Chapter 26

Varies with weight, age, and sex:Early embryo (97%)Newborn (77%)Adult male (60%)Adult female (54%)Elderly (45%)

Adipose tissue versus skeletal muscle largely determines adult differences.

ElectrolytesCations: sodium, potassium, hydrogen, magnesium, and calciumAnions: chloride, bicarbonate, phosphate, and sulfate

Non-electrolytesGlucoseUreaProteinLipidsCreatinine

Exchange between blood and interstitial spaces

Exchange between extracellular fluids and cells

Obligatory Water LossesSkin and lungsUrine and feces

Fluid intake

Diet

Levels of Anti-diuretic hormone (ADH)

Dehydration

Hypotonic hydration

Edema

Regulation of Sodium Balance

Regulation of Potassium Balance

Regulation of Calcium and Phosphate Balance

Regulation of Magnesium Balance

Regulation of Anions

AldosteroneANPBaroreceptorsOther hormones: Estrogen=enhances Na+ reabsorptionProgesterone=decreases Na+ reabsorptionGlucocorticoids=enhances Na+ reabsorption

75-80% of sodium (NaCl) in renal filtrate is reabsorbed in proximal tubules of kidneys.Aldosterone aids in actively reabsorbing remaining Na+Cl- in distal convoluted tubule/collecting tubule by increasing tubule permeability; therefore aldosterone promotes both sodium and water retention Mechanism: increase in K or decease in Na in blood plasma renin-angiotensin Mechanism stimulates adrenal cortex to release aldosterone aldosterone targeted towards the kidney tubules increase in Na reabsorption increase in K secretion restores homeostatic plasma levels of Na and K

Influences on aldosterone synthesis and release:Elevated potassium levels in ECF directly stimulates adrenal cells to secrete aldosteroneJuxtaglomerular apparatus of renal tubes release renin in response to:decreased stretch (due to decrease in blood pressure) decreased filtrate osmolarity sympathetic nervous system stimulation Cardiovascular system

As blood volume (and pressure) rises, the baroreceptors in the heart and in the large vessels of the neck and thorax (carotid arteries and aorta) communicate to the hypothalamusSympathetic nervous system impulses to kidneys decrease, allowing afferent arterioles to dilate; as the glomerular filtration rate rises, sodium and water output increases (causing pressure diuresis)Reduced blood volume and pressure results

Influence of ADH

Amount of water reabsorbed in the distal segments of the kidney tubules is proportional to ADH release (increase in ADH secretion = increase in water resorption)Osmoreceptors of the hypothalamus sense the ECF solute concentrations and trigger or inhibit ADH release from the pituitaryMechanism:decrease in sodium concentration in plasma (decreased osmolarity)stimulates osmoreceptors in hypothalamusstimulates posterior pituitary to release ADHADH targeted toward distal and collecting tubules of kidneythe effect is increased water resorptionplasma volume increases, osmolarity decreasesscant urine produced

Influence of atrial natriuretic factor (ANF) Reduces blood pressure and blood volume by inhibiting nearly all events that promote vasoconstriction and sodium and water retention

The regulatory site of potassium is in the renal tubules

Influence of aldosterone

Influence of plasma potassium concentrations

Regulation of Potassium BalancePotassium is the chief intracellular cation Relative intracellular-extracellular potassium concentrations directly affects a cell's resting membrane potential, therefore a slight change on either side of the membrane has profound effects (ie. on neurons and muscle fibers)Potassium is part of the body's buffer system, which resists changes in pH of body fluids; ECF potassium levels rise with acidosis (decrease pH) as potassium leave cells and fall with alkalosis (increase pH) as potassium moves into cells Potassium balance is maintained primarily by renal mechanismsPotassium reabsorption from the filtrate is constant - 10-15% is lost in urine regardless of need; because potassium content of ECF is low (compared to sodium concentration), potassium balance is accomplished by changing amount of potassium secreted into the filtrate; therefore regulated by collecting tubules

Influence of Parathyroid Hormone

Influence of Calcitonin

Regulation of Calcium Balance99% of calcium found in bones as an apatiteCalcium needed for blood clotting, nerve transmission, enzyme activation, etc...Calcium ion concentration is regulated by interaction of two hormones: parathyroid hormone and calcitonin Calcium ion homeostasis: effects of PTH and calcitonin PTH - released by the parathyroid cells, promotes increase in calcium levels by targeting...bones - PTH activates osteoclasts, which breakdown the matrixsmall intestines - PTH enhances intestinal absorption of calcium ions indirectly by stimulating the kidneys to transform vitamin D to its active form which is a necessary cofactor for calcium absorptionKidneys - PTH increases calcium reabsorption by renal tubes while simultaneously decreasing phosphate ion reabsorption Calcitonin - targets bone to encourage deposition of calcium salts and inhibits bone reabsorption (therefore an antagonist of PTH

Influence of Parathyroid HormoneDecreases plasma phosphate concentrations while increasing calcium concentrations

Influence of CalcitoninIncreases plasma phosphate concentration while decreasing calcium concentrations

MagnesiumPTH increases plasma magnesium concentrations by causing a decrease in the amount of magnesium excreted by the kidneys

AnionsChloride is indirectly increased by Aldosterone because it passively follows sodium

Respiratory acidosisIncreased CO2=increased H+=decreased pHHypoventilationTo compensate: increase excretion of H+ or by increased reabsorption of HCO3-

Respiratory alkalosisDecreased CO2=decreased H+=increased pHHyperventilationTo compensate: decreased H+ excretion or by decreased reabsorption of HCO3-

Metabolic acidosisDecreased HCO3-=increased H+=decreased pHDiarrhea, ketosis, renal dysfunctionHyperventilation

Metabolic alkalosisIncreased HCO3-=decreased H+=increased pHVomiting, diuretics, alkaline drug useHypoventilation

Three Types:Bicarbonate BuffersPhosphate BuffersProtein Buffers

Major extracellular buffering system

HCO3- functions as a weak base while H2CO3 functions as a weak acid.

Example:HCl + NaHCO3- H2CO3 + NaCl

Important in urine and intracellular buffering systems

However NaH2PO4 acts as the weak acid and Na2HPO4 serves as the weak base.

Example:HCl + Na2HPO4NaH2PO4 + NaCl

Most abundant buffering system in the body including intracellular and extracellular compartments.

Carboxyl groups (COOH) and amine groups (NH3) act as either an acid or a base respectively.

Two Types:Respiratory Buffering SystemRenal Buffering System

Respiratory SystemRising plasma H+ causes deeper, rapid breathing which decreases CO2 blood thereby decreasing H+ ions.

Renal SystemTo counteract acidosis, H+ is secreted into the renal tubules and excreted in urine or NH4+ is excreted rather than reabsorbed.

To counteract alkalosis, bicarbonate ions are secreted into the filtrate and H+ is reabsorbed.