Ch 26, L3

Fluid and Electrolyte & Acid Base Balance

• Sodium, potassium, calcium, and phosphate levels are tightly regulated.

Routes of Electrolyte Entry and Loss

• Electrolyte balance usually refers to salt balance in the body
• Electrolytes control fluid movement
• Electrolytes provide minerals essential for excitability, secretory activity, and membrane permeability.
• Salts enter the body in foods and fluids, with small amounts generated during metabolic activity
• Obtaining enough electrolytes is usually not a problem (NaCl)
• Salts are lost from the body in sweat, feces, urine, and vomit
• Gastrointestinal disorders can lead to large salt losses in feces or vomitus
• Renal mechanisms regulate the electrolyte balance of the blood
• Hyper refers to too much salt, while hypo refers to too little salt

Importance of Sodium

• Sodium is central to fluid and electrolyte balance and overall body homeostasis
• Sodium is the most abundant cation in the ECF and the only one exerting significant osmotic pressure
• Cellular plasma membranes are relatively impermeable to Na+
• Sodium control ECF volume and water distribution in the body
• Water follows salt - a change in plasma Na+ levels affects plasma volume, blood pressure, ICF, and IF volumes
• Concentration of Na+ in the ECF determines the osmolality of ECF fluids and influences electrical excitability of neurons and muscles.
• Water moves by osmosis into or out of the ICF, counteracting changes in Na+ concentration, leading to stable state
• ADH and thirst mechanisms control ECF Na+ concentration by controlling water loss or gain
• The total amount of Na+ in the body determines the ECF volume and therefore blood pressure.
• Renin-angiotensin-aldosterone and ANP mechanisms, which control Na+ reabsorption and excretion, regulate Na+ content

Aldosterone & Angiotensin II

• Aldosterone regulates sodium ions and part of the renin-angiotensin-aldosterone mechanism
• When aldosterone concentrations are high virtually all the remaining filtered Na+ is actively reabsorbed in the distal convoluted tubules and collecting ducts
• Water follows Na+ when aldosterone concentrations are high
• Virtually no Na+ reabsorption occurs beyond the distal convoluted tubule when aldosterone concentrations are low
• Urinary excretion of large amounts of Na+ results in the excretion of large amounts of water
• The reverse is not true
• Substantial amounts of nearly sodium-free urine can be eliminated to achieve water balance
• The renin-angiotensin-aldosterone mechanism, mediated by the juxtaglomerular complex (JGC) of nephrons, triggers aldosterone release from the adrenal cortex
• Elevated K+ concentration in the ECF can directly stimulate adrenal cortical cells to release aldosterone
• Release of aldosterone results in increased reabsorption of Na+ and increased secretion of K+
• Aldosterone brings about its effects slowly

Mechanisms of Sodium Balance

• Low blood volume and blood pressure trigger renin release from the granular cells of the JGC via sympathetic stimulation
• Low blood volume and blood pressure trigger renin release from the granular cells of the JGC when there is decreased filtrate NaCl concentration
• Low blood volume and blood pressure trigger renin release from the granular cells of the JGC due to decreased stretch of the granular cells of the afferent arterioles
• Renin catalyzes the production angiotensin II
• Angiotensin II stimulates the adrenal cortex to release aldosterone and also directly increases Na+ reabsorption by kidney tubules

Atrial Natriuretic Peptide (ANP)

• ANP reduces blood pressure and blood volume by inhibiting vasoconstriction and Na+ and water retention
• Is a hormone released by certain cells of the heart atria when they are stretched by elevated blood pressure with diuretic and natriuretic effects

Potassium, Calcium, & Anion Balance

• Potassium is the chief intracellular cation, normal neuromuscular functioning essential metabolic activities
• Small changes in K+ concentration in the ECF have effects on neurons and muscle (AP/Contraction)
• The heart is sensitive to K+ levels and hyperkalemia and hypokalemia can disrupt electrical conduction in the heart, leading to sudden death
• Potassium is part of the body’s buffer system, which resists changes in the pH of body fluids
• K+ balance is maintained by renal mechanisms

Calcium & Anions

• About 99% of the body’s calcium is found in bones as calcium phosphate salts, a dynamic reservoir from which calcium and phosphate can be withdrawn to maintain the balance of these electrolytes in the ECF
• ECF calcium ion levels are closely regulated by parathyroid hormone (PTH) and rarely deviate from normal limits
• Chloride is the major anion accompanying Na+ in the ECF
• Most other anions, such as sulfates and nitrates, have transport maximums
• When concentrations in the filtrate exceed the amount that can be reabsorbed, the excess spills into urine