Fluid, electrolyte, acid base balance

body water content

• infants are 73% more water (low body mass, low bone mass)

• adults males are ~60% water

• adult females are ~50% water (higher fat content, less skeletal muscle mass(

• adipose tissue is least hydrated of all

• total body water in adults averages ~40L

• water content declines to ~45% in old age

intracellular fluid compartment

fluid inside cells accounts for 2/3 of total body fluid, about 25L of 40L total

extracellular fluid compartment

fluid in two main compartments outside cells accounts for one-third of total body fluid:

→ plasma

→ interstitial fluid

plasma

extracellular fluid compartment

→ accounts for 3L of total body fluid

interstitial fluid

extracellular fluid compartment

→ accounts for 12L of total body fluids in spaces between cells

what is water?

the universal solvent

solutes

substances dissolved in water

what are solutes classified as?

nonelectrolytes and electrolytes

nonelectrolytes

• most are organic molecules

• do not dissociate in water

• examples: glucose, lipids, creatinine, and urea

• no charged particles are created

electrolytes

• disassociate into ions in water

• examples: inorganic salts, all acids and bases, some proteins

• ions conduct electrical current

• greater osmotic power than nonelectrolytes

• greater ability to cause fluid shifts due to ability to dissociate into two or more ions

comparing extracellular and intracellular fluids

each fluid compartment has a different pattern of electrolytes

• ECF: electrolyte contents are all similar except for higher protein, lower Cl- content of plasma; major cation is Na+ major anion is Cl-

• ICF: contains more soluble proteins than plasma; low Na+ and Cl-, major cation is K+, major anion is (HPO4)2- (hydrogen phosphate)

How much space dissolved solutes take

• 90% in plasma

• 60% in IF

• 97% in ICF

metabolic water

(water of oxidation); water produced by cellular metabolism

insensible water loss

water that is lost through skin and lungs

obligatory water lossess

unavoidable output of certain amounts of water; why we cannot live without water for very long

dehydration

• extracellular fluid compartment water loss due to hemorrhage, severe burns prolonged vomiting or diarrhea, profuse sweating, water deprivation, dieretic abuse, endocrine disturbances

• signs and symptoms include sticky oral mucous, thirst, dry skin, less urine

• may lead to weight loss, fever, confusion

hypotonic hydration

• cellular over hydration, or water intoxication

• occurs with renal insufficiency or rapid excess water ingestion

• ECF osmolarity decreases, causing hyponatremia

hyponatremia

• results in net osmosis of water into tissue cells and swelling of cells

• symptoms include severe metabolic disturbances, nausea, vomiting, muscular cramping, cerebral edema, and possible death

edema

• atypical accumulation of IF, resulting in tissue swelling (not cell swelling)

• can impair tissue function by increasing distance for diffusion of oxygen and nutrients from blood into cells

electrolyte balance

• usually refers only to salt balance even though electrolytes also include acids, bases, and some proteins

• salts control fluid movements, provide minerals for excitability, secretory activity and membrane permeability

• salts enter body by ingestion and metabolisn and are lost via perspiration, feces, urine and vomit

sodium concentration vs. sodium content

concentration of Na+

• determines osmolality of ECF and influences excitability of neurons and muscles

content of Na+

• total body content determines ECF volume and therefore blood pressure

what hormone is the main regulator of sodium in the kidneys?

aldosterone

how much Na+ is always reabsorbed no matter what

• 65% in proximal tubules

• 25% in nephron loops

(Na+ is never secreted)

what happens when aldosterone levels are high?

more Na+ reabsorbed in distal convoluted tubule and collecting duct → water follows, higher ECF volume

what happens when aldosterone levels are low?

less Na+ reabsorbed → more Na+ and water lost in urine

what is the main trigger for aldosterone release?

renin-angiotensin-aldosterone system

what does renin do?

converts precursors into angiotensin II, which causes aldosterone release

what does aldosterone do overall?

increases Na+ reabsorption (and water follows)

influence of atrial natriuretic peptide on sodium balance

• released by atrial cells in response to stretch caused by increased blood pressure

• it decreases blood pressure and blood volume, increases excretion of Na+ and water

estrogen - female sex hormone

increases NaCl reabsorption (like aldosterone); leads to H2O storage during menstrual cycle and pregnancy

progesterone - female sex hormone

decreases Na+ reabsorption (blocks aldosterone), promotes Na+ and H2O loss

glococoriticoids

increase Na+ reabsorption and promotes edema

cardiovascular baroreceptors

• alert brain to increase in blood volume and pressure

• sympathetic nervous system impulses to kidneys decline, causing:

→ afferent arterioles to filate

→ GFR increases

→ increases Na+ and water output

→ reduced blood volume and pressure

hyperkalemia

increases in ECF (K+) cause decreased resting membrane potential, causing depolarization, followed by reduced excitability

hypokalemia

decreases in ECF (K+) cause hyper polarization and non-responsiveness

where is potassium (K+) regulated in the nephron?

distal convoluted tube (DCT) and collecting duct

how do kidneys control K+ balance

by changing how much K+ is secreted into the filtrate

what happens when ECF K+ is high?

principal cells increase K+ secretion into filtrate

what happens when ECF K+ is low

principal cells reduce K+ secretion to a minimum

which cells can reabsorb leftover K+

Type A intercalated cells

influence of plasma potassium concentration

• most important factor affecting K+ secretion is its concentration in ECF

• high K+ diet leads to increased K+ content of ECF

→ K+ entry into principal cells leads to increased K+ secretion

influence of aldosterone on potassium balance

• aldosterone stimulates K+ secretion (and Na+ reabsorption) by principal cells

• increased K+ in adrenal cortex causes release of aldosterone

what regulates calcium levels in the body

parathyroid hormone (PTH)

what does parathyroid hormone do

increases blood calcium levels

how does parathyroid hormone raise calcium

• kidneys break down down to release calcium

• kidneys raise calcium reabsorption

• small intestine increase calcium absorption

what regulates phosphate reabsoprtion

mostly parathyroid hormone, also insulin and glucagon

alkalosis or alkalemia

arterial pH is more than 7.45

acidosis or acidemia

arterial pH is less than 7.35

three mechanisms for hydrogen ion regulation

1. chemical buffer systems

2. brain stem respiratory centers

3. renal mechanisms

chemical buffer systems

rapid, first line of defense for acid-base balance

brain step respiratory centers

acid-base balance that acts within 1-3 minutes

renal mechanisms

acid-base balance that is most powerful but requires hours to days to effect pH changes

chemical buffer

is a system of one or more compounds that act to resist pH changes when strong acid or base is added

three systems:

1. bicarbonate buffer system

2. phosphate buffer system

3. protein buffer system

bicarbonate buffer system

• It uses carbonic acid (H₂CO₃) (weak acid) and sodium bicarbonate (NaHCO₃⁻) (weak base) to control pH.

• It works in both ICF and ECF, but is the main buffer in the ECF.

• When a strong acid is added, bicarbonate (HCO₃⁻) grabs the extra H⁺, turning it into carbonic acid (H₂CO₃).

• This prevents a big drop in pH. pH only falls a little unless all the bicarbonate is used up.

• The kidneys control bicarbonate levels, keeping the system working.

phosphate buffer system

• action nearly identical to bicarbonate buffer

• components are sodium salts of: dihydrogen phosphate (a weak acid) and monohydrogen phophate (a weak base)

• effective buffer in urine in ICF, where phosphate concentrations are high

protein buffer system

• protein molecules are amphoteric (can function as both weak acid and weak base)

• most important buffer in ICF; also in blood plasma

hypoventilation

causes carbon dioxide maintenence and respiratory acidosis

hyperventilation

causes carbon dioxide elimination and respiratory alkalosis

volatile

lungs eliminated ______ carbonic acid by eliminating CO2

nonvolatile

kidneys eliminate _______ (fixed) acids produced by cellular metabolism to prevent metabolic acidosis

two mechanisms in proximal convoluted tubule (PCT) and type A intercalated cells generate a new HCO3- by ridding body of new H+

1. via excretion of buffered H+

2. via NH4+ excretion

excretion of buffered H+

• most important urine buffer is phosphate buffer system

• intercalated cells actively secrete H+ into urine, which is buffered by phosphates (monohydrogen phosphates) and excreted in urine

• new HCO3- is generated in process and moves into interstitial space

NH4+ excretion

• more important mechanism for excreting acid

• involves metabolism of glutamine in PCT cells

• each glutamine produced 2NH4+ and 2 new HCO3-

• HCO3- moves to blood and NH4+ is excreted in urine

respiratory acidosis and alkalosis

• caused by failure of respiratory system to perform pH-balancing role

• single most important indicator is blood P_co2

metabolic acidosis and alkalosis

• all abnormalities other than those caused by P_co2 levels in blood

• indicated by abnormal HCO3- levels

respiratory acidosis

PCO2 above 45 mmHg

respiratory alkalosis

PCO2 below 35mmHG → common result of hyperventilation

metabolic acidosis

low blood pH and HCO3

metabolic alkalosis

rising blood pH and HCO3-

respiratory compensation

lungs try to compensate for metabolic pH problems by changing respiratory rate and depth

renal compensation

kidneys try to compensate for pH problems caused by lungs by adjusting bicarbonate levels