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.

Electrolytes• Cations: sodium, potassium, hydrogen,

magnesium, and calcium• Anions: chloride, bicarbonate, phosphate, and

sulfate

Non-electrolytes• Glucose• Urea• Protein• Lipids• Creatinine

Exchange between blood and interstitial spaces

Exchange between extracellular fluids and cells

Obligatory Water Losses• Skin and lungs• Urine 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

Aldosterone ANP Baroreceptors Other hormones:

• Estrogen=enhances Na+ reabsorption• Progesterone=decreases Na+ reabsorption• Glucocorticoids=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 aldosterone• Juxtaglomerular 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 hypothalamus

Sympathetic 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 pituitary

Mechanism:• decrease in sodium concentration in plasma (decreased

osmolarity)• stimulates osmoreceptors in hypothalamus• stimulates posterior pituitary to release ADH• ADH targeted toward distal and collecting tubules of

kidney• the effect is increased water resorption• plasma volume increases, osmolarity decreases• scant 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 Balance Potassium 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 mechanisms

Potassium 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 Balance 99% of calcium found in bones as an apatite Calcium 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 matrix small 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 absorption

Kidneys - 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 Hormone• Decreases plasma phosphate concentrations

while increasing calcium concentrations

Influence of Calcitonin• Increases plasma phosphate concentration

while decreasing calcium concentrations

Magnesium• PTH increases plasma magnesium

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

Anions• Chloride is indirectly increased by

Aldosterone because it passively follows sodium

Respiratory acidosis Increased CO2=increased H+=decreased pH Hypoventilation To compensate: increase excretion of H+ or by

increased reabsorption of HCO3-

Respiratory alkalosis Decreased CO2=decreased H+=increased pH Hyperventilation To compensate: decreased H+ excretion or by

decreased reabsorption of HCO3-

Metabolic acidosis Decreased HCO3

-=increased H+=decreased pH Diarrhea, ketosis, renal dysfunction Hyperventilation

Metabolic alkalosis Increased HCO3

-=decreased H+=increased pH Vomiting, diuretics, alkaline drug use Hypoventilation

Three Types:Bicarbonate Buffers

Phosphate 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 + Na2HPO4 NaH2PO4 + 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 System

Renal Buffering System

Respiratory System• Rising plasma H+ causes deeper, rapid breathing which

decreases CO2 blood thereby decreasing H+ ions.

Renal System• To 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.