Unit 5. Chapter 25 The Body Compartments: Extracellular and Intracellular Fluids; EdemanSaturday, December 19, 202010:54 AM GUYTON AND HALL MEDICAL PHYSIOLOGY
Unit 5. Chapter 25 The Body Compartments: Extracellular and Intracellular Fluids; Edema
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Saturday, December 19, 2020
10:54 AM
#Fluid intake and output are balanced during steady state conditions
The relative constancy of the body fluids is remarkable because there is continuous exchange of fluid and solutes with the external environment as well as within the different compartments of the body.
*daily intake of water
Two major sources
- Ingested in the form of liquid or water in the food. About 2100ml/day
- Synthesized in the body as a result of oxidation of carbohydrates adding about 200ml/day
*daily loss of body water
-insensible water loss
There is water loss through the skin which averages about 300 to 400 mL per day. This skin loss is minimized by the cholesterol filled cornified layer. Without this layer such as is the case in pts with burns the loss increases to 3 to 5 L per day. That is why burn victims must be given large amounts of water usually intravenously.
Then there is water loss through the respiratory system. Because the inhaled vapor pressure is lower than the vapor pressure of exhaled air, about 300 to 400 mL gets breathed out per day. In cold weather this increases because the atmospheric vapor decreases to nearly 0. That is why there is the dry feeling in the respiratory passages in cold weather
-fluid loss in sweat
Highly variable. About a 100 mL normally but even 1 to 2 L per hour in heavy exercise or very hot weather
-water loss in feces
Not much. 100mL per day.
-water loss by the kidneys
The most important means by which the body maintains a balance between water intake and output, as well as a balance between intake and output of most electrolytes in the body is by controlling the rates at which the kidneys excrete these substances.
#body fluid compartments
Intracellular fluid + extracellular fluid (interstitial fluid + plasma + transcellular fluid[1-2L synovial, peritoneal, pericardial, intraocular, cerebrospinal fluid, etc])
Total body fluid is about 60% of the body weight
42L in an average 70 Kg man
*intracellular fluid compartment
28L of the 42L
40% of the total body weight in an average person
#Extracellular fluid compartment
14L in a normal 70 kilogram man
20% of the total body weight
#blood volume
*hematocrit(packed red cell volume)
0.40 in men and 0.36 in women
Severe anemia as low as 0.10
Polycythemia can rise to 0.65
#constituents of extracellular and intracellular fluids
*ionic composition of plasma and interstitial fluid is similar
Donnan effect
*intracellular fluid constituents
Contains only small quantities of sodium and cl ions and almost no calcium ions.
Contains large amounts of potassium and phosphate ions plus moderate amount of magnesium and sulfate ions, all of which have low concentrations in the extracellular fluid.
Also contains almost four times as much protein as in the plasma
#measurement of fluid volumes in the different body fluid compartments - the indicator-dilution principle
#Determination of volumes of specific body fluid compartments
*measurement of total body water
*measurement of extracellular fluid volume
*calculation of intracellular volume
*measurement of plasma volume
*calculation of interstitial fluid volume
*measurement of blood volume
#regulation of fluid exchange and osmotic equilibrium between intracellular and extracellular fluid
#basic principles of osmosis and osmotic pressure
*relation between moles and osmoles
One osmole (osm) is equal to 1 mole(6.02 time 1023) of solute particles
1 mol of nacl will have an osm of 2osm/L because na and cl come apart
*osmolality and osmolarity
Osmolality is osmoles per kilogram of water
Osmolarity is osmoles per Liter of solution
*calculation of the osmolarity and osmotic pressure of a solution
*osmolarity of the body fluids
*corrected osmolar activity of the body fluids
#osmotic equilibrium is maintained between intracellular and extracellular fluids
*isosmotic, hyperosmotic and hypoosmotic fluids
The terms isotonic, hypotonic and hypertonic refer to whether solutions will cause a change in cell volume. The tonicity of solutions depends on the concentration of impermeant solutes. Some solutes however can permeate the cell membrane. Solutions with an osmolarity the same as the cell are called isoosmotic, regardless of whether the solute can penetrate the cell membrane. The terms hyperosmotic and hypoosmotic refer to solutions that have a higher or lower osmolarity, respectively, compared with the normal extracellular fluid without regard for whether the solute permeates the cell membrane.
*osmotic equilibrium between intracellular and extracellular fluids is rapidly attained
#volume and osmolality of extracellular and intracellular fluids in abnormal states
One can calculate both the changes in intracellular and extracellular fluid volumes and types of therapy that should be instituted if the following basic principles are kept in mind.
- Water moves rapidly across cell membranes
Even when a solution is added and disequilibrium is caused, water moves back and forth and make equilibrium in a matter of minutes
- Cell membranes are almost completely impermeable
The changes in the number of osmoles usually occur in extracellular fluid according to how much is added or lost. Otherwise the numbers in the intracellular and extracellular remain constant
*effect of adding saline solution to the extracellular fluid
-calculation of fluid shifts and osmolarities after infusion of hypertonic saline
Step 1. Initial conditions
Step 2. Instantaneous effect of adding 2 liters of 3.0 percent sodium chloride
Step 3. Effect of adding 2 liters of 3.0 percent sodium chloride after osmotic equilibrium
#glucose and other solutions administered for nutritive purposes
Care must be taken that these nutritive and therapeutic solutions are adjusted close to isotonic as much as possible or are administered slowly enough that they do not upset the osmotic equilibrium of the body fluids.
Additional water that is left after the substances have been metabolized is excreted through the kidneys. So net addition are the nutrients.
#clinical abnormalities of fluid volume regulation: hyponatremia and hypernatremia
*causes of hyponatremia: excess water or loss of sodium
- Loss of sodium chloride : diarrhea, vomiting, overuse of diuretics that inhibit the ability of the kidneys to conserve sodium, certain types of sodium-wasting kidney disease, Addison's disease: which results from decreased secretion of aldosterone, impairs the ability of the kidneys to reabsorb sodium and can cause a modest degree of hypnatremia
- Excess water retention: excessive secretion of antidiuretic hormone
*consequences of hyponatremia : cell swelling
Brain cell edema and neurological symptoms including headache, nausea, lethargy, and disorientation. If plasma sodium concentration rapidly falls below 115 to 120 mmol/L, brain swelling may lead to seizures, coma, permanent brain damage and death.
If this state persists, the brain cells emits sodium, chloride, potassium and organic solutes to the extracellular fluid. But if hyponatremia is corrected too quickly by giving sodium excessively this can outpace the brain's ability to recapture the solutes lost from the cells and could cause osmotic injury that involves demyelination of the brain cells.
This osmotic injury could be avoided by limiting the correction of hyponatremia to less than 10 to 12 mmol/L in 24 hours and to less than 18mmol/L in 48 hours.
*causes of hypernatremia: water loss or excess sodium
- Loss of water: inability to secrete antidiuretic hormone, diabetes insipidus
Certain types of renal diseases, the kidneys cannot respond to antidiuretic hormone also causing a type of nephrogenic diabetes insipidus,
Less intake of water than output as in heavy exercise
- Excessive sodium chloride added
Excessive secretion of the sodium retaining hormone aldosterone but this does not create severe case of hypernatremia because there is more water absorption as well
*consequences of hypernatremia : cell shrinkage
Hypernatremia is not as common as in hyponatremia but severe cases can occur in patients with hypothalamic lesions that impair their sense of thirst, in infants who can't drink water when they want to, or elderly patients with altered mental status
Be prudent and correct hypernatremia slowly in patients who have had chronic increases in plasma sodium concentration because hypernatremia activates defense mechanisms that protect the cell from changes in volume and take in more solutes such as sodium into the cells.
#Edema: Excess fluid in the tissues
*intracellular edema
- Hyponatremia
- Depression of the metabolic systems of the tissues
- Lack of adequate nutrition to the cells
When the metabolic system goes down due to the oxygen lack or more the sodium potassium pump doesn't function properly, cause more sodium to enter into the cells. This brings in more water into the cells.
Also in inflamed tissues, inflammation increases cell membrane's permeability allowing sodium and other ions to diffuse into the cells which is followed by water.
*extracellular edema
- Abnormal leakage of fluid from the plasma to the interstitial spaces across the capillaries
- Failure of the lymphatics to return fluid from the interstitium back into the blood
-factors that can increase capillary filtration
-lymphedema-failure of the lymph vessels to return fluid and protein to the blood
Infection or surgery like radical mastectomy
*summary of causes of extracellular edema
- Increased capillary pressure
- Excessive Kidney retention of salt and water
-acute or chronic kidney failure
-mineralocorticoid excess
- High venous pressure and venous constriction
-heart failure
-venous obstruction
-failure of venous pumps
1. paralysis of muscles
2. immobilization of parts of the body
3. failure of venous valves
- Decreased arteriolar resistance
-excessive body heat
-insufficiency of sympathetic nervous systems
-vasodilator drugs
- Decreased plasma proteins
- burns
- wounds
- Failure to produce proteins
- liver disease (e.g.cirrohosis)
- serious protein or caloric malnutrition
- Increased capillary permeability
- immune reactions that cause release of histamine and other immune products
- toxins
- bacterial infections
- vitamin deficiency, esp C
- prolonged ischemia
- burns
- Blockage of lymph return
- cancer
- infections (eg filaria nematodes)
- surgery
- congenital absence or abnormality of lymphatic vessels
-edema caused by heart failure
In heart failure, arterial pressure tends to fall causing decreased excretion of salt and water by the kidneys
Decreased blood flow to the kidneys causes the secretion of renin, angiotensin2 and aldosterone. -> more edema
In cases where right heart still functions normally, lung edema becomes an issue
-edema caused by decreased kidney excretion of salt and water
Interstitial fluid volume is increased, hypertension is caused because of the increase in blood volume, children who develop acute glomerulonephritis
-edema caused by decreased plasma proteins
Loss of proteins in the urine in certain kidney diseases, this combined with the loss of body function to produce proteins causes a reduction in plasma protein. If the plasma protein concentration falls below 2.5g/100mL, serious generalized edema occurs.
Cirrhosis(development of large amounts of fibrous tissue among the liver parenchymal cells) - fails to produce sufficient plasma proteins. Also sometimes the liver fibrosis compresses the abdominal portal venous drainage vessels. Ascites occurs.
*safety factors that normally prevent edema
-safety factor caused by low compliance of the interstitium in the negative pressure range
- Importance of interstitial gel in preventing fluid accumulation in the interstitium
- Importance of the proteoglycan filaments as a spacer for the cells and in preventing rapid flow of fluid in the tissues
-increased lymph flow as a safety factor against edema
Safety factor caused by increased lymph flow has been calculated to be about 7mmHg.
-washdown of the interstitial fluid protein as a safety factor against edema
*summary of safety factors that prevent edema
The capillary pressure in a peripheral tissue could theoretically rise by 17mmHg or approximately double the normal value before marked edema would occur.
#fluids in the potential spaces of the body
Pleural cavity, pericardial cavity, peritoneal cavity, synovial cavity (joint and bursae)
*fluid is exchanged between the capillaries and the potential spaces
*lymphatic vessels drain protein from the potential spaces
*edema fluid in the potential spaces is called effusion
It is especially interesting that the normal fluid pressure in most or all of the potential spaces in the nonedematous state is negative in the same way that this pressure is negative(subatmospheric) in loose subcutaneous tissue. For instance, the interstitial fluid hydrostatic pressure is normally about - 7 to - 8 mmHg in the pleural cavity, -3 to -5 mmHg in the joint spaces and -5 to -6 mmHg in the pericardial cavity.