Human Body Composition Flashcards
2 major fluid compartments of the body
Intracellular fluid (ICF) Extracellular fluid (ECF)
2 subcompartments of the ECF
Intravascular
Interstitial (extravascular)
Percent of body weight occupied by water in an average man
60%
Percent body weight occupied by ICF
40%
Percent body weight occupied by ECF
20%
Percent body weight occupied by water in old age
50%
Percentage of ECF water in the interstitial compartment
75 - 80%
Percentage of ECF water in the vascular compartment
20 - 25%
Name of fluid in the intravascular compartment
Plasma
Via which compartment does water enter and leave the body?
Intravascular compartment
What determines the volume of water that ends up in a given compartment?
Osmotic pressure (the number of osmotically active particles within) Intravascular hydrostatic pressure
Define an osmole
The number of osmotically active particles in a solution
Define an osmotically active particle
A dissolved molecule (solute) that cannot penetrate through a water-permeable membrae
Semi-permeable membrane separating the ECF and ICF
Cell membrane
4 naturally occurring molecules that can freely penetrate cell membranes
Oxygen
Carbon dioxide
Water
Urea
Solute composition of the ICF
Sodium and chloride = very low
Potassium, magnesium and phosphate = very high
Protein = very high (approx. 200 g/L)
Solute composition of the ECF
Sodium = high (140 mmol/L) Chloride = high (105 mmol/L) Potassium = very low (4 mmol/L) Protein = very low (relative to ICF)
Semi-permeable membrane separating the interstitial and intravascular compartments of the ECF
Capillary wall
2 components of the bloodstream
Fluid (plasma)
Cells (blood cells)
What do the capillaries retain in their lumen (despite being porous)
Blood cells
High molecular weight proteins
One of the main reasons why capillaries retain high molecular weight proteins
To create an osmotic pressure to counter-balance the hydrostatic pressure that would otherwise force plasma water out of them into the interstitial compartment and deplete the intravascular compartment
Approximate hydrostatic pressure inside the capillaries relative to the arterial pressure
Considerably less than half (but far from zero)
Pressure in the interstitial compartment
Approximately zero
Define colloid oncotic pressure
Inward directed osmotic pressure (in the capillaries)
WHat exerts the colloid oncotic pressure?
The concentration difference of high molecular weight proteins across the capillary wall
Main high molecular weight protein in the capillary responsible for the colloid oncotic pressure
Albumin
Define the hematocrit
Content of blood occupied by red blood cells (40% normally)
Percentage of blood occupied by plasma
60%
Most acutely critical body compartment
Intravascular compartment
Purpose of the interstitial compartment in terms of water balance
Buffer to minimize volume fluctuations in the intravascular compartment (since it is a zero pressure system)
Speed of equalization between the ECF sub-compartments’ osmolarity and why
Rapid (minutes) because the ECF’s main osmoles (sodium and chloride) permeate easily through the capillary wall
Speed of equalization between the ECF and ICF osmolarity and why
Long (hours) because it occurs as a result of water shifts alone
Standard osmolarity of all fluid compartments at equilibrium
300 mOsm/L
Normal serum sodium concentration
140 mmol/L
Importance of serum sodium concentration measurement
Indication of osmolarity in all of the fluid compartments
What fluid does the intracellular compartment contain
Water inside the body’s cells (mostly skeletal muscle cells)
Anatomic correspondence of the intracellular compartment
Body cell mass (BCM)
Reason why the volume of the ICF compartment is large
1) A lot of cells in the body
2) These cells exert a strong osmotic pressure
Why do the cells of the body exert a strong osmotic pressure?
High concentrations of potassium, magnesium, phosphate and soluble proteins
Define a hypotonic solution
Solution that has a relatively low osmolarity
Define a hypertonic solution
Solution that has a relatively high osmolarity
Effect of putting a cell in a hypotonic environment
Unequal osmotic pressures will drive water into the cell –> swell to a larger volume
Effect of putting a cell in a hypertonic environment
Unequal osmotic pressures will drive water out of the cell –> shrink in size
Define simple dehydration
A medical condition accompanying hypertonicity of the body’s fluid compartments
Define relative dehydration
A disproportionately large amount of sodium in relation to the volume of water in the ECF (hyperosmolarity)
Define hypernatremia
Abnormally high serum sodium concentration
Most frequent cause of simple dehydration
Failure or inability of the patient to drink enough water to compensate for body losses of water
Key symptom pointing to relative dehydration
Thirst
2 clinical symptoms and signs of moderate dehydration (thirst aside)
Lethargy
Uneasiness
Cause of symptoms in severe dehydration
Shift of water out of brain cells –> shrink them enough to depress consciousness and cause coma and death
Water buffer for the ECF compartment
ICF
Effect of simple dehydration on ECF osmolarity
Hyperosmolarity
Effect of ECF hyperosmolarity on fluid distribution
Shift water from the ICF into the ECF –> protect from drastic depletion
Define simple (ECF) volume depletion
A roughly iso-osmolar loss of electrolytes and water from the ECF
2 common causes of volume depletion
Severe diarrhea
Protracted vomiting
Intravascular compartment effects of a lowered ECF volume (acutely)
Reduced volume
Lowered arterial blood pressure
Compensatory response to reduced intravascular volume
Increased HR to maintain the rate of blood flow through the circulation
Describe the compensatory response in SUBTLE forms of ECF depletion and the importance of this
HR accelerates only upon assuming an upright position. Measurement of BP and HR, first on lying, then upright posture = important diagnostic maneuver
Describe the physical manifestation of low venous pressure
Veins that are normally visible, such as jugular veins, become undetectable
Describe the contribution to ECF depletion by each of its compartments
Interstitial fluid volume and intravascular volumes decrease together, but most decrease occurs in the interstitial
Place in the body to examine the interstitial compartment
Skin
Describe the skin of a patient with normal interstitial fluid volume
Juicy
Medical term for skin juiciness
Turgor
Describe decreased skin turgor
If you pinch up the skin over the deltoid muscle or sternum or a patient who is volume depleted, the pinched up skin will remain tented, like putty, for a few moments
Describe the mouth of a person with volume depletion
Small pool of saliva normally present under the tongue disappears
Urine excretion under volume depletion
Decreased (compensatory mechanism)
2 signs and symptoms of a baby experiencing volume depletion
Dry diaper
Crying without tears
Consequence of blood volume falling >50%
Inability to perfuse organs adequately
Define hypovolemic shock
Inability to perfuse organs adequately
Earlier stage of shock
Pre-shock
Define pre-shock
An important and dangerous intravascular volume depletion (>15% of blood volume)
5 symptoms and signs of pre-shock
BP very low HR very fast Collapsed veins Reduced (or zero) urine output Patient feels terrible
Body’s response to severe toxic-inflammatory states
Cytokines and hormones are released from certain cells to make the capillary wall much more permeable to large molecular weight proteins
Consequence on serum albumin in the event of toxic-inflammatory states
Albumin can diffuse out of the vascular compartment –> interstitial compartment –> patient’s serum albumin concentration decreases dramatically (NOTE: total albumin unchanged in ECF, just redistributed)
Reliable indicator of tissue damage and systemic inflammation
Hyperalbuminemia
Define over-hydration
Decreased serum sodium per L of plasma
When can over-hydration AND hypovolemia occur together?
In a patient who has been experiencing ECF depletion for a long enough time for their body to sense their low ECF volume and respond by sending a thirst signal to induce them to drink more water. Kidney also reabsorbs more free water.
Why does the body allow over-hydration to occur in the setting of hypovolemia?
The body permits its fluid compartments to become hypo-osmolar because its survival priority is preservation of the intravascular volume
Amount of water that the skin can soak up and release without visible change
2 - 3 kg (10 pounds)
The most sensitive way to detect small changes in the ECF volume
Record and follow the patient’s weight
Characteristic and specific sign of ECF excess
Palpable and usually visible accumulation of fluid within and under the skin (edema)
Most dangerous consequence of over-expansion of the total ECF compartment
Intolerable over-expansion of the intravascular compartment
4 signs and symptoms of over-expansion of the intravascular compartment
Feeling of distress
Normal (or above-normal) BP
Swollen veins (high venous pressure)
Expanded interstitial compartment (edema)
2 commonest settings for ever-expansion of the intravascular compartment
Acute RHF Acute LHF (--> pulmonary edema)
Define effusion
Abnormal accumulation of fluid in potential spaces of the body (i.e. pleural space or pericardium)
Define ascites
Excessive ECF accumulation in the abdomen (peritoneum)
Define transudate
An effusion with the chemical composition of normal interstitial fluid (same electrolyte pattern and a LOW protein concentration)
Cause of transudates
Pathological expansion of the ECF compartment, developing from an increased outward hydrostatic P, negative interstitial tissue P, and decreased plasma oncotic P
Define exudate
Protein-rich and often cell-rich outpouring of inflammatory fluid
Cause of exudate
Infection or tissue damage
4 anatomic compartments of the human body
Fat
Non-fat soft tissues (body cell mass)
Extracellular fluids
Structural compartment (skeleton and associated connective tissue)
Define the body cell mass
Hydrated, metabolically active protein tissues of the body
Approximate percent body weight occupied by bones and supporting extracellular ligaments and structural protein
20%
Ideal percent of body weight occupied by fat
20% in men
30% in women
Most variable compartment
Fat
Two techniques to determine contribution of pure fat to total body weight
Densitometry
Measuring total body water (isotope dilution or bioelectrical impedance analysis)
Describe densitometry
Measurement of total body fat by relying on different densities of pure fat (0.900 kg/L) and the rest of the body (1.100 kg/L). Every density in between these boundaries corresponds to a unique fraction of fat/total body weight
Differentiate between pure fat and the fat seen and felt on a person
Adipose tissue = physical fat, comprised of 85% pure fat and 15% water
For precise calculations of body fat loss, what must one consider?
The 15% of body water that accompanies any important change in pure fat
Equation for BMI
Weight (kg) / height^2 (m)
Conversion from kg to lbs
1 kg = 2.2 lbs
Conversion from inches to cm
1 inch = 2.54 cm
2 exceptions to BMI
1) Professional athletes and body builders may have an abnormally high muscle mass and low fat, but high BMI
2) ECF compartment expansion or depletion under pathological conditions
Define BMI >25
Implies that the body’s fat content is greater than normal
Define BMI <18 - 20
Implies the body’s fat content is below normal
Define BMI >30
Association with increasing health risk that increases in rough proportion to the increase in BMI
Approximate percent of BCM occupied by skeletal muscle
80%
Define the peripheral protein store
Portion of the BCM containing skeletal muscle
Approximate percent of BCM occupied by all other non-adipocyte cels of the body (i.e. organs, bone marrow, blood cells)
20%
Define the central/visceral protein store
Proteins from the portion of the BCM containing organs, blood cells and bone marrow (non-adipocyte cells that are non-skeletal muscle)
When does BCM decrease
Muscle mass loss
Define BCM loss of >10 - 15%
Association with reduced strength and a variety of physiological derangements
Define BCM loss of >25%
Association with severe disability and an increased risk of death from several possible complications
Define BCM loss of >45%
Commonly incompatible with survival
Most important way to assess a patient’s BCM
Examine their muscle groups
Method to measure the rate of change of BCM
Calculate changes in body nitrogen content
Percent weight of protein occupied by nitrogen
16%
1 g of N lost corresponds to what mass of protein lost?
6.25 g of protein
1 g of protein lost corresponds to what mass of normally hydrated lean tissue lost?
5 g
Equation for the estimation of daily N output
Urinary urea N excretion : 2 g N (urinary non-urea N) + 2 g fecal N
Define sarcopenia
The decrease of muscle mass with age (old people’s fat mass tend to increase)
