs, kidneys, GI tract Kidneys: ~1500 mL/day Skin: sweat evaporates ~600 mL/day Lungs: 300 mL/day; more if fever GI tract: ~100 mL/day; more if diarrhea


Water Balance


Regulation of Gain Thirst center in hypothalamus ~2% dehydration will cause BP

Increase in body osmolality dry mouth thirst Hormonal responses

High osmolality hypothalamus releases ADH water retention by kidneys

BP renin released from kidney angiotensin II aldosterone water retention by kidneys

Sensation of thirst may be decreased, especially in elderly


Regulation of Gain


Regulation of Salt and Water Loss Urinary NaCl loss mainly determines body fluid volume

Na+ = main solute in ECF determining osmosis Fluid intake varies so loss must vary also

ANP, angiotensin II and aldosterone regulate ADH regulates water loss


Regulation of Salt and Water Loss


Movement of Fluid ICF and ECF are normally at the same

osmolality Water moves freely interstitial fluid osmolality cell swelling

and vice versa Most often due to Na+ change ADH responds rapidly: prevents significant

cell change


Electrolytes in Body Fluids Functions of electrolytes

1. Confined to compartments; control osmosis

2. Help maintain acid-base balance

3. Carry electrical currents

4. Serve as cofactors for enzymes


Electrolyte Distribution Electrolyte content of ICF and ECF differ

significantly ICF: K+ major cation; protein, HPO4

2-: anions ECF: Na+ major cation; Cl- major anion

Na+/K+ pump maintains the cation difference The two ECF fluids are similar

Electrolytes in plasma similar to those in IF One difference: plasma contains more protein

than interstitial fluid (IF) Colloid osmotic pressure (due largely to plasma

proteins) “holds onto” fluid in capillaries


Electrolyte Distribution


Other Electrolytes K+ high in ICF, low in ECF

Regulated by aldosterone Mg2+ and SO4

2- high in ICF, low in ECF Ca2+ high in ECF, low in ICF

Regulated in plasma (PTH, calcitriol, and calcitonin)

Bones serve as Ca2+ reservoir


Acid- Base Balance Input: diet, products of metabolism

Such as lactic acid, ketones Output

Lungs: exhale CO2

Kidney: can eliminate H+ or HCO3-

Regulatory mechanisms1.Buffers: fastest but incomplete

2.Respiratory responses: fast but incomplete

3.Renal responses: slowest but compete elimination


1: Buffer Systems Protein in cells or plasma

Carboxyl and amino groups of amino acids Hemoglobin (protein) in red blood cells

Carbonic acid-bicarbonate Especially important in plasma CO2 + H2O H2CO3 ↔ HCO3

- + H+

Phosphate H2PO4

- H+ + HPO42-


2: Exhalation of Carbon Dioxide H+ + HCO3

- ↔ H2CO3 ↔ CO2 + H2O

Decrease of CO2 ↔ decrease of H+

Increase of CO2 ↔ increase of H+

Change of rate and depth of ventilation rapidly alters plasma pH

Negative feedback loop regulates


2: Exhalation of Carbon Dioxide


3. Renal Responses Kidney Excretion of H+

Slow but only way to actually eliminate acid or base

Secrete H+ and replace with HCO3-


Imbalances Acidosis: arterial blood pH < 7.35

Depresses CNS Below pH 7.0 can be fatal

Alkalosis: arterial blood pH > 7.45 Overexcitation of CNS Muscle spasms, convulsions

Compensation Respiratory or renal mechanisms Respiratory very rapid; renal slower


Aging Decrease in control of water and electrolyte

balance can lead to pH problems Decreases in respiratory and renal

functioning Decreased capacity to sweat

Education

Lecture 12 fluid, electrolyte and acid base balance