clinical biochemistry - chapter 12 water balance and electrolytes

What is the major determinant causing water to be retained within a tissue

osmotic pressure

What is the predominant intracellular electrolyte?

potassium

Electrolytes

substances whose molecules have dissociated into ions when placed in solution

Sodium (Na⁺)

the predominant cation of ECF (extracellular fluid). The major role of sodium is in maintaining the normal water distribution and osmotic pressure of plasma. Changes in sodium concentration result in changes in plasma volume

Aldosterone

The reabsorption of sodium at the DCT in the kidneys is primarily controlled by this. 

Antidiuretic Hormone (ADH)

serves an important role in sodium regulation and is one of the primary regulators of renal water excretion. It is synthesized in the hypothalamus and stored in the posterior pituitary gland. is secreted in response to an increased plasma osmolality and decreased circulating blood volume. increases the permeability of the collecting ducts, leading to increased water reabsorption and excretion of concentrated urine.

Atrial natriuretic peptide (ANP)

a peptide produced in the atrium of the heart and is also involved in sodium regulation. is released in response to persistent hypervolemia and chronic renal failure and acts as a mechanism to promote rapid diuresis (secretion and passage of large amounts of urine) and natriuresis (excretion of abnormal amounts of sodium in the urine)

Hyponatremia

is a condition in which the plasma sodium is < 135 mmol/L. occurs when there is an actual loss of total body sodium. It may be due to either renal or nonrenal losses. Some renal causes may be due to:

• diuretics  which block the reabsorption of Na⁺ and Cl^-, leading to Na⁺ excretion with water

• hypoaldosteronism (decreased aldosterone), causing an impairment of the RAAS system

• aldosterone deficiency, either primary or secondary (renin deficiency)

• Addison's disease, an adrenocortical insufficiency due to the autoimmune destruction of adrenal tissue

Hypernatremia

occurs when the concentration of Na⁺ exceeds > 145 mmol/L as a result of water loss or sodium gain

Potassium

the major intracellular cation with the majority (98%) in the ICF. Only 2% is in the ECF (3.5-5 mmol/L). There are two major physiologic functions of potassium, cell metabolism and neuromuscular excitation

Hypokalemia

a condition that occurs when the plasma K⁺ < 3.5 mmol/L. The symptoms of this disorder is related to the effects of the decrease of muscle and renal function, as well as decreased cardiac conduction. The patient may demonstrate muscle weakness. In the most severe cases, death may result from respiratory failure

Hyperkalemia

concentration of K+ exceeds 5 mmol/L. The symptoms experienced during hyperkalemia include muscle weakness and abnormal cardiac conduction. The effects of marked hypokalemia are most severe on cardiac function and can result in cardiac arrest

Chloride (Cl-)

is the major extracellular anion, with a normal concentration of 99 -109 mmol/L. The two major functions are the maintenance of fluid balance and osmotic pressure.

Chloride shift

a mechanism plays an important role in helping to maintain the ionic balance in erythrocytes when O₂ and CO₂ are exchanged

Hypochloremia

Cl- level is < 99 mmol/L

caused by Gastrointestinal loss and Diuretics

Hyperchloremia

Cl- level is > 109 mmol/L.

originates from the same causes as hypernatremia with one exception: an elevated Cl^- level with a normal Na⁺ level is due to an acid-base disturbance, resulting in metabolic acidosis

Sweat chloride

instead of measurement of plasma chloride, chloride content in sweat is measured. This is used for the diagnosis of cystic fibrosis (CF) and is performed on newborns and pediatric patients.

Bicarbonate/Carbon Dioxide

the second-largest anion fraction in the ECF and is a major component of the total CO₂. functions as the major component of the bicarbonate-carbonic acid buffer system and acts promptly to buffer any sudden changes in blood pH. It also assists in transporting the CO₂ produced from metabolic processes in tissues and delivered to the lungs for exhalation.

Magnesium

the second most abundant intracellular cation. It is not one of the traditional electrolytes routinely measured. catalyzes numerous enzyme reactions involved in the transfer, storage, and use of energy

What is a cofactor in the production of ATP?

Magnesium

an increased anion gap

may indicate metabolic acidosis (the presence of organic acids), diabetic ketoacidosis, ketoacidosis from alcohol abuse, or the presence of toxins such as methanol, ethylene glycol, and aspirin

a decreased anion gap

may indicate hypoalbuminemia

What constituent contributes most to the anion gap?

sodium

what affects the colligative properties?

the number of particles

Osmolality

the number of moles of particles (solute) per kilogram of water (solvent).  This measurement is independent of the shape, size, or weight of the solute particles.

osmolarity

the number of moles of particles (solute) per liter of water. This measurement is affected by changes in water content, as well as temperature and pressure, unlike osmolality is independent of temperature and pressu

FPO advantages

• Rapid & inexpensive measurements

• Small sample size

• Simple & reliable performance

FPO disadvantages

Samples must be of low viscosity

VPO advantages

• Rapid & inexpensive measurements

• Small sample size

VPO disadvantages

• Less accurate than FPO

• Samples must be of a viscosity

• Cannot be used for volatile solutes like alcohol

MO advantages

• Direct measurement of osmotic pressure

• Good for colloidal solutions

• No limitation on sample concentration

• Determines MW of macromolecules

MO disadvantages

• Time-consuming & difficult

• Requires large sample volume

• Not applicable for small molecules

• Irreproducible results due to clogging of membrane pores