Body Fluids and Water Movement

Distribution of Body Fluids

• Total body water (TBW) comprises intracellular fluid, extracellular fluid, interstitial fluid, intravascular fluid, lymph, synovial fluid, intestinal fluid, cerebrospinal fluid (CSF), sweat, urine, pleural fluid, peritoneal fluid, pericardial fluid, and intraocular fluids.
• In pediatrics, the body consists of 75% to 80% water, making them highly susceptible to changes in body fluids and dehydration, especially newborns.
• Aging causes a decreased percentage of total body water, decreased free fat mass and muscle mass, renal decline, and diminished thirst perception.

Water Movement Between Fluid Compartments

• Oncotic pressure refers to the pressure exerted by plasma proteins on capillary walls.
• Osmotic pressure is the external pressure needed to prevent solvent movement across a membrane.
• Aquaporins are pores in cell membranes that selectively conduct water molecules while preventing ion passage (sodium and potassium).
• Starling forces dictate that net filtration equals forces favoring filtration minus forces opposing filtration.

Fluid Movement Between Plasma and Interstitial Space

• Capillary fluid movement occurs via net filtration.
• Capillary hydrostatic pressure drives fluid out through capillary pores into the interstitial space.
• Interstitial oncotic pressure displaces fluid, as plasma proteins in the interstitial space draw fluid out of the capillaries.
• Interstitial hydrostatic pressure increases and drives fluid to be reabsorbed into the capillaries.
• Capillary oncotic pressure concentrates plasma protein in the capillary.

Net Filtration

• Capillary hydrostatic pressure (blood pressure) and interstitial oncotic pressure (water-pulling) favor filtration.
• Plasma (capillary) oncotic pressure (water-pulling) and interstitial hydrostatic pressure favor reabsorption.

Edema

• Edema involves fluid accumulation within interstitial spaces.
• Edema is caused by increased capillary hydrostatic pressure, decreased plasma oncotic pressure, increased capillary permeability, and lymph obstruction (lymphedema).
• Edema can be localized or generalized.

Sodium and Chloride Balance

• Sodium is the primary extracellular fluid (ECF) cation that regulates osmotic forces and water balance.
• Sodium's roles include neuromuscular irritability, acid-base balance, cellular chemical reactions, and membrane transport.
• Chloride is the primary ECF anion that provides electroneutrality.

Renin-Angiotensin-Aldosterone System

• Sodium balance is controlled and regulated by aldosterone.
• Aldosterone increases sodium resorption from urine back into the blood at the distal tubule of the kidney.
• Aldosterone is a hormone secreted by the renal cortex that acts on kidneys to promote sodium retention and potassium excretion.
• Increased sodium causes increased water and increases blood pressure.
• Renin, secreted by the kidneys in response to low blood pressure, activates angiotensin to promote aldosterone secretion, which leads to increased blood pressure.

Naturetic Peptides

• Atrial natriuretic hormone (peptide) (ANP) is released in response to high blood pressure.
• Increased pressure stretches the heart wall, causing cardiac muscle in the atria to release ANP.
• ANP acts as an antagonist to aldosterone and increases sodium excretion in the urine and reduces blood volume and pressure.

Water Balance

• Antidiuretic hormone (ADH) is secreted by the posterior pituitary in response to low blood pressure, increased plasma osmolality, or decreased circulation blood volume.
• ADH promotes water retention by the kidneys to regulate water balance.
• Osmolality receptors detect hyperosmolality and plasma volume depletion, initiating the thirst mechanism.
• Volume receptors consist of baroreceptors that also contribute to thirst perception.

Alterations in Sodium, Chloride, and Water Balance

• Isotonic alterations involve total body water changes with proportional electrolyte and water changes but without concentration changes.
• Isotonic fluid loss causes dehydration and hypovolemia with loss of ECF, causing weight loss, dry skin and mucous membranes, and decreased urine output.
• Signs of hypovolemia include rapid heart rate, flattened neck veins, and normal or decreased blood pressures.
• Isotonic fluid excess causes hypervolemia, which results from excess IV fluid administration, hypersecretion of aldosterone, and drugs like cortisone.
• Hypervolemia results from weight gain with diluting effects leading to decreased hematocrit.
• Indicators of hypervolemia are distended neck veins, increased blood pressure, and edema.

Hypertonic Alterations

• Hypernatremia is an excess of sodium, where serum sodium is greater than 145 mEq/L and relates to sodium gain or water loss.
• Hypernatremia causes water movement from the ICF to the ECF, leading to intracellular dehydration.
• Clinical manifestations of hypernatremia include thirst, weight gain, bounding pulse, and increased blood pressure.
• Central nervous system manifestations include muscle twitching and hyperreflexia (hyperactive reflexes), confusion, coma, convulsions, and cerebral hemorrhage.
• Hyperchloremia is an excess of chloride with serum chloride greater than 105 mEq/L, occurring alongside hypernatremia or bicarbonate deficit that occurs in metabolic acidosis.
• No specific treatment or symptoms exist other than the management of blood glucose.

Hypotonic Alterations

• Hyponatremia is a decreased sodium state with serum sodium <135 mEq/L, decreased osmolality (hypoosmolality), and decreased extracellular solutes (Na+, Cl-).
• Hyponatremia causes decreased sodium, which decreases the ECF osmotic pressure.
• Water moves into the cell via osmosis, causing cells to swell via hypotonicity.
• Clinical manifestations of hyponatremia include muscle twitching, depressed reflexes, weakness, nausea, and vomiting.
• The central nervous system shows life-threatening cerebral edema and increased intracranial pressure.
• Free water excess is caused either by compulsive water drinking, syndrome of inappropriate ADH, or ectopic ADH secretion due to hypovolemia or hyperosmolality or hyponatremia with hypervolemia.
• Manifestations: Cerebral edema (with confusion and convulsions), weakness, muscle twitching, nausea, headache, and weight gain.
• Hypochloremia presents due to hyponatremia or elevated bicarbonate concentration, develops from vomiting and the loss of HCl, and occurs in cystic fibrosis due to lack of adequate chloride pump.
• Treatment involves addressing its underlying cause.

Potassium Balance

• Potassium is a major intracellular cation.
• Concentration maintained by Na+/K+ pump.
• It is essential for transmission and conduction of nerve impulses, normal cardiac rhythms, and skeletal and smooth muscle contraction.
• Changes in pH impact K+ balance, where hydrogen ions accumulate in the ICF during acidosis, causing K+ shifts out to maintain cation balance, resulting in hyperkalemia.
• Serum potassium levels are influenced by aldosterone, insulin, and epinephrine.
• The kidney is the most efficient regulator.
• Gradual changes are easier to tolerate than rapid changes.
• Hypokalemia presents at potassium levels of <3.5 mEq/L.
• Potassium balance is reflected in plasma potassium levels.
• It is caused by reduced potassium intake, increased potassium entering the cells, and increased potassium loss.
• Manifestations depend on rate and severity, including membrane hyperpolarization that decreases neuromuscular excitability, skeletal and smooth muscle weakness, and cardiac dysrhythmias.
• Hyperkalemia presents at potassium levels of >5.5 mEq/L
• Generally rare because of efficient renal excretion.
• It results from increased uptake of potassium, or the shift of K+ from ICF into ECF, decreased renal excretion, insulin deficiency, or cell trauma.
• Mild attacks of hyperkalemia cause increased neuromuscular irritability, restlessness, intestinal cramping, and diarrhea.
• Severe attacks decrease the resting membrane potential, muscle weakness, loss of muscle tone, and paralysis.

Calcium and Phosphate

• Calcium and phosphate concentrations are rigidly controlled by parathyroid hormone (PTH), vitamin D, and calcitonin.
• Hypocalcemia is caused by inadequate intestinal absorption, deposition of ionized calcium into bone or soft tissue, decreased PTH and vitamin D, and nutritional deficiencies from inadequate sources of dairy products or green.
• Hypocalcemia results in increased neuromuscular excitability, tingling, muscle spasms (particularly in hands, feet, and facial muscles), intestinal cramping, and hyperactive bowel sounds.
• Severe cases show convulsions and tetany, with prolonged QT intervals and cardiac arrest.
• Hypercalcemia is caused by hyperparathyroidism, bone metastases with calcium resorption from breast, prostate, renal, and cervical cancer, sarcoidosis, excess vitamin D, as well as many tumors that produce PTH.
• Hypercalcemia causes many nonspecific effects: e.g. fatigue, weakness, lethargy, anorexia, nausea, and constipation.
• Impaired renal function and kidney stones may result.
• Most phosphate are located in the bone.
• Provides energy for muscle contraction
• Regulated by parathyroid hormon, vitamin D3, and calcitonin.
• Normal Value = 2.5-5.0 mg/dl
• Hypophosphatemia is caused from Intestinal malabsorbtion (vitamin D deficiency, Magnesium- and Aluminum containing antacids, Long-term alcohol abuse( and malabsorbtion syndromes, respiratory alkolosis, and increased renal excretion associated with hyperparathyroidism.
• Reduced capacity of O2 transport by red blood cells, thus disturbed energy metabolism as well as Leukocyte, platelet dysfunction.

Acid-Base Balance

• is carefully regulated to maintain a normal pH.
• H+ must be neutralized or excreted to maintain normal arterial blood pH of 7.35-7.45.
• Kidneys are major organs in the regulation of ph as well as Lungs,and Bones.
• Volatile acids - H2CO3 (carbonic acid) are eliminated as CO2 gas.
• Nonvolatile acids - Sulfuric, phosphoric, and other organic acids are eliminated by the renal tubules with the regulation of HCO3- (bicarbonate acid).
• A buffer - a chemical that can bind excessive or OH– without significant change of pH

Carbonic Acid-Bicarbonate Pair

• Works in both lungs and kidneys
• The greater the partial pressure of carbon dioxide, the more carbonic acid is formed
• The ratio of bicarbonate to carbon acid at 7.4 is 20.1
• The resiratory system compensates by increasing the rate of breathing

Other balance mechanisms

• Protein buffering (hemoglobin)
• Proteins have negative charges, so they can serve as buffers for H+
• Renal buffering - secretion of h+ in the urine, and reabsorbtion of HC03
• Normal blood PH is between 7.35 and 7.45 and determined by arterial blood gas sampling
• Acidosis is a systemic increase in h+ or decrease in bicarbonate base.
• Acidosis causes nervous system depression.
• Alkalosis is a systemic decrease in h+ or increase in bicarbonate base.
• Alkalosis can cause nervous system irritability.
• Respiratory acidosis—elevation of pco2 as a result of ventilation depression
• Respiratory alkalosis—depression of pco2 as a result of alveolar hyperventilation
• Metabolic acidosis—depression of HCO3– or an increase in noncarbonic acids
• Metabolic alkalosis—elevation of HCO3- usually caused by an excessive loss of metabolic acids
• Metabolic acidosis - kidneys cant get rid of acid buildup.
• Respiratory acidosis occurs when lungs do nt properly illuminate CO2.
• Alkalosis is body fliuds have excess base(alkali) opposite of ccess of an acid.