Intracellular fluid compartment (ICF)
The fluid contained within all the cells of the body
Contains about 2/3 of the body water in healthy adults
Contains moderate amounts of magnesium and large amounts of potassium
Contains almost no calcium, small amounts of sodium, chloride, bicarbonate, a
Extracellular fluid compartment (ECF)
Contains all the fluids outside the cells, including those in the interstitial spaces and the plasma in the blood vessels
Contains about 1/3 of the body water in healthy adults
Contains large amounts of sodium chloride, moderate amounts of bicarbonate
Con
Most of the water in the body is in the intracellular or extracellular fluid compartment?
Intracellular fluid compartment (ICF)
Potassium
~28 times more concentrated inside the cell than outside
Sodium and potassium rely on transmembrane proteins to cross the lipid bilayer
How does water cross the cell membrane?
Osmosis
Electrolytes
Substances that dissociate in solution to form ions (charged particles)
ex. sodium chloride, NaCl, dissociates to form a positively charged Na+ and a negatively charged CL-
ex. of electrolytes:
Potassium
Sodium
Chloride
Nonelectrolytes
Particles that do not dissociate into ions
ex. glucose, urea
Cations
Positively charged ions
Anions
Negatively charged ions
Cations and anions
May be exchanged for one another, providing they have the same charge
Both the interstitial fluid and extracellular fluid contain equal amounts of anions and cations
Osmosis
The movement of water across a semipermeable membrane
Water diffuses down a concentration gradient from the side that has fewer non-diffusable particles to the side that has more, creating osmotic pressure
The magnitude of the osmotic pressure represents
Osmolality
Refers to the osmotic activity that nondiffusable particles exert in pulling water from one side of the semipermeable membrane to the other
Tonicity
The tension or effect that the osmotic pressure of a solution with nondiffusable solutes exerts on cell size because of water movement
Isotonic solution
Same osmolality as the intercellular fluid
ex. .9% sodium chloride
Cells placed in an isotonic solution will neither shrink nor swell
Hypotonic solution
Lower osmolality than the intercellular fluid
When cells are placed in a hypotonic solution, they swell as water moves into the cells
Hypertonic solution
Greater osmolality than intercellular fluid
When cells are placed in a hypertonic solution, they shrink as water moves out of the cells
Body water constitutes approximately ____% of body weight
60%
ICF= 40%, ECF= 20%
Two major subdivisons of the ECF compartment
Interstitial compartment= 14% of body weight
Plasma compartment= 5% of body weight
Transcellular compartment= 1%
Fluid in the interstitial compartment
Acts as a transport vehicle for gases, nutrients, wastes, and other materials
Produces a reservoir from which vascular volume can be maintained during periods of hemorrhage or loss
Interstitial gel, a sponge-like material supported by collagen fibers, fil
Transcellular compartment
Includes the cerebrospinal fluid and the fluid contained in the vascular body spaces, such as the peritoneal, pleural, and pericardial cavities, and joint spaces
Normally 1% of extracellular fluid, but can increase in conditions such as ascites, in which
Four main forces that control the movement of water between the capillary and interstitial spaces
1. Capillary filtration pressure
2. Capillary colloidal osmotic pressure
3. Interstitial or tissue hydrostatic pressure
4. Interstitial colloidal osmotic pressure
Capillary filtration
Refers to the movement of water through capillary pores because of hydrostatic pressure, rather than osmotic force
Capillary filtration pressure
The pressure pushing water out of the capillary into the interstitial spaces
30-40 mm Hg at the arterial end
10-15 mm Hg at the venous end
25 mm Hg in the middle
Hydrostatic pressure
Pressure that results from the weight of water
Inside the capillaries, the hydrostatic pressure is the same as the capillary filtration pressure, about 30 mm Hg at the arterial and and 10 mm Hg at the venous end
Colloidal osmotic pressure
The pulling force created by the presence of evenly dispersed particles, such as the plasma proteins, that cannot pass through the pores of the capillary membrane
Capillary colloidal osmotic pressure
Osmotic pressure generated by the plasma proteins that are too large to pass through the pores of the capillary wall
Pulls fluid back into the capillary
About 28 mm Hg
Interstitial fluid pressure (-3 mm Hg)
Interstitial colloidal osmotic pressure (8 mm Hg)
Contribute to movement of water into and out of interstitial spaces
Interstitial fluid pressure
Force of fluid in the interstitial spaces pushing against the outside of the capillary wall
Normally negative (-3 mm Hg), contributing to the outward movement of fluid from the capillary
Contributes to the outward movement of water into the interstitial s
Interstitial colloidal osmotic pressure
Reflects the small amount of plasma proteins that normally escape into the interstitial spaces from the capillary
Lymphatic system
Represents an accessory route whereby fluid from the interstitial spaces can return to the circulation
Provides a means for removing plasma proteins and osmotically active particulate matter from the tissue spaces
Without it, excessive amounts of fluid ac
Edema
Palpable swelling produced by an increase in interstitial fluid volume
Factors that can lead to edema
1) Increase of capillary filtration pressure
2) Decrease in capillary colloidal osmotic pressure
3) Increase in capillary permeability
4) Obstruction of lymph flow
Cause of edema- increased capillary pressure
Usually due to increased vascular volume (e.g. heart failure- fluid retention, kidney disease)
Venous obstruction (e.g. thrombophlebitis)
Liver disease with portal vein obstruction
Acute pulmonary edema
Commonly causes dependent enema- accumulation of flu
Cause of edema- decreased colloidal osmotic pressure
Usually due to increased loss of plasma proteins, mainly albumin (e.g. liver failure- impaired synthesis of albumin, kidney diseases- glomerular capillaries become permeable to the plasma proteins, particularly albumin & large amounts of albumin are filte
Cause of edema- increased capillary permeability
When the capillary pores become enlarged or the integrity of the capillary wall is damaged, capillary permeability is increased ---> plasma proteins and other particles leak into the interstitial spaces, pulling fluid out of the capillary into the interst
Cause of edema- obstruction of lymphatic flow
Osmotically active plasma proteins and other large particles that cannot be reabsorbed through the pores in the capillary membrane rely on the lymphatic system for the movement back into the circulatory system
Edema due to impaired lymph flow is commonly
Plasma proteins
Exert the osmotic force needed to pull fluid back into the capillary from tissue spaces
Mixture of albumin, globulins, and fibrinogen
Synthesized in the liver
Albumin
Smallest of the plasma proteins, most abundant
Greatest effect on colloidal osmotic pressure
Third space accumulation
Represents the loss or movement and trapping of ECF in a transcellular space, such as in the pericardial sac, the peritoneal cavity, or the pleural cavity
Effusion
Transudation of fluid into the serous cavities
Can contain blood, plasma proteins, inflammatory cells (i.e. pus), and extracellular fluid
The distribution of body fluids between the ICF and ECF compartments relies on...
The concentration of ECF water and sodium
Water- 90-93% volume of body fluids
Sodium salts- 90-95% of ECF solutes
TBW
Total body water
Young men- ~60% of body weight
Elderly men- ~52% of body weight
Young women- ~50% of body weight
Elderly women- ~46% of body weight
Infants- 75-80% of body weight
Obesity decreases TBW, with levels sometimes as low as 30-40% in adults
Obligatory urine output
Even when oral or parenteral fluids are withheld, the kidneys continue to produce urine as a means of ridding the body of metabolic wastes
Insensible water losses
Water losses that occur through evaporative losses from skin and to moisten the air in the respiratory system
Occur without a person's awareness
Most plentiful electrolyte in the ECF compartment
Sodium (Na+)
135-145 mEq/L
Does not readily cross the cell membrane; as a result, only a small amount is in the intracellular fluid
The major cation in the ECF compartment
Na+
Along with its attendant Cl- and HC03- anions, account for approximately 90-95% of the osmotic activity in the ECF
Thus, serum osmolality usually varies with changes in serum sodium concentration
Most sodium losses occur through...
The kidney
When sodium intake is limited or conservation of sodium is needed, the kidneys are able to reabsorb almost all the Na+ that has been filtered through the glomerulus, resulting in essentially sodium-free urine
Two major physiologic mechanisms for regulating body levels of water
Thirst
Antidiuretic hormone (ADH)
ADH
Controls the output of water by the kidney
Effective circulating volume
The portion of the ECF that fills the vascular compartment and is "effectively" perfusing the tissues
Monitored by sensors that are located both in the vascular system and the kidney
Two things that regulate sodium balance by the kidneys
Sympathetic nervous system
Renin-angiotensin-aldosterone system
Sympathetic nervous system
Responds to changes in arterial pressure and blood volume by adjusting the glomerular filtration rate and the rate at which sodium is filtered from the blood
Also regulates renal reabsorption of sodium and renin release
Renin-angiotensin-aldosterone system
Exerts its action through angiotensin II and aldosterone
Angiotensin II acts directly on the renal tubules to increase sodium reabsorption. It also acts to constrict renal blood vessels, thereby decreasing the glomerular filtration rate and slowing renal
Inappropriate ADH secretion would cause a person to exhibit:
A small volume of urine and low serum sodium
Thirst
The conscious sensation of the need to obtain and drink fluids high in water content
Controlled by the thirst center in the hypothalamus
One of the earliest symptoms of hemorrhage and is often present before other signs of blood loss appear
Two stimili for true thirst based on water needs
1) Cellular dehydration caused by an increase in ECF osmalality
2) A decrease in the effective circulating volume, which may or may not be associated with a decrease in serum osmolality
Sensory receptors, called osmoreceptors, which are located in or near
Third "backup" system for thirst
Production of angiotensin II by the renin-angiotensin mechanism in the kidney
Angiotensin II increases in response to low blood volume and low blood pressure
Elevated levels of angiotensin II may lead to thirst in conditions such as congestive heart failu
Hypodipsia
A decrease in the ability to sense thirst
Commonly associated with lesions in the area of the hypothalamus (e.g. head trauma, meningiomas, occult hydrocephalus, subarachnoid hemorrhage)
Polydipsia
Excessive thirst
Normal when it accompanies conditions of water deficit, but abnormal when it results in excess water intake
Increased thirst/drinking behavior can be classified into two categories
1) Inappropriate or false thirst that occurs despite norm
Vasopressin (ADH)
The antidiuretic hormone
Controls the reabsorption of water by the kidneys
Synthesized in the hypothalamus and stored in the posterior pituitary gland
Increased osmolality ---> nerve impulses from hypothalamus travel to the pituitary gland ---> ant pit re
How ADH exerts its effects
Through vasopressin receptors located in the collecting tubules of the kidney
In the presence of ADH, highly permeable water channels called aquaporins are inserted into the tubular membrane. The increased water permeability allows water from urine to be
ADH acts on the ______ to _______ water excretion
Kidneys; decrease
ADH levels are high---> most or all of the filtered water is reabsorbed and a small amount of concentrated urine is excreted
ADH levels are low---> less water is reabsorbed and dilute urine is excreted
Diabetes insipidus
Caused by a deficiency of ADH or a decreased renal response to ADH
People with DI are unable to concentrate their urine during periods of water restriction and they excrete large volumes of urine- accompanied by excessive thirst
Neurogenic/central DI- occ
SIADH
Syndrome of inappropriate antidiuretic hormone
Results from a failure of the negative feedback system that regulates the release and inhibition of ADH
In people with the syndrome, ADH secretion continues even when the serum osmolality is decreased, causin
Disorders of water and sodium balance can be divided into two main categories
1) Isotonic contraction or expansion of the ECF volume brought about by proportionate changes in sodium and water
2) Hypotonic dilution or hypertonic concentration of the ECF brought about by disproportionate changes in sodium and water
Isotonic fluid volume deficit
Results when water and electrolytes are lost in isotonic proportions
Almost always caused by a loss of body fluids and is often accompanied by a decrease in fluid intake
Often follows loss of GI fluids with severe vomiting, diarrhea, or gastrointestinal s
Isotonic fluid volume excess
Represents an isotonic expansion in the ECF compartment ---> increases both interstitial and vascular volumes
Usually results in an increase in total body sodium accompanied by a proportionate increase in body water
Excessive sodium intake or a decrease i
Hyponatremia
Sodium deficit
Decreased serum sodium osmolality
Dilutional decrease in blood components, including hematocrit, blood urea nitrogen (BUN)
One of the most common electrolyte disorders seen in hospitalized patients
Can present as hypovolemic, (low blood pla
Hypernatermia
Sodium excess
Increased serum sodium osmolality
Increased concentrations of blood components, including hematocrit, BUN
Results in cellular dehydration---> excessive thirst
What is the main determinant of water and sodium balance?
The effective circulating blood volume
Monitored by
-Stretch receptors in the vascular system that exert their effects through thirst, which controls water intake
-ADH, which controls urine concentration
Sodium (Na+)
Most abundant cation (positively charged ion) in the ECF
Chloride (Cl-)
Most abundant anion (negatively charged ion) in the ECF
Potassium (K+)
Most abundant cation in the intracellular fluid- 98% located in ICF
Second most abundant cation in the body
High ICF concentration of potassium is required for many cell functions, including
-Maintenance of osmotic integrity of cells and acid-base balance
Hypokalemia
Potassium deficit
Can result from inadequate intake, excessive losses, or redistribution from the ICF to ECF compartments
Manifested by alterations in kidney, skeletal muscle, gastrointestinal, and cardiac function
Hyperkalemia
Potassium excess
Can result from decreased elimination of potassium by the kidney, a transcellular shift in potassium from the ICF to ECF compartment, or excessively fast IV administration of potassium
Manifested by alterations in neuromuscular and cardia
Hydrogen phosphate (HPO4-)
Most abundant anion in the intracellular fluid
Most water is excreted via the
Kidneys
Aldosterone
Key regulator of sodium reabsorption in the kidney
Which of the following would increase sodium excretion?
-Progesterone
-Glucocorticoids
-Aldosterone
-Estrogen
Progesterone
Calcium, phosphorus, magnesium
The most divalent (two bonds formed) cations in the body
Levels regulated by intestine, kidney, and bone, primarily by the interaction of PTH and vitamin D
Calcium
Most deposited into bone- only a small amount in the ECF
99% of calcium found in the bone, less than 1% in ECF compartment
Of the three forms of ionized calcium, only the ionized form can cross the cell membrane, contributing to
-Neuromuscular function
-B
Hypocalcemia
Decreased levels in ionized calcium
Produces an increase in muscular excitability
Hypercalcemia
Increased levels of ionized calcium
Produces a decrease in muscular excitability
Phosphorous
Like calcium, largely an ICF anion, being incorporated into nucleic acid, ATP, and RBCs
Hypophosphatemia
Abnormally low levels of phosphate in the blood
Associated with:
-Decreased intestinal absorption
-Transcompartmental shifts
-Disorders of renal elimination
Causes:
-Signs and symptoms of neural dysfunction
-Disturbed musculoskeletal function
-Hematologic
Hyperphosphatemia
Abnormally high levels of phosphate in the blood
Occurs with renal failure and PTH deficit
Associated with decreased plasma calcium levels
Magnesium
Second most abundant ICF cation (after potassium)
Acts on many intracellular enzyme reactions
Required for cellular energy metabolism
Hypomagnesemia
Deficiency of magnesium in the blood
Produces:
-Decrease in calcium due to suppression of PTH release
-Decrease in serum potassium due to renal wasting
Both contribute to increase in neuromuscular excitability
Hypermagnesemia
Overabundance of magnesium in the blood
Causes neuromuscular dysfunction, muscle weakness, confusion
Parathyroid hormone
Main function: to maintain extracellular fluid calcium concentrations
-Released in response to low calcium levels
-Causes the release of calcium + phosphorusfrom the bones
-Increased absorption of calcium + phosphorus from small intestine (through its eff
Hypoparathyroidism
Diminished concentration of PTH in the blood
Acute- causes hypocalcemia, manifested by muscle spasms, muscle tetany (cramps)
Chronic- manifested by lethargy and fatigue
Hyperparathyroidism
Abnormally high PTH concentration in the blood
Primary disorder- causing elevated calcium levels and increased excretion of both calcium and phosphorus ---> weak bones, potential for kidney stones
Secondary- associated with chronic kidney disease, exerts
Vitamin D
Synthesized by the skin and converted to its active form, calcitriol, in the kidney
Calcitrol acts on the intestines, kidney, bone and increases calcium and phosphorus concentration
-Stimulates absorption of calcium from the intestine
-Increases calcium a
Acid-base balance
Metabolic activities of the body require precise regulation of acid-base balance as reflected in the pH of the ECF- normally maintained within a very narrow range of 7.35-7.45
Acid
A compound that can dissociate and release a hydrogen (H+) ion
Base
A compound that can accept or combine with H+
Most of the body's acids and bases are...
Weak
H2C03
Weak acid derived from carbon dioxide
HC03-
Weak base
The concentration of hydrogen in body fluids is _____ compared to other ions
Low
Because of this, hydrogen concentration is expressed as pH
pH
Represents the negative logarithm of H+ concentration
Low pH = acidic = high H+ concentration
High pH = alkaline = low H+ concentration
Carbon dioxide
Body metabolism results in a continuous production of carbon dioxide
Diffuses out of body cells and into tissue spaces, then into the circulation
Carbon dioxide is transported into circulation in three forms:
1) Dissolved in plasma
2) As a bicarbonate
3) Attached to hemoglobin
Carbon dioxide dissolved in plasma
Small portion (10%) of CO2 that is produced by body cells is transported in the dissolved state to the lungs and then exhaled
The amount of CO2 that can be carried in plasma is determined by the partial pressure of the gas (PCO3) and the solubility coeffi
Carbon dioxide in bicarbonate
CO2 in excess of that which can be carried into the plasma moves into the RBCs, where the enzyme carbonic anhydrase (CA) catalyzes its conversion to carbonic acid (H2CO3)
The H2CO3 then dissociates into hydrogen (H+) and bicarbonate (HCO3-) ions
H+ combin
Carbon dioxide in hemoglobin
The remaining CO2 in the RBCs combines with hemoglobin to form carbaminohemoglobin (HbCo3)
The combo of CO2 with hemoglobin is a reversible reaction characterized by loose blood, so that Co2 can be easily released in the alveolar capillaries and exhaled f
Regulation of acid-base balance
Metabolic processes produce volatile carbonic acid (H2CO3) in equilibrium with dissolved carbon dioxide (PCO2), which is eliminated through the lungs, and nonvolatile acids, which are excreted by the kidneys
pH in body fluids is regulated by three major mechanisms
1) Chemical buffer systems in body fluids, which immediately combine with excess acids or bases to prevent large changes in pH
2) The lungs, which control the elimination of CO2
3) The kidneys, which eliminate H+ and both reabsorb and generate HCO3-
Respiratory regulation of pH
Relies on ventilation for release of CO2 into the environment
Is rapid but does not return the pH completely to normal
Renal regulation of pH
Rely on the elimination of H+ ions and the conservation of HCO3- ions
Takes longer but returns pH to normal or near-normal levels
Metabolic acidosis
Decrease in pH due to a decrease in HCO3-
Metabolic alkalosis
Increase in pH due to an increase in HCO3-
Respiratory acidosis
Decrease in pH due to an increase in PCO2 levels
Result of elevated blood levels of CO2 due to hypoventilation, shallow breathing, suffocation, or lung diseases that impede O2 and CO2 exchange.
Respiratory alkalosis
Increase in pH due to a decrease in PCO2 levels
Caused by conditions that produce hyperventilation
If you gave a normal patient 1 L of distilled water IV (which you would never do), once that water had equilibrated across the total body water the plasma volume would be expanded by about:
Less than 100 mL
If you gave a normal patient 1 L of isotonic saline IV, once that fluid had equilibrated across the total body water the intracellular volume would be expanded by about:
0 mL
Which of the following would NOT cause edema:
a) a decrease in arterial blood pressure.
b) an increase in venous blood pressure.
c) a decrease in albumin production by the liver.
d) an increase in capillary permeability to proteins.
a) a decrease in arterial blood pressure
Inappropriate ADH secretion would cause a person to exhibit:
A small volume of urine and low serum sodium
A person who is dehydrated because of lack of water access would exhibit:
A small volume of urine and high serum sodium
A person with untreated diabetes insipidus would exhibit:
A large volume of urine and high serum sodium
In diabetic ketoacidosis we expect to see a:
A low serum bicarbonate level and a low pH
Which of the following would be most likely to produce a small reduction in pH with an elevated PCO2 and an elevated serum bicarbonate?
Chronic emphysema
Buffers
A substance that consists of acid and base forms in a solution and that minimizes changes in pH when extraneous acids or bases are added to the solution.
At least 3/4 of the body's buffering capacity is via intracellular proteins