finals ia - water and electrolyte balance
Post on 07-Feb-2016
Embed Size (px)
Water Electrolyte Balance
Ricardo R. Santos, M.D.
Normal daily intake and output of water (in ml/day)
Fluid ingested 2100
From metabolism 200
Total intake 2300
Insensible skin 350
Insensible lungs 350
Total output 2300
Sources of water
1. Preformed water
*Water found in food and drink
2. Metabolic water
*Water produced through the catabolic breakdown of nutrients during cellular
Body Fluid Compartments
Extracellular Fluid - 14.0 L (20%)
Plasma - 3.0 L Interstitial fluid 11.0 L
Intracellular fluid 28.0 L (40%)
Transcellular fluid 1 to 2 L
2. Transcellular fluids
- Cerebrospinal fluid
- Gastrointestinal fluids (digestive juices)
- Synovial fluid
- Eye fluids (aqueous & vitreous humors)
- Ear fluids (perilymph & endolymph)
- Pleural, pericardial and peritoneal fluids
In a 70-kg man, total body water is 60% of body weight or 42 L.
As person grows older %age of body fluid gradually decreases.
Increase in fat decreases body water.
Women have less %age of body water than men
Obligatory H20 reabsorption
Facultative H20 reabsorption
Site in the nephron
DCT and CD
High plasma osmolarity
Urine volume depends decreased
Types of Water Reabsorption in the Kidney
The mechanisms for the regulation of body fluids are centered in the hypothalamus.
The regulation of body fluid volume and extracellular osmolarity is under the control of ADH and aldosterone.
Primary factors that trigger release of ADH:
2.Baroreceptors (pressure receptors)
Secondary factors: stress, pain, hypoxia, K
Connected to the response of the osmoreceptors.
- increased plasma osmolarity stimulates osmosreceptors which in turn stimulates sensation of thirst
Thirst center is in the hypothalamus.
Other factors involved:
1. degree of dryness of mucosal linings of mouth and pharynx
2. stretch receptors in the GIT
Cations sodium potassium
Anions chloride phosphate
Normal plasma level = 135 145 mEq/L
Predominant cation in the ECF
Plays a crucial role in the excitability of muscles & neurones
Important in regulating fluid balance
Sodium regulation at the cellular level is controlled by the Na-K pump
Body levels of Na (retention/excretion) are controlled by aldosterone
Aldosterone is controlled by renin-angiotensin system
Secretion, Transport and Metabolism of Aldosterone
Renin-angiotensin system - involved in regulation of blood pressure and electrolyte metabolism - primary substance in this process is angiotensin II
Angiotensinogen Angiotensin I renin converting enzyme
Normal plasma level = 3.5 5.5 mEq/L
Critical for electrical conduction of nerve impulses - particularly cardiac electrical conduction
Major cation in the ICF
Potassium balance at the cellular level is maintained by the Na-K pump
Kidney can excrete K and in exchange for Na controlled by aldosterone
Body is much more sensitive to small changes in serum K levels than to small changes in other serum electrolytes
Normal level: 4.0 5.5 mEq/L or 8.5 10 mg/dl
Required for normal skeletal, cardiac and smooth muscle contraction
Needed for blood clotting
Intestinal absorption of dietary calcium requires vitamin D
Calcium metabolism is regulated by parathyroid hormone and vitamin D
Calcitonin from the thyroid gland causes ECF levels of calcium levels to decrease by inhibition of bone resorption, inhibits vitamin D absorption and increases renal excretion of calcium.
Normal value: 1.5 2.5 mEq/L
Needed to prevent overexcitability of muscles
Has a sedative effect on neuromuscular junction, inhibits acetylcholine release, and diminishes muscle cell excitability
Acts as a cofactor in enzyme reactions
Participates in bone and teeth production
Ionic composition of plasma and interstitial fluid
Ionic compositions are similar.
Protein concentration is higher in the plasma.
Because of Donnans effect, concentrations of cations is slightly greater in the plasma by 2%.
Proteins are negatively charge more proteins in the plasma bind more cations.
Interstitial fluids have more anions than plasma.
Basic principles osmosis and osmotic pressure
Osmosis is the net diffusion of water across a semipermeable membrane from a region of high water conc. to one that has a lower water conc.
In osmosis, water diffuses from a region of low solute conc. to one that has a high solute concentration.
Cell membrane is relatively impermeable to most solutes but highly permeable to water.
Important in order to describe the concentration of solute particles.
Total number of particles in a solution is measured in terms of osmoles.
One osmole (osm) is equal to 1 mole (mol) (6.02 x 1023) of solute particles.
A soln. containing 1 mole of glucose in each liter has a conc. of 1 osm/L.
A soln with 1 mole of NaCl per liter will have an osmolar conc of 2 osm/L
Osmole is too large a unit for expressing osmotic activity of solutes in the body.
Therefore, the term milliosmole (mOsm), which equals 1/1000 osmole, is commonly used.
Osmolal concentration of soln is called:
1. Osmolality - when conc is expressed as
osmoles per kg of water.
2. Osmolarity when conc is expressed as
osmoles per liter of solution
The two terms can be used synonymously
It is easier to express body fluid quantities in liters of fluid than in kg of water.
Osmotic pressure the precise amount of pressure required to prevent the osmosis.
It is an indirect measurement of the water and solute concentrations of a solution.
The higher the osmotic conc of a solution, the lower the water conc but the higher the solute conc.
Osmotic pressure of a solution is directly proportional to the concentration of osmotically active particles in that solution.
Total osmolarity of the body fluid compartments is 300 mOsm/L
80% of total osmolarity of the ECF is due to sodium and chloride.
Corrected osmolar activity 282 mOsm/L
Total osmotic pressure 5443 mm Hg
Impermeant solute one that will not permeate the cell membrane
The terms isotonic, hypotonic and hypertonic refer to whether solutions will cause a change in cell volume.
Tonicity of solutions depends on the conc of impermeant solutes.
Isosmotic solutions with an osmolarity the same as the cell.
Hyperosmotic and hypo-osmotic refers to solutions that have a higher osmolarity or lower osmolarity, respectively compared with normal ECF.
Isotonic solutions 0.9% soln of NaCl or a 5% glucose solution.
- have osmolarity of 282 mOsm/L
- cell will not change in volume
Hypotonic solution < than 0.9% NaCl
- osmolarity less than normal
- water will diffuse into the cell (swell)
Hypertonic solution - > than 0.9% NaCl
- osmolarity more than normal
- water will flow out of the cell (shrink)
Clinical problems with fluid balance
Hypotonic contraction of ECF
Fluid has fewer solutes than normal plasma
Relatively uncommon loss of more solute (usually Na) than water
Causes fluid to shift from the blood stream into the cells, leading to decreased vascular volume and eventual shock
Increased cellular swelling
Cerebral edema causes increased intracranial pressure, headache and confusion
Seen in heat stroke or exhaustion
Isotonic contraction of ECF
Fluid has the same osmolarity as normal plasma
Most common form of dehydration
Occurs when fluids and electrolytes are lost in even amounts
There are no intercellular shifts
- diuretic therapy
- excessive vomiting
- excessive ur