PHS 3341 Lecture Notes - Extracellular Fluid, Body Fluid, Homeostasis
Midterm 1 – Objectives
Body Fluids
1- Homeostasis
1.1 Define homeostasis and explain the concept of dynamic constancy. Recognize that numerous
variables must be maintained homeostatically and explain how the loss of homeostasis in one variable can
affect others
• Homeostasis: the ability of the body to maintain constant internal factors even when the
environment is constantly changing
o Is not a strict value, but it is a dynamic phenomenon that tries to keep the body within a
small range
• Dynamic constancy: can vary over the short term but is constant over a long period
• Many variables in the body need to be kept in a certain range
o Examples are blood pressure, temperature, concentration of solutes in the blood system,
etc.
1.2 Describe the general characteristics and components of homeostatic control systems. Differentiate
between steady state and equilibrium and explain the concept of set point regulation
• Components of a homeostatic control system
o Receptor: notices a change in a variable (caused by a stimulus) and sends a message
through the afferent pathway towards the integrating center
o Integrating center: Receives a message from the receptor via the afferent pathway,
interprets the message, and responds by sending a message through the efferent pathways
to the effector
o Effector: receives a message from the integrating center and acts accordingly to return
the body to the optimal state
• Steady state: a system in which a particular variable is not changing, but energy must be added
continuously to maintain a constant condition
• Equilibrium: a particular variable is not changing but no input of energy is required
• Set point: the numerical value of the variable measured at steady state
1.3 Differentiate between negative and positive feedback, and feedforward regulation
• Negative feedback: a change in the variable being regulated brings about responses that tend to
push the variable in the direction opposite to the original stimulus
o Minimizes changes from the setpoint of the system, leading to stability
• Positive feedback: accelerates a process and moves the value away from the set point
• Feedforward: a response designed to prevent an anticipated change in a controlled variable
2- Body Fluid Compartments
2.1 Explain the concept of body fluid compartmentalization and briefly describe the relevant proportions
• Water is present with and around all cells and within all blood vessels (60-70% water)
• Collectively, the fluid present in the blood (plasma) and between cells (interstitial fluid) is
called extracellular fluid (ECF)
o Consists of 20-25% plasma and 75-80% ISF
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Midterm 1 – Objectives
• The ECF is roughly a homogeneous mixture because there is a constant exchange of nutrients
between the plasma and the ISF
o The only difference is the composition of proteins
• Fluids are enclosed in compartments, which are defined and maintained by barriers and the
properties of the barriers determine which substances can move between compartments
2.2 Distinguish the different compositions of the intracellular and extracellular fluids
• Plasma (3L) is about 7% water
• Interstitial fluid (11 L) is about 26% water
• Intracellular fluid (28 L) is about 67% water
3- Movement of Molecules Across Cell Membranes
3.1 Diffusion
3.1.1 Define diffusion and explain the concept of diffusion equilibrium. Explain what determines
the magnitude of net flux across a membrane
• Diffusion: the movement of molecules from one location to another solely as a result of their
random thermal motion
o Molecules tend to diffuse from an area of high solute concentration to an area of low
solute concentration
o Flux: the amount of material crossing a surface per unit time
• Diffusion equilibrium: if the fluxes are equal in magnitude and opposite in direction, then the net
flux will be zero and the system will be at diffusional equilibrium
• Net flux is difference between fluxes of two compartments
o Always proceeds from a higher concentration to a lower concentration
3.1.2 List the factors that determine the magnitude of solute flux
• Factors that will affect the magnitude of the flux are:
o Concentration
▪ The greater the difference in concentration, the larger the net flux is
o Temperature (kinetic energy)
▪ An increase in kinetic energy causes an increase in movement of particles and
therefore an increase in flux
o Molecular mass
▪ The larger the solute, the slower the net flux will be
o Surface area between compartments
▪ The larger the surface area, the large the net flux
o The medium through which the molecules are moving
▪ The more dense the medium is, the smaller the net flux will be
3.2 Mediated transport systems
3.2.1 Contrast ligand-gated, voltage-gated, and mechanically-gated ion channels
• Channel gating: the process of opening or closing channels
• Ligand-gated ion channel: a binding of a specific ligand/solute will cause a change in
conformation, causing the channel to open or close
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Midterm 1 – Objectives
• Voltage-gated ion channels: a change in potential around the channel will cause the channel to
open or close
• Mechanically-gated ion channels: a change in pressure causes the channel to open or close
3.2.2 Distinguish between diffusion, facilitated diffusion and active transport
• Diffusion: a movement of a solute from an area of high concentration to an area of low
concentration
• Facilitated diffusion: the movement of solutes across a membrane down their concentration
gradient
• Active transport: the movement of a solute across a membrane against its concentration gradient
with the use of energy (ATP)
3.2.3 Contrast primary and secondary active transport
• Primary active transport: the direct use of ATP to move a solute across a membrane using a
protein against its concentration gradient
• Secondary active transport: the indirect use of ATP to move a solute across a membrane against
its concentration gradient
o Uses the movement of another solute down its concentration gradient to cross
o Antiport: the two solutes are moving in opposite direction
o Symport: the two solutes are moving in the same direction
3.2.4 Describe the factors that affect diffusion across membranes
• Simple diffusion is limited by:
o Concentration gradient
o Permeability of the membrane
• Facilitated diffusion is limited by:
o Concentration gradient
o The amount of transporters available
o How fast the transporters can move the solute
o How long the channel stays open
• Active transport is limited by:
o The amount of transporters available
o How fast the transporters can move the solute
o The extent to which the binding sites are saturated
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Document Summary
1. 1 define homeostasis and explain the concept of dynamic constancy. 1. 2 describe the general characteristics and components of homeostatic control systems. 2. 2 distinguish the different compositions of the intracellular and extracellular fluids: plasma (3l) is about 7% water. Interstitial fluid (11 l) is about 26% water. Intracellular fluid (28 l) is about 67% water. 3. 1. 1 define diffusion and explain the concept of diffusion equilibrium. Midterm 1 objectives: voltage-gated ion channels: a change in potential around the channel will cause the channel to open or close, mechanically-gated ion channels: a change in pressure causes the channel to open or close. 3. 3. 2 define osmotic pressure and explain the relationship between extracellular osmolarity and cell volume. If the osmolarity of the extracellular fluid is more than the intracellular fluid (hypertonic) then water will rush out of the cell and the cell will shrivel.
