Internal Environment: Fluid Balance (3)

Which of the following body fluid compartments contains the largest volume of water?

• Intracellular (2x the size of extracellular, know ratios)

• Extracellurar

• Interstitial

• Plasma

• Transcellula

Describe the role of the ascending linb of the loop of Henle in producing a high renal interstitial fluid osmolarity. Beginning with the loop of Henle, contrast the tubular fluid and interstitial fluid osmolarity changes that either dilute or concentrated urine to be produced and excreted

kidneys alter water volume in the body

• control of water volume in the body is through control intake & control of urine production

Contrast the relative contribution of neural and renal mechanisms in blood pressure and blood volume regulation

• long-term with fluid balance

• short-term with baroreceptor reflex

Inputs

• Endocrine

  • Vasopressin (ADH)

    • don’t have ADH, will die within a day

    • alcohol (ethanol inhibits vasopressin)

      • urine output exceeds intake

      • dehydrated next morning bc they’ve lost water

        • CSF pressure goes down

  • Atrial Natriuretic Peptide

    • Na+ loss through kidneys

    • both hormones lead to temporary diabetes insipidus

  • Aldosterone

  • Cortisol

• CNS

  • Sympathetic

    • B1- Stimulates renin release (brings pressure back up)

      • inc. HR & strength of contraction

      • inc. CO

        • both short-term

      • granular cells that release renin

        • renin release → long-term regulation to bring pressure back up

  • Parasympathetic (no effect on volume)

• posterior pituitary → neurohormone

Body Fluid Compartments

Constituents

• Extracellular fluid

  • Plasma

    • measure things in plasma, what we have access to

    • wtv. plasma conc. is, same that it is in interstitial fluid

      • not same as intracellular

  • Interstitial fluid

• Intracellular fluid

  • phospholipid bilayer around cells

    • compounds easily cross

Distribution of water

• 60% of body weight is water (42 kg of water, a kg of water is a liter)

  • 40% is intracellular

  • 20% is extracellular (ECF)

    • 20% of ECF is plasma

    • 80% of ECF is interstitial

• as you get older, get drier

• men have a slightly higher %age of body water than women of the same age based on fat distribution

• 2 patients, you are an oncologist:

  • 70 kg female that is 72 years old; 70 kg male that’s 22 years old

    • have same cancer, use same drug

    • don’t give them same dose, conc. higher in one than the other

    • she’s going to need less drug bc she’s got less water

    • give him her dose → not high enough to kill cancer

    • give her his dose → he’s got more water, may be too high in her

Indicator Dilution

• A known quantity of a substance in an unknown volume in liters yields a concentration (put a known mass in it)

  • M/V = C

• Example: 2 g of a substance in an unknown volume yields a concentration of 20 ug/ml

  • M = 2 g

  • V = X L (always solving for volume, plasma volume, total body water)

  • C = 20 ug/ml

Indicator Dilution in Body Fluids

• Total Body water = radio-labeled water (deuterium oxide, heavy water)

  • indicator is most important thing

  • how indicator behaves tells you your fluid compartment

  • volume

• Extracellular Fluid = inulin

  • small, but polar… will not go into cells

  • stays in extracellular space

  • subtract from total body water to find intracellular volume

• Plasma = albumen

  • big, does not come out of capillaries at all

    • plasma volume

    • interstitial fluid

• what body compartment do my indicators measure?

Regulation of Body Fluids

Total Body Water

• Water in = water out

  • Water in

    • consumption 2.25 L/day (both food & drink)

    • metabolic water- 0.35 L/day (body produces 350 mL/day)

  • Water out

    • Insensible loss (skin & lungs)- 0.9 L/day

      • Sweat- 0.1 L/day

      • Urine- 1.5 L/day (only one we control)

      • Feces- 0.1 L/day (weak control, dehydrated → dec. in fecal water loss)

Movement of water through the various compartments of the body

Constituents of Body Fluid

• Extracellular (plasma)

  • Cations

    • Sodium

    • Potassium

    • Calcium

  • Anions

    • Chloride

    • Bicarbonate

    • Phosphate

• ECF

  • Other

    • Glucose

    • Protein

    • Urine (has lipid solubility)

Constituents of Body Fluid 2

• Intracellular fluid

  • Cations

    • Sodium

    • Potassium

• Intracellular

  • Anions

    • Chloride

    • Bicarbonate

    • Phosphates

• Other

  • Hexose monophosphate

  • Protein

  • ATP

  • Urea

Relative concentrations of cations and anions in the extracellular and intracellular body fluid compartments

Maintenance of Volume

• Only partially dissociated from salt balance

• Water loss

  • Insensible

  • Balance intake

  • Renal excretion

    • 1.5 L/day

• we can reabsorb water without reabsorbing Na+

• can reabsorb Na+ without reabsorbing water (times we just need water → get back in balance, overhydrated → pull in Na+ without water)

  • need to be able to do these things separately

  • don’t always do them separately

• dehydrated → high Na+ value

  • kidneys need to hold onto water

    • requirement for them to pull water & they also have to pull in Na+

Diagram of process of correction for hypertonicity and hypotension

• vasopressin and osmoreceptors more important than heart

  • prioritize

    • what’s problem/situation that’s going to kill patient fastest?

• #1 osmotic particle in body is Na+ (what’s role of Na+ in body → osmotic agent)

  • body doesn’t actually pay attention to Na+ itself, pays attention to conc. of particles

    • not really a Na+ detector in body

• Ex: ramen noodles

  • lots of Na+

  • osmolarity goes up, from hypothalamus, going to trigger both thirst & vasopressin

• osmolarity is most important stimulator of vasopressin release

  • just a slight change in osmolarity causes cells in posterior pituitary, if osmolarity goes up, the cells shrink a bit, vasopressin goes up

Maintenance of Volume (cont’d)

• Increased volume is measured as an increase in blood pressure

• body assumes that if BP is high there is too much volume

• assumption not always correct

• someone can have hypertension & high BP & not have too much volume

• diuretic → inc. urine production

• trying to get rid of water/volume

• if we can get volume down, pressure will come down

  • Baroreceptor reflex (movement by moment control, vasopressin not tied in)

    • ADH release (less sensitive)

    • Increase parasympathetic tone (if pressure is high_

    • Decrease sympathetic tone

      • decrease arteriolar constriction

      • increase flow through the afferent arteriole of the nephron

      • increase GFR (glomerular filtration rate, inc. formation of urine, urine volume will go up to bring pressure down)

• pressure is high, will constrict but GFR will still say somewhat elevated

  • filtration rate will still stay high bc we’re trying to lose that volume, get pressure down

Maintenance of Volume (cont’d 2)

• Increased pressure

  • Fluid shift

    • From plasma to interstitial fluid

    • Short-term control

    • Based upon pressure changes and osmolarity changes

• Decreased osmolarity (very tightly regulated)

  • Inhibition of ADH release

  • 1% change alters ADH release (sensitive)

Maintenance of Volume (cont’d 3)

• Decreased volume = decreased pressure

  • Renin-angiotensin-aldosterone system

  • Baroreceptor reflex

    • Increased sympathetic stimulation

      • increased arteriolar constriction (inc. resistance)

      • decreased GFR (dec. urine formation, short-term)

• Ex: donate blood

  • pressures are low

  • baroreceptor reflex is going to compensate, will not be able to fix

The Renin-Angiotensin-Aldosterone

Maintenance of Osmolarity

• Increased osmolarity (inc. ADH)

  • Osmoreceptor cells

    • Located near third ventricle

    • Outside the blood/brain barrier

    • Sensitive to minute changes in osmolarity

  • Direct stimulation of thirst

  • Direct ADH release

• Decreased osmolarity (less vasopressin)

The relationship between osmolarity and vasopressin release

The relationship between blood pressure and vasopressin release

Combination of pressure and osmotic change

A nephron

Fig. 14-17, p. 524

Urine Formation

• Isotonic (same tone as plasma), hypotonic (less salt/solute), and hypertonic (more)

  • Based upon need

  • Regulation of blood volume

• Osmotic gradient

  • Established by the counter-current multiplier system

  • Maintained by countercurrent exchange with capillaries

Medulla

• very hypertonic

• isotonic: 300 milliosmoles/liter

• need to know gradient exists & this is how we can move water without moving Na+ bc Na+ has already been moved, sitting in interstitial space concentrated & when water gets by it, water gets pulled out through aquaporins

Urine Formation (cont’d)

• Vasopressin (ADH), variable release

  • Produced in hypothalamus

  • Acts at the level of the basolateral membrane in the distal and collecting tubules only

    • Open channels for water

    • Brings filtrate osmolarity to 1200 mOsm

  • Hypotonic urine produced without ADH

  • Isotonic urine produced with low levels of ADH

Action of Vasopressin on Principle Cells

Formation

Formation 2

Table 14-4: Handling of Sodium and Water by Various Tubular Segments of the Nephron

Fig. 10-34, p. 376

Which of the following is/are mechanisms to increase plasma volume?

• Decrease vasopressin release (body would not conserve water as much & urine output would go up → BP is going down… not going to help us)

• Shift fluid from the interstitial space into the plasma

• Increase venous blood flow to the kidneys (more blood would be filtered, glomerular filtrate formation is going to go up & plasma volume is going to go down)

• Inhibit renin release (no renin → no angiotensin II → no aldosterone → not going to retain Na+ → not going to get vasopressin release… everything is going to go down, low pressure triggered renin release → pressure goes back up)

• All of these will increase plasma volume

ICQ

• if the outward driving force of BP is lower, then less fluid is going to go into the interstitial space & more is going to stay into plasma

• diagram that shows hemorrhage

• vasopressin will not always be released if aldosterone is released

  • main thing that causes vasopressin release is change in osmolarity

  • if aldosterone is released & osmolarity goes too high, we would get vasopressin release

  • what if osmolarity is too low, they release aldosterone & they come back up to normal, then we’re not going to get vasopressin release

  • going back to set point, wtv is normal

    • once correction occurs, system shuts off

Control

• Short-term

  • Baroreceptor reflex

• Long-term

  • Osmoreceptors & Volume receptors

    • Hypothalamus, kidney, atria

    • Alters salt and water balance to alter volume

Fig. 10-40, p. 384

• blue → consequences

• pink → compensation

  • what does body do to bring things back up?