Ch. 21 - Fluid, Electrolyte, and Acid-Base Balance

Question 1

Explain what is meant by the term fluid balance

Answer 1

Refers mainly to water balance.

Total water intake = total water output.

Maintained by homeostatic mechanisms.

Average adult consumes approx. 2,500ml H2O per day, broken down as:

• 60% - water and beverages
• 30% - moist foods
• 10% - ‘water of metabolism’ - byproduct of the oxidative metabolism of nutrients

Thirst is primarily responsible for regulation of water intake.
Intense thirst comes from osmotic pressure affecting extracellular fluids on the thirst centre in the hypothalamus.
Loss of body water increases osmotic pressure to stimulate osmoreceptors in the thirst centre.
Whenever total body water decreases by as little as 1%, thirst is triggered.
Drinking fluids triggers nerve impulses that inhibit the thirst mechanism, stopping drinking before the swallowed fluids are absorbed, keeping a person from drinking too much fluid.

The body loses water in:

• urine (60%)
• faeces (4%)
• sweat - known as ‘sensible perspiration’ (8%)
• evaporation from the skin - known as ‘insensible perspiration’
• from the lungs during breathing - known as ‘insensible water loss’ (28% from skin and lungs combined)

These percentages of loss may vary due to environmental temp, relative humidity and physical exercise.

The distal convoluted tubules of the nephrons and collecting ducts are important in the regulation of water excreted in the urine.
Their epithelial linings are mostly impermeable to H2O unless ADH is present, when it will increase water reabsorption to reduce urine output.

In the healthy individual the osmolality of the body fluids is maintained between 280-300mOsm.
Increased plasma osmolality triggers thirst and causes release of ADH.
The kidneys then conserve water, excreting concentrated urine.

Oppositely, decrease in osmolality inhibits thirst and ADH release.
The kidneys excrete large amounts of dilute urine.

Sodium attracts water, yet the ADH and thirst mechanisms in control of osmolality regulate water independently of sodium’s effects.

Question 2

Explain what is meant by the term electrolyte balance

Answer 2

No’s. of electrolytes the body gains = no’s. of electrolytes the body loses.

Maintained by homeostatic mechanisms.

Most important electrolytes(ions) needed for cellular function are:

• sodium
• potassium
• calcium
• magnesium
• chloride
• sulfate
• phosphate
• bicarbonate
• hydrogen

These are mostly found in food, water and other beverages, and as byproducts of metabolic reactions.

Sodium, potassium and calcium are among the most important, imbalance of these electrolytes can be significant.

Severe deficiency of electrolytes may produce desire to eat salty foods/’salt craving’.

Salts help control fluid movement in the body and provide needed minerals for excitability, membrane permeability, and secretory activities.

Electrolytes are lost through sweating on warm days and during exercise, and in the faeces, but the greatest output is because of kidney function and urine production.

The kidneys control electrolyte output to maintain balance.

Positive ions such as calcium, potassium and sodium are essential for maintaining cell membrane potential, muscle fibre contraction, and nerve impulse conduction.

Nearly 90%of positively charged ions in the extracellular fluids are sodium ions, which are regulated by the kidneys and the hormone aldosterone.

Question 3

Compare the composition of intracellular and extracellular fluids

Answer 3

Intracellular fluid:

• made up of water and electrolytes enclosed by cell membranes
• contains more magnesium, phosphate, potassium and sulphate ions than extracellular fluid
• represents approx. 63%, by volume, of total body water
• primary cation is potassium
• primary anion is hydrogen
• large quantities of soluble proteins, approx. 3x those found in plasma
• proteins and some non-electrolytes like cholesterol, phospholipids, triglycerides are large molecules that are present, accounting for 97% of the mass of dissolved solutes

Extracellular fluid:

• made up of all fluid outside of cells, which includes plasma(in blood vessels), lymph(in lymphatic vessels), interstitial fluid(in tissue spaces)
• transcellular fluid is separated from other extracellular fluids, but includes:
  
  • aqueous/vitreous humors:in the eyes
  • cerebrospinal fluid:CNS
  • secretions:exocrine glands
  • serous fluid:body cavities
  • synovial fluid:joints
• contains high amounts of chloride, bicarbonate, and sodium ions, as well as more calcium than is found in intracellular fluid
• represents approx. 37%, by volume, of total body water
• primary cation is sodium
• primary anion is chloride
• plasma proteins usually anions
• proteins and some non-electrolytes like cholesterol, phospholipids, triglycerides are large molecules that are present. In plasma they account for approx. 90% of the mass of dissolved solutes, and approx. 60% in interstitial fluid

Question 4

Describe the movement of fluid within the extracellular and intracellular fluids

Answer 4

Hydrostatic pressure and osmotic pressure regulate the movement of water and electrolytes from one fluid compartment to another.

Hydrostatic pressure inside cells and surrounding interstitial fluid is normally equal and stable, therefore a change in osmotic pressure usually causes net fluid movement.

The net inward force is called ‘colloid osmotic pressure’

When sodium ions levels decrease in the extracellular fluid, water moves from the extracellular compartment in to intracellular compartment via osmosis, and the cells ‘swell’ as a result

The opposite is true when sodium ion concentration in interstitial fluid increases, meaning the cells ‘shrink’.

Water moves freely between compartments, but solutes are not equally distributed, due to their electrical charges, sizes or need to use transport proteins.

Substances must pass through the plasma and interstitial fluid to reach the intracellular fluid.

Plasma composition and volume are both altered.

Plasma is medium that allows substances to be delivered to all areas of the body.

Balance is quickly restored by the body’s adjustments between the plasma, extracellular and intracellular fluid.

2 key points:

1 - Exchanges between plasma and interstitial fluid occurs across capillary walls

• the blood’s hydrostatic pressure forces plasma that almost entirely lacks proteins into the interstitial space
• the highly filtered fluid then is almost totally reabsoped into the bloodstream because of the colloid osmotic pressure of the plasma proteins
• lymphatic vessels pick up small amounts of net leakage remaining behind in the interstitial space, returning it to the blood

2 - Exchanges between interstitial fluid and intracellular fluid occur across plasma membranes

• based on permeability
• generally, substantial 2-way osmotic flow of water
• restriction of ion changes is based on ions moving selectively through channels or by active transport
• nutrients, respiratory gases and wastes usually found to move only in 1 direction, eg. metabolic wastes move out of cells, glucose and O2 move into cells
• except during the first minutes after a change in one type of body fluid, osmolalities of body fluids are equal

Question 5

Explain how the body responds when the pH of body fluids varies outside normal limits

Answer 5

When normal pH levels of arterial blood are not maintained, the body responds by producing acidosis or alkalosis.

Survival may be impossible if pH is below 6.8 or above 8.0 for more than a few hours.

Question 6

Describe metabolic alkalosis

Answer 6

Results from excessive loss of Hydrogen ions, or gain of bases or bicarbonate ions.

Less common than metabolic acidosis.

This results in an increase in blood pH or alkalemia.

It may follow gastric drainage/lavage, use of certain diuretics, prolonged vomiting or taking too many antacids.

Loss of acidic gastric juice leaves body fluids more basic.

A condition called ‘alkaline tide’ may occur, caused by many bicarbonate ions moving into the extracellular fluid.

This movement is related to secretion of hydrochloric acid from the gastric mucosa.

Temporary elevation of bicarbonate ions in the extracellular fluid occurs during eating, but serious metabolic alkalosis may occur because of repeated vomiting as the stomach generates more stomach acids to replace those regurgitated.

This means bicarbonate ion concentrations in the extracellular fluid rise continually.

Symptoms include:

• decreased breathing rate and depth
• increased blood CO2

Compensatory factors for metabolic alkalosis include reduced breathing rate, with a loss of bicarbonate ions in the urine.

Question 7

Compare respiratory acidosis with metabolic acidosis

Answer 7

Respiratory acidosis.

• may be caused by increased CO2 concentration as well as respiratory/carbonic acid
• generally indicated by a partial pressure of CO2 above 45mmHg, known as Hypercapnia, and by lowered blood pH
• usually caused by hypoventilation/low rate of breathing
• as carbonic acid forms and dissociates, the partial pressure of CO2 rises in the extracellular compartment, hydrogen and bicarbonate ion concentrations also rise
• when the buffer systems cannot keep up the pH falls rapidly
• just a few minutes of hypoventilation can cause acidosis
• pH of the extracellular fluid may reduce to as low as 7.0
• when the body’s chemical and physiological buffers return pH to normal, the acidosis is ‘compensated’, this is normally accomplished by chemoreceptors that stimulate an increase in the rate of breathing
• in ‘uncompensated acidosis’ the pH continues to drop, the pt can become comatose and eventually die
• Acute Respiratory acidosis develops when decline in pH is severe, it is especially dangerous when the pt’s tissues generate large amounts of CO2 or when normal respiratory activity is not possible
• Chronic Respiratory Acidosis occurs because normal respiratory function is compromised but the compensatory mechanisms have not completely failed, these individuals often develop acidosis because of chronic hypoventilation.
• when normal pulmonary responses are disabled, the kidneys increase hydrogen ion secretion into the tubular fluid, slowing the rate of pH change
• the kidneys are not however, able to return pH to normal levels on their own. The underlying circulatory/respiratory problems must be corrected
• respiratory acidosis causes metabolic acidosis as lactic acid is generated in tissues that do not have sufficient O2
• may result in injury to the brain stem’s respiratory centre, obstruction of air passages, pneumonia, emphysema, or other respiratory conditions

Metabolic acidosis.

• indicated by bicarbonate levels below/above the normal range, which is 22-26mEq/L- may be caused by accumulation of nonrespiratory acids or loss of bases such as following conditions:
  
  • lactic acidosis - strenuous exercise/prolonged tissue hypoxia, also known as oxygen starvation
  • diabetes mellitus - ketoacidosis caused by fatty acids being turned ketone bodies/acetone
  • overconsumption of alcohol, which is metabolised to acetic acid
  • prolonged vomiting causing the stomach to continually generate stomach acid, as a result bicarbonate concentration in the blood continues to rise
  • prolonged diarrhoea causing excessive loss of bicarbonate ions
  • kidney disease, reabsorption of sodium ions stops, secretion of hydrogen ions also stops
• the body generally compensates for metabolic acidosis via the lungs and kidneys:
  
  • the lungs eliminate CO2 molecules formed by the interaction of Hydrogen ions with bicarbonate ions
  • the kidneys excrete additional hydrogen ions into the urine while generating bicarbonate ions, which are released into the extracellular fluid

Metabolic and Respiratory Acidosis are often linked because oxygen-starved tissues generate lactic acid in massive amounts and because sustained hypoventilation results in decreased arterial partial pressure of O2.