Electrolyte Balance

Electrolyte Balance

• state in which amount of electrolytes absorbed by the small intestine balances the amount lost from the body through urine

Major cations

• Na+

• K+

• Ca2+

• Mg2+

• H+

Major anions

• Cl-

• HCO3-

• Pi

Functions of Electrolytes

• Chemically reactive/participate in metabolism 

• Determine electrical potential across membranes 

• Strongly effect osmolarity of body fluids

ECF Concentrations (mEq/L)

• Na+: 145 

• K+: 4 

• Cl-: 103

• Ca2+:5 

ICF Concentrations (mEq/L) 

• Na+: 12

• K+: 150 

• Cl-: 4

• Ca2+: <1 

Sodium Functions

• Resting membrane potential 

• Depolarization for nerve/muscle function 

• Principal cation for ECF

• generates heat 

• cotransport for other solutes

What is the most significant solute in determining total body water and the distribution of water among fluid compartments? 

• sodium 

what cells in the nephron have aldosterone receptors?

• ascending nephron loop

• DCT

• collecting duct 

what hormones effect sodium osmolarity and BP

• aldosterone

• ADH

• naturetic peptides

What are the only cells that have aldosterone receptors?

• ascending limb

• DCT

• cortical part of collecting duct 

What happens when aldosterone binds to nuclear receptors and activates transcription of Na+ -K+ pump?

• Na + concentration in urine decreases

  • Na+ is reabsorbed

• K+ concentration in urine increases

  • K+ is secreted

Aldosterone also supports the movement of what along with sodium?

• Cl- and water

  • both will follow Na+ reabsorption

What is the PRIMARY effect of aldosterone? 

• urine will have less NaCl and more K+

Why does aldosterone have little effect on plasma sodium concentration? 

• water is reabsorbed along with sodium so concentration should not change

what can inhibit the renin-angiotensin-aldosterone mechanism?

• increased Blood pressure

How does ADH differ from aldosterone in its impact on sodium? 

• ADH modifies water excretion independently of sodium excretion 

  • ADH can change sodium concentration 

When is ADH released

• when sodium concentrations in blood increase 

What does ADH do

• cause kidneys to reabsorb more water

• slows down increase in blood sodium concentration 

What can inhibit ADH release 

• drop in sodium concentration 

What do natriuretic peptides do?

• inhibit sodium and water reabsorption 

• inhibit secretion of renin and ADH

What happens when naturetic peptides are secreted?

• kidneys eliminate more water and sodium

• blood pressure decreases 

Hypernatremia causes

• administration of intravenous saline

Hypernatremia effects

• water retention, hypertension, edema

Hyponatremia causes

• excess body water rather than excess sodium excretion 

Hyponatremia effects

• usually quickly corrected by excretion of excess water 

Potassium Functions

• most abundant cation in ICF

• greatest determinant of intracellular osmolarity

• produces RMP and AP of nerve/muscle cells

• cotransport, heat production, cofactor for protein synthesis

How much potassium is reabsorbed by the PCT

• 90%

• the rest is excreted in urine 

How does the kidney regulate potassium secretion 

• changing amount of K+ returned to tubular fluid by DCT and CD

What happens when potassium concentrations are high

• cortical CD and DCT secrete more potassium

• urine contains more potassium than the glomerulus filters from blood 

What happens when potassium drops to low?

• cortical CD and DCT secrete less potassium 

How does aldosterone affect potassium? 

• increase in K+ stimulates aldosterone 

  • causes renal secretion of potassium 

  • stimulates reabsorption of sodium 

What causes a very quick rise in potassium resulting in hyperkalemia? 

• crush injury

• hemolytic anemia 

• transfusion with outdated blood 

What happens when there is a quick sudden increase in potassium?

• make nerve/muscle cells more excitable

  • more K+ remains in cell 

  • less negative resting membrane potential 

  • closer to threshold which sets of action potentials

what is a very deadly consequence of a quick sudden increase in potassium 

• cardiac arrest 

  • caused by the less negative resting membrane potential that will allow action potentials to be set off 

What causes a slower onset of hyperkalemia?

• aldosterone hypersecretion

• renal failure

• acidosis

What can occur due to a slow rise in potassium (hyperkalemia)

• extracellular potassium levels rise slowly

• nerve/muscle become LESS excitable

• slow depolarization of cells inactivates voltage gated sodium channels

• channels don’t become excitable and dont produce action potentials

Hypokalemia causes

• heavy sweating

• chronic vomiting

• diarrhea

• excessive laxatives

• aldosterone hypersecretion 

• alkalosis 

Result of Hypokalemia

• ECF K+ concentration falls, more K+ moves to ECF

• cells become hyperpolarized and nerve muscle cells are less excitable 

What happens in hypokalemia

• muscle weakness, loss of muscle tone, depressed reflexes, irregular electrical activity of heart 

Calcium Functions

• strengthen skeleton 

• second messenger for hormones

• activates exocytosis

• essential factor for blood clotting 

• sustains ventricular contraction 

How do cells maintain high phosphate concentration but avoid crystallization of calcium phosphate? 

• cells pump Ca2+ out 

  • helps maintain low intracellular calcium concentration 

What hormones regulate calcium levels?

• PTH

• calcitriol

• calcitonin 

Hypercalcemia causes

• alkalosis 

• hyperparathyroidism 

• hypothyroidism 

Hypercalcemia effects

• reduces Na permeability of plasma membrane

• inhibits depolarization of nerve/muscle cells 

• causes muscle weakness, depressed reflexes, cardiac arrhythmias

Hypocalcemia causes

• vitamin D deficiency

• diarrhea

• pregnancy

• lactation acidosis

• hypoparathyroidism 

• hyperthyroidism 

Effects of hypocalcemia

• increase sodium permeability, cause nerve/muscular systems to be over excitable

Why cells usually must maintain low intracellular calcium level? 

• Cells require a high concentration of phosphate ions 

• If calcium and phosphate were both concentrated in a cell, calcium phosphate crystals would precipitate into cytoplasm