Fluid, Electrolyte, and Acid-Base Homeostasis (Ch 20)

What are body fluids made up of?

Intercellular and extracellular fluid

Body fluids in lean adult human

55-60% of body mass in fluids

Intracellular fluid

That within the cells

Extracellular cells

- That which surrounds the cells

- Includes plasma and interstitial

Water gain

- Ingestion

- Metabolic synthesis

Water loss

Equal to the water gain

Methods of water loss

- Urine

- Perspiration

- Lung exhalation

- Feces

Where is the thirst center located?

Hypothalamus

What does dehydration cause?

- Decrease blood volume -> decreased blood pressure

- Increased osmolarity

What is thirst center stimulated by?

- Osmoreceptors in the hypothalamus

- Baroreceptors in blood vessels

- Volume receptors in atria

- Angiotensin II

- Sensory neurons in mouth

What is the major regulator of water loss?

Antidiuretic hormone (vasopressin)

What stimulates the release of antidiuretic hormone (ADH)?

Osmoreceptors in the hypothalamus

What protein does antidiuretic hormone (ADH) insert into principal cells?

Aquaporin-2

What effect does antidiuretic hormone (ADH) have on water reabsorption?

Increases water reabsorption

What conditions stimulate the release of antidiuretic hormone (ADH)?

A decrease in blood volume or blood pressure

What substance inhibits the release of antidiuretic hormone (ADH)?

Alcohol

What does aldosterone activate?

Renin-angiotensin-aldosterone pathway

What does aldosterone do to blood pressure and Na+?

Decreased blood pressure or

decreased Na+

What does aldosterone do for reabsorption?

Increases Na+ reabsorption, which

promotes water reabsorption

What is atrial natriuretic peptide stimulated by?

Release stimulated by volume receptors that detect increased stretch of atria

What does atrial natriuretic peptide do?

Increases excretion of Na+ in urine (prevents reabsorption of Na+) leads to loss of more water in urine

Electrolytes in body fluids

Ions dissolved in body fluids are electrolytes

What do electrolytes do?

- Certain ions control the osmosis of water between fluid compartments

- Maintain acid-base balance

- Carry electrical current

- Serve as enzyme cofactors

What is the most abundant cation in extracellular fluid?

Sodium

What do sodium electrolytes do?

- Accounts for ½ of extracellular osmolarity

- Plays a role in action potential generation/conduction

What is sodium electrolyte concentration controlled by?

ADH, Aldosterone, ANP

What is the most abundant anion in extracellular fluid?

Chloride

What do chloride electrolytes do?

- Easily moves across most membranes via Cl- leak channels

- Helps balance anion levels in different fluid compartments

What is the most abundant cation in intracellular fluid?

Potassium

What does potassium electrolyte do?

Plays key role in establishing the resting membrane potential, and repolarization phase of action potential

What does bicarbonate electrolyte do?

- Helps regulate blood pH

- Mechanism of CO2 homeostasis

What does calcium electrolyte do?

- Large amount stored in bone

- Hormonally regulated

- Major signaling molecule (Ex. Neurotransmitter release)

What is phosphate electrolyte?

Important buffer

What does magnesium electrolyte do?

Functions as a cofactor for many enzymes

Acid-base balance

- [H+]/(pH) of body fluids must be kept in homeostasis

- pH levels critically affect cellular function

- pH affects protein structure

- pH must be kept at normal levels ~7.4

Major mechanisms for controlling blood pH

- Buffer systems

- Exhalation of CO2

- Kidney excretion of H+

Buffer systems

Quickly but temporarily bind to H+ and increase pH

Exhalation of CO2

Increase rate and depth of breathing releases excess CO2, which reduces carbonic acid

levels, increasing pH

Kidney excretion of H+

- Slowest mechanism

- Only way to eliminate acids (other than carbonic acid)

- Increases pH

Carbonic acid-bicarbonate buffer system

- If there is too little H+, H2CO3 can provide H+

- If there is too much H+, HCO3- can remove it

Protein buffer system

- Most abundant in ICF and plasma

- IF pH is increased (above 7.45)

- IF pH is decreased (below 7.35)

- Hemoglobin is an important buffer in RBCs

Protein buffer in ICF

Hemoglobin

Protein buffer in plasma

Albumin

pH increase in protein buffer

NH2 - C - COOH -> NH2-C-COO- + H+ -> adds H+ and lowers pH

pH decrease in protein buffer

NH2 - C - COOH -> +NH3-C-COOH -> removes H+ and raises pH

Phosphate buffer system

- Important buffer in intracellular fluid

- Dihydrogen phosphate (H2PO4-) and monohydrogen phosphate

(HPO42-)

- H2PO4- can serve as a weak acid that buffers OH-

- HPO42- can serve as a weak base that removes H+

Formula for how OH- is buffered with phosphate

OH- + H2PO4- -> H2O + HPO42-

Formula for how H+ is removed with phosphate buffer

H+ + HPO42- -> H2PO4-

What does exhalation of CO2 do?

Helps eliminate H+ ions

Respiratory regulation of pH

- If pH decreases -> RR increases

- If PCO2 increases -> RR increases

- If pH increases -> RR decreases

- If PCO2 decreases ->RR decreases

Proximal tubule

- Na+/H+ antiporters secrete H+

- HCO3- transporters reabsorb HCO3-

Types of intercalated cells

First and second types

Why are Intercalated cells the most important?

Primary regulators of systemic acid-base balance

First type intercalated cells

- Proton pumps (H+ ATPases) secrete H+

- HCO3- reabsorbed by Cl-/HCO3-

antiporters (New bicarbonate produced in

intercalated cell by dissociation of

H2CO3)

Second type intercalated cells

- Proton pumps (H+ ATPases) reabsorb H+

- HCO3- secreted by Cl-/HCO3- antiporters

Acidosis

- When the blood pH drops below the normal range because of an excess H+

load

- Usually a blood pH below 7.35

Alkalosis

- When the blood pH rises above the normal range because of a deficit in H+

load

- Usually a blood pH above 7.45

Respiratory Acidosis

- Abnormally high PCO2 levels (and consequent H+)

- Caused by an inadequate exhalation of CO2 (hypoventilation)

Metabolic Acidosis

- HCO3- drops below normal, and important buffer is decreased

- HCO3- loss due to devere diarrhea, accumulation of acid (other than carbonic), renal dysfunction

Respiratory Alkalosis

- Abnormally low PCO2 levels

- Caused by an increased ventilation (hyperventilation)

Metabolic Alkalosis

- HCO3- levels increase above normal values

- HCO3- level increase due to excess vomiting (loss of HCl), endocrine disorders, excessive intake of antacids