Fluids and Acid Base Physiology

Dr. Meg-angela Christi Amores

• maintenance of a relatively constant volume and a stable composition of the body fluids is essential for homeostasis

• Daily Intake of Water:• (1) it is ingested in the form of liquids or water in the

food, which together normally add about 2100 ml/day to the body fluids• (2) it is synthesized in the body as a result of oxidation

of carbohydrates, adding about 200 ml/day– variable

• Daily Loss of water– Insensible water loss – 700 mL/day• cannot be precisely regulated• continuous loss of water by evaporation from the

respiratory tract and diffusion through the skin– Sweating – 100 mL/day– Feces – 100 mL/day– Urine - variable

Water in the body

• Total body water (TBW) - ~42 liters– Compartments: 1. Intracellular Fluid - 75%2. Extracellular Fluid (ECF) – 25%

– Plasma 25%– Interstitial Fluid 75%

Body Fluid Compartments

– extracellular fluid compartment is balanced between the principal cation—sodium and the principal anions—chloride and bicarbonate

– intracellular fluid compartment is comprised primarily of the cations, potassium and magnesium, and of the anions, phosphate and proteins

Fluid Electrolyte

• primary measurement that is readily available to the clinician for evaluating a patient's fluid status is the plasma sodium concentration

• Na (Sodium)– Hyponatremia – when plasma Na concentration

falls below 142 mEq/L– Hypernatremia

Hyponatremia

• Causes:– Loss of NaCl – diarrhea and vomiting, diuretics• Addison's disease

– excess water retention• excessive secretion of antidiuretic hormone

Hypernatremia

• Causes:– loss of water• Dehydration due to prolonged sweating or exercise

– excess sodium in the extracellular fluid

pH• Precise H+ regulation is essential

because the activities of almost all enzyme systems in the body are influenced by H+ concentration

• Acids - molecules containing hydrogen atoms that can release hydrogen ions in solutions

• Bases - molecules that can accept an H+

pH

• alkalosis refers to excess removal of H+ from the body fluids

• in contrast to the excess addition of H+, which is referred to as acidosis

• pH is inversely related to the H+ concentration

pH

• normal pH of arterial blood is 7.4• The lower limit of pH at which a person can

live more than a few hours is about 6.8, and the upper limit is about 8.0

Regulators of H concentration

• 1) the chemical acid-base buffer systems of the body fluids, which immediately combine with acid or base to prevent excessive changes in H+ concentration;

• (2) the respiratory center, which regulates the removal of CO2 (and, therefore, H2CO3) from the extracellular fluid; and

• (3) the kidneys, which can excrete either acid or alkaline urine, thereby readjusting the extracellular fluid H+ concentration toward normal during acidosis or alkalosis

Acid Base Disturbances

• Acidosis • Alkalosis

• Metabolic• Respiratory

Respiratory Acidosis

• pH below 7.4 caused by respiratory problems• Decreased Ventilation and Increased PCO2

– Increased H2CO3 and H+ concentration, thus resulting in acidosis

• Conditions that damage the respiratory centers or that decrease the ability of the lungs to eliminate CO2

• What are possible causes of decreased ventilatory rate?

• Central area of respiratory control• Peripheral• Voluntary

Respiratory Acidosis

– damage to the respiratory center in the medulla oblongata

– obstruction of the passageways of the respiratory tract

– pneumonia, emphysema, or decreased pulmonary membrane surface area

• compensatory responses:– (1) the buffers of the body fluids and – (2) the kidneys

Respiratory Alkalosis

• caused by overventilation by the lungs• major means for compensation are the

chemical buffers of the body fluids and the ability of the kidneys to increase HCO3

- excretion

Metabolic Acidosis

• (1) failure of the kidneys to excrete metabolic acids

• (2) formation of excess quantities of metabolic acids in the body

• (3) addition of metabolic acids to the body by ingestion or infusion of acids

• (4) loss of base from the body fluids

Metabolic Acidosis

• Renal Tubular Acidosis– defect in renal secretion of H+ or in reabsorption

of HCO3

– impairment of renal tubular HCO3- reabsorption

– inability of the renal tubular H+ secretory mechanism

– renal failure, insufficient aldosterone secretion (Addison's disease),

Metabolic Acidosis

• Severe diarrhea• loss of large amounts of sodium bicarbonate into the

feces

• Vomiting of intestinal contents• Diabetes Mellitus• Ingestion of Acids• Chronic Renal Failure

Metabolic Alkalosis

• excess retention of HCO3- or loss of H+ from

the body• Administration of Diuretics• Excess Aldosterone• Vomiting of Gastric Contents• Ingestion of Alkaline Drugs