Internal Environment and Homeostasis Flashcards
What percentage of the body is fluid (approximately)?
60%
How is body fluid divided?
Divided into compartments:
- Approximately 70% of body fluid is within cells - ICF
- Approximately 30% of body fluid is outwith cells - ECF
- Compartments are spearated by membranes
Describe the basic composition of blood.
Approximately how much blood is there in the body?
Contains both ECF (plasma) and ICF (fluid in RBC).
Approximately 5L (7% of body weight).
Describe the composition of ECF.
Approximately how much is there in the body?
Made up of:
- Plasma (in blood) ~3L
- Interstitial fluid (in spaces between cells) ~11L
- Approximately 14L (20% of body weight)
What is the difference between plasma and interstitial fluid?
Proteins
Why is there a higher conentration of proteins in plasma than interstitial fluid?
Plasma and interstitial fluid exchange substances through highly permeable capillary membranes, but:
- Most proteins are too big to permeate the membrane so they are maintained in a higher concentration in plasma than in interstitial spaces.
Describe the composition of ICF.
Approximately how much is there in the body?
- ICF is separated from ECF by membranes highly permeable to water but impermeable to most electrolytes.
- ~28L (40% of body weight)
What is the ion concentration of potassium in ICF?
140mM
What is the ion concentration of sodium in ICF?
15mM
What is the ion concentraion of chloride in ICF?
5mM
What is the ion concentration of potassium in ECF?
5mM
What is the ion concentration of sodium in ECF?
140mM
What is the ion concentration of chloride in ECF?
110mM
What determines the rate of osmosis (ie the net movement of water)?
Concentration of dissolved substances in the water
Describe osmosis
The process by which molecules of water pass through a semipermeable membrane from a less concentrated solution to a more concentrated one.
Water molecules move randomly, so there is always some water movement in both directions in physiologic systems.
What is the osmolality of a fluid?
Osmoles / Kg
A measure of the number of osmotically active particles per kg
What is the osmolarity of a fluid?
Particles / L
A measure of the number of osmotically active particles per litre.
Describe tonicity
The mechanical effects (eg on a cell) in terms of shrinkage or swelling caused by placing a cell in a solution of impermeant osmotically active particles.
Describe isotonic ECF and ICF
Water concentration in ECF and ICF is equal; solutes cannot leave or enter; cell volume stays the same.
Describe hypertonic ECF and ICF
A cell in a solution with low water concentration and high concentration of impermeant particles; water diffuses out; the cell shrinks.
Describe hypotonic ECF and ICF
A cell in a solution with high water concentration and lower concentration of impermeant particles; water diffuses into the cell; the cell swells (bursts).
Some solutes can permeate the cell membrane.
What is an iso-osmotic solution?
Solution with the same osmolarity as the cell, regardless of whether the solute can permeate the cell.
Some solutes can permeate the cell membrane.
What is a hyper-osmotic solution?
A solution with a higher osmolarity than usual ECF
Some solutes can permeate the cell membrane.
What is a hypo-osmotic solution?
A solution with a lower osmolarity than usual ECF
How is homeostasis maintained?
By control systems
- Within cells
- Genetics
- Within / between tissues / organs / systems
- Nervous system - quick response, short duration
- Endocrine system - longer response, longer duration
- Most control systems use negative feedback
Describe an open loop system
- No feedback
- Could result in runaway reactions
- Controller → Controlled component → Output
- The output is not influenced by the resulting conditions
Describe a closed loop system
- Output of the system feeds back and influences the controller
- Controller → Controlled component → Output → Sensor → Feedback → Controller
- The output is influenced by the resulting conditions
What is negative feedback?
Return to set point
What is positive feedback?
Moves away from set point
What is feedforward?
A response is generated in anticipation of a change
Describe hunting in a negative feedback system
- Negative feedback conrolls the system within limits
- Over time, the process of hunting takes place within a safe limit, around a set point.
- The depth and extent of hunting depends upon the properties of the elements of the feedback system.
- The set point may be altered at times of metabolic or physiologic stress.
Describe the body’s response to infection
- Often, temperature increase (fever)
- Results from increase in temperature set point
- Bacteria / waste products phagocytosed by leukocytes (WBC)
- Progens (cytokines) released
- Binds at hypothalamus
- Change in set point
Describe the negative feedback control of arterial blood pressure
What is gain?
- The degree of effectiveness of the negative feedback control system
- Gain = correction / error
