PS120: Body Water & Homeostasis Flashcards
Water consistutes…
60% of body weight
That means the volume of body water in a 70kg adult male is…
42 L
What are the compartments of total body water (TBW)?
- intracellular fluid (ICF): 40% of body weight and 2/3 of TBW
- extracellular fluid (ECF): 20% of body weight and 1/3 of TBW
What is ECF divided into?
- ISF (interstitial fluid): fluid present between the cells; 15% of body weight
- intravascular fluid (plasma): fluid present inside blood vessels; 5% of body weight.
15% of body weight is…
interstitial fluid (ISF)
5% of body weight is…
intravascular fluid (plasma)
40% of body weight is…
intracellular fluid (ICF)
20% of body weight is…
extracellular fluid (ECF)
60% of body weight is…
total body water (TBW)
what are the interstitial fluid and the ECF as a whole considered?
they are called the internal environment of the body because it is the environment in which cells are living.
what is ICF?
the fluid present inside the cells
what is ECF?
fluid present outside the cells
What are factors for normal variation in TBW?
- age
- sex
- fat content in body
TBW change with increase in age:
decreases with old age; goes down from 60% to 45% of body weight.
When does TBW reach 60%?
TBW reaches adult level of 60% at puberty.
Regarding age, who has the most TBW?
infants and children, with about 75% of body weight being TBW.
Why is there an increase in TBW in children?
where?
mainly in ECF
- the ECF to ICF ratio is much greater in children
How does the difference in ECF/ICF ratio affect children?
ECF is much easier to lose than ICF, so dehydration and fever develops much more rapidly and severely in children compared to adults.
In regards to sex, TBW is less in…
and why
females (50% of body weight)
- due to high fat content in our bodies.
- body fat is relatively free of water
In regards to fat content…
TBW is less in obese persons.
The concentration of cations in ICF:
in mEq/L
- sodium (Na+): 14
- potassium (K+): 140
- calcium (Ca2+): 0.0001
The concentration of cations in ECF:
in mEq/L
- sodium (Na+): 140
- potassium (K+): 4
- calcium (Ca2+): 2.4
The concentration of anions in ICF:
in mEq/L
- chlorine (Cl-): 4
- bicarbonate (HCO3 -): 10
- phosphates (PO4 3-): 75
- Proteins: 40 (16gm%)
The concentration of anions in ECF:
in mEq/L
- chlorine (Cl-): 104
- bicarbonate (HCO3 -): 28
- phosphates (PO4 3-): 4
- Proteins: 4 (2gm%)
How does ECF ionic composition differ between plasma and ISF?
the ion/electrolyte composition is virtually the same, however protein ratio is higher in plasma.
- plasma proteins: 17mEq/L, 7gm%
- ISF: 4mEq/L, 2gm%
What are the funcions of body water?
- Medium for most biological processes
- Medium for exchange process
- Medium for chemical and enzymatic reactions as they occur in metabolic processes of the cell
- Transport medium
- Regulation of body temperature
- Essential in homeostatic processes
- Lubricant
- Refractive medium in the eye
- Mechanical buffer (shock absorber)
Where is water a lubricant?
- mouth
- joints
- potential spaces (serous cavity): pleura, pericardium, peritoneum
How does water act as a mechanical buffer?
It distributes mechanical trauma applied to any part of the body to a larger area so it becomes less harmful.
Why is water essenntial for homeostatic processes?
essential in regulation of pH & osmolarity.
How is water relevant for transport medium?
Transports substances (nutrients, O2, waste products like CO2, hormones, vitamins, etc.) to different parts of the body.
Where is water a medium for exchange processes?
- Lungs: exchange of oxygen & CO2 through aveolar membrane
- Capillary walls: exchange of nutrients, waste products, & CO2
- Cell membrane: exchange of ions & nutrients
Where is water a medium for biological processes?
- Digestion in GIT: an essential component for all digestive secretions; all secretions won’t work without water.
- Absorption: GIT, renal tubules, & venous end of capillaries.
- Excretion: GIT, kidney, & skin (sweat)
- Filtration: glomeruli of kidneys & arterial end of capillaries
water in GIT serves as…
a medium for digestion, absorption, & excretion
Water acts as a medium for filtration in…
arterial end of capillaries & glomeruli of kidneys
In the venous end of capillaries, water is…
a medium for absorption
Water is a medium for excretion in…
GIT, kidney, & skin
How does water regulate body temperature?
heat distribution & evaporation
How are the body water compartments measured?
using indicator dilution principle
mechanism of body water measurement:
- a known amount of substance is injected intravenously
- given time to distribute uniformly in the compartment that it is known to stay in exclusively
- a blood sample is taken
- concentration of substance in plasma is determined
- volume of water compartment is calculated using an equation
Volume of compartment= amount of substance injected/ concetration of the substance in plasma
This works because the more of the substance left in the plasma, the smaller the volume of compartment is since there’s less to absorb the substance.
Indicators for TBW:
- antipyrine
- deuterium oxide (“heavy water”)
- tritium oxide (isotopically labeled water)
How can antipyrine be used to measure TBW?
it is a very lipid soluble substance so it can rapidly penetrate cell membrane
How do deuterium & tritium oxide measure TBW?
they are forms of water so they mix with the TBW.
How is ECF measured?
using a substance that disrtibutes in plasma & ISF but do not readily penetrate the cell membrane
Indicators for ECF:
- insulin
- sucrose
- mannitol
- thiosulphate
- thiocyanate
Indicators for ICF:
none, it cannot be measured directly.
- calculated by finding the difference between TBW & ECF:
ICF= TBW-ECF
How is plasma measured?
using substances that do not readily penetrate capillary membrane but remain in the vascular system.
Indicators for plasma:
- Evan’s Blue
- albumin labeled with radioactive iodine
Indicators for ISF:
none, it cannot be measured directly.
- calculated via:
ISF= ECF- plasma
Define homeostasis:
Keeping the composition of internal environment (ISF/ ECF) of the body constant.
What are the 4 factors of homeostasis?
- volume (water content)
- ions (Na+, K+, Cl-, HCO3-)
- temperature
- pH
What makes the ECF and interstitial fluid so important?
being the internal environment of the body, they must stay within the narrow limits of change in the chemical or physical properties of internal environment that allow for life to be possible.
How do biological system in the body work to maintain homeostasis?
Both directly or indirectly, all to keep the internal environment optimum for cellular function.
Stages of organs contributing to homeostasis:
- Heart pump
- Blood
- Respiratory system
- Kidneys
- GIT
- Skin
- Musculoskeletal system
Kidneys regulate…
- water
- ions
- pH
Respiratory system role:
- supply oxygen
- remove carbon dioxide
role of GIT:
supply nutrients
Role of skin:
- regulate temperature
- remove some waste products
role of muscoskeletal system:
movement to…
- obtain food
- escape from dangers
role of blood:
- circulating fluid through all parts of the body
role of ISF:
- allow nutrients & O2 into cells from blood
- take CO2 & waste products of metabolism out from cell to blood
What regulates the functions of these biological systems?
- Nervous system [for rapid control]
- Endocrinal (hormonal) system [ slow control]
What is feedback control?
the control of certain functions by the resultant affect of this function.
Types of feedback control:
- Negative feedback control (most common)
- Positive feedback control
What is negative feeback control?
- when the resultant effect of a function inhibits that function
- leads to stability of internal environment
examples of negative feedback control include:
- regulation of blood glucose levels
- regulation of CO2 in blood
- regulation of arterial blood pressure
Mechanism for regulation of blood glucose levels:
- increase in blood glucose
- increase in insulin secretion
- increase in glucose utilization by tissues
- decrease of blood glucose
blood glucose is back to normal.
Mechanism of CO2 in blood regulation:
- increase in CO2 in blood
- hyperventilation
- CO2 wash
- decrease in CO2
CO2 in blood levels are back to normal!
mechanism of arterial blood pressure regulation:
- increase in ABP
- decrease in heart rate & vasoldilation
- decrease in ABP
ABP is back to normal!
What is positive feedback control?
where the resultant effect of a function stimulates that function
- not common
What are effects of positive feedback control?
leads to instability of internal environment & often death due to vicious circles (death cycles)
What are examples of a vicious cycle?
- Heart failure
- Heat stroke
mechanism of heart failure:
- weakness of heart
- decrease in cardiac output
- decrease in ABP
- decrease in coronary blood flow (coronary arteries)
- decrease heart rate
repetition, making heart even weaker
what is the mechanism of heat stroke?
- increase in temperature
- increase in metabolic rate
- increase in heat production
- increase in temperature
vicious cycle where heat causes for more metabolic activity and that activity itself releases thermal energy.
When can positive feedback be useful?
the process of parturition (labour).
1. stretch of uterine cervix
2. reflex uterine contraction
3. descend of baby
4. more stretch of cervix
5. more uterine contration
6. more descend of baby
and so on until labour is complete.
