Chapter 24 Water & Electrolytes Outline

How much fluid is intracellular fluid?

65% (most)

How much fluid is extracellular fluid?

35%

• 25% tissue fluid

• 8% blood plasma & lymphatic fluid

• 2% transcellular fluid (cerebrospinal, synovial, pericardial)

Electrolytes

(salt) help govern water distribution and total water content

osmosis restores balance

fluid balance: gains = losses

2,500 mL/day

Water is gained from what 2 sources?

preformed water (2,300 mL/day)

• food and drink

metabolic water (200 mL/day)

• from aerobic metabolism & dehydration synthesis

Sensible water loss

(observable)

1,500 mL/day in urine

200 mL/day in feces

100 mL/day in sweat

Insensible water loss

(unnoticed)

300 mL/day in BREATH

400 mL/day in CUTANEOUS TRANSPIRATION

Obligatory water loss

Sensible (urine, feces, sweat) and insensible (breath, cutaneous transpiration) water loss combined

What mechanism triggers thirst in response to dehydration?

osmoreceptors detecting increased plasma osmolarity

(angiotensin II is produced when BP drops)

Dehydration

decreases blood volume and BP

increases blood osmolarity

Hypothalamus produces what in response to dehydration?

ADH!

salivation inhibited (sympathetic)

Cerebral cortex role in dehydration?

makes us “feel” thirsty

intense sense of thirst if only 2-3% increase in plasma osmolarity

1.) Which hormone increases water reabsorption without altering sodium reabsorption?

ADH

Long-term inhibition of thirst

absorption of water from small intestine reduces blood osmolarity

• stops osmoreceptors

Short-term inhibition of thirst

cooling/moistening of mouth

distension of stomach and small intestine

DESIGNED TO PREVENT OVERDRINKING

variation in urine volume =

significant control of water output (kidneys can only slow rate of loss)

changes in urine volume linked to adjustments in sodium reabsorption

• as sodium reabsorbed or excreted, water follows

can concentrate urine with ADH, independently of sodium

• in CDs, water reabsorbed but sodium still excreted

volume

both sodium and water lost/retained

concentration

more water than sodium lost/retained

fluid deficiency examples

hypovolemia (volume) or dehydration (concentration)

fluid excess examples

volume excess or hypotonic hydration (concentration)

hypovolemia

sodium and water decreased (volume); osmolarity CONSTANT

ex: hemorrhage, severe burns, chronic vomiting/diarrhea

dehydration

body loses significantly MORE water than sodium

osmolarity RISES (concentration)

ex: diabetes mellitus, diabetes insipidus, profuse sweating, diuretics

who are most vulnerable to dehydration?

INFANTS

high metabolic rate, greater body surface-to-mass ratio, immature kidneys cannot concentrate urine effectively

dehydration from excessive sweating

sweat from capillaries

blood volume and BP drop, osmolarity rises

blood absorbs tissue fluid to replace loss!

tissue fluid pulled from ICF

all 3 compartments lose water

fluid excess

kidneys so effective at excreting more urine to compensate for excess intake

renal failure can therefore cause fluid retention

volume excess

both sodium and water retained

ECF isotonic (volume)

cause: aldosterone hypersecretion or renal failure

Hypotonic hydration

more water than sodium retained/ ingested

ECF becomes hypotonic (concentration)

cause: overdrinking water or ADH hypersecretion

Fluid sequestration

excess fluid accumulates in particular location

EDEMA (abnormal accumulation of interstitial (in-between cells) fluid)

Hemorrhage

blood can pool in tissues during fluid sequestration (excess fluid)

Effusion

fluid in cavity (ex: pleural) during fluid sequestration (excess fluid)

Physiological functions of electrolytes

• many metabolic processes

• determine electrical potential across cell membranes

• affect osmolarity of body fluids, therefore water content

sodium functions

critical ion for RMPs, APs (Na+—K+ pump)

principal cation in ECF

• 90-95% osmolarity of ECF

• most significant solute in determining total body water

How much sodium do we need each day and how much does the American diet have?

need= 0.5 grams of sodium/day

get= 3-7 a day

Aldosterone

“salt-retaining hormone”

• secreted in response to hyponatremia and hypotension (low BP)

• if hypotension, via renin—angiotensin—aldosterone mechanism

Natriuretic peptides (ANP)

inhibit sodium and water reabsorption

Hypernatremia

high concentration of sodium

consequences: hypertension, edema

Hyponatremia

low concentration of sodium

• drinking too much plain water after profuse sweating

• quickly corrected by excretion of excess water

Potassium functions

critical ion for RMPs, APs

principal cation of ICF

Potassium homeostasis closely linked to that of sodium

most potassium reabsorbed by PCT

DCT and collecting duct secrete potassium in response to blood levels, mediated by aldosterone

Most dangerous imbalances

hyperkalemia and hypokalemia (potassium)

Hyperkalemia

if extracellular potassium rises quickly (injury), makes nerve and muscle cells more excitable

if rises slowly (renal failure), nerve & muscle cells become less excitable

Hypokalemia

from sweating, chronic vomiting, or diarrhea

neurons and muscle cells hyperpolarized, less excitable