Cell physiology Flashcards
What is the water distribution breakdown of a 70kg man?
60% of man is water
40% ICF- cannot be measured-> measure TBW with deuterium oxide
20% ECF (14litres)
of ECF; 25% vascular, 75% interstitial
ECF is isosomotic with ICF at osmotic equilibrium
ECF best measured using insulin, a polysaccharide, MW5200
Transcellular water= component of ECF that has been processed by cells into special compartments eg.CSF
What are the key facts about total body water?
60% of body weight Higher in men than women Reduces with age Used to measure ICF Inversely proportion to fat-> decreased in Cushing's disease
What is osmosis?
The diffusion of a solvent into a region where there is a higher concentration of a solute to which the membrane is impermeable
What are the fundamental properties of the plasma membrane?
Semipermeable; permeable to H20, K+, Na+ etc
Preferential permeability to lipid soluble substances
Low surface tension
High electrical resistance-> impermeable to organic ions
What are the 4 ways a substance can cross a membrane?
Diffuse freely Endocytosis Exocytosis Via transport proteins How well a substance crosses membrane depends on ; size, electrical charge, fat solubility
What is the Donnan effect?
When an ion on one side of a membrane cannot diffuse across, the other ions which are permeable will move to balance the charge.
This effect means that:
1. there are more osmotically active particles in cells than in interstitial fluid, hence Na-K+ ATPase is essentially to prevent cell swelling
2. an electrical difference exists across the membrane
3. there are more proteins in plasma than interstitial fluid, there is Donnan effect ion movement on capillary walls
what is the equilibrium potential of a membrane?
There is no net passive movement of ion across the membrane
NOT related to concentration being the same on both sides or potential difference being zero
What is the normal resting membrane potential in the cell membrane of nerve and muscle?
-70 to -90mV
Potassium permeability is greater than sodium permeability
Insulin causes an increase in resting membrane potential (hyperpolarization)
What role to Na/K+ ATPase play in maintaining the cell membrane potential/
The concentration gradient of K+ facilitates its movement out of the cell via K+ channels, however its electrical gradient is in the opposite direction
Na/K+ATPase pumps 3 Na+ out and 2K+ into the cell, which keeps the intracellular concentration of sodium low and acts and a electrogenic pump
What are the key facts about the capillary membrane?
Different tissues have differing permeability
Liver capillaries are permeable to plasma proteins
Glucose transfer across capillary wall occurs by passive diffusion
How much sodium and potassium is there in the body?
More total Na than K
Total exchangeable Na is 3000ml (70% of total body Na)
Total body K is 3000ml (90% exchangeable)
What is normal serum osmolality and pH of ECF?
Serum osmolality is 300mmol/kg
pH of ECF 7.36-7.44
What are the main cations in intracellular fluid?
K, Mg, Na
K concentration 150-160
Organic phosphates are present in high concentrations
More H ion intracellular than extracellular
Describe the concentration of ions in plasma
Higher sodium concentration than in interstitial fluid
Higher magnesium concentration than in interstitial fluid
Higher protein concentration than in interstitial fluid
Chloride concentration is LOWER in plasma due to Donnan’s effect
What happens to a cell when extracellular K+ is reduced to 3.0?
K+ will diffuse out
H+ will diffuse in
Intracellular net charge=unchanged due to Donnan’s effect
What are the key functions of tight junctions?
Surround apical margins of cells in epithelia
Restrict molecular movement across epithelium
Facilitate cell to cell adhesion (NOT communication)
Found just below luminal surface
What are the functions of gap junctions?
Spanned by corresponding connexons
Connexons line up and are the only way things can pass between cells without entering ECF
Allows ions, sugars, amino acids and other small molecules
Facilitates cell to cell communication
Allows synchronous contraction of cardiac+smooth muscle
Defective connexin gene in Charot-Marie Tooth
What are the 4 ways cells communicate through the ECF?
Autocrine
Paracrine, endocrine and neural (all use chemical messengers that bind to receptors)
How do receptor numbers get downregulated in the case of an excessive hormone?
Ligand binds to receptor->ligand receptor complex gets endocytosed-> decreased number of available receptors
Alternatively gets chemically desensitized so they are less sensitive
What is an example of a stimulus that acts to open or close ion channels in a cell membrane?
ACh on nictonic receptor
Noradrenaline on K+ channel in heart
What is an example of a stimulus that acts via cytoplasmic/nuclear receptors to increase transcription of mRNAs?
Thyroid hormones
retinoic acid
Steroid hormones
What is an example of a stimulus that acts to increase phospholipase C?
ANGII
Noradrenaline via alpha adrenergric receptor
ADH via V1 receptor
What is an example of a stimulus that acts on adenylyl cyclase to modify intracellular levels of cAMP?
Noradrenaline via B1 receptor (increases cAMP)
Glucagon
ADH
Noradrenaline via A2 receptor (decreases cAMP)
Which 2 substances increase cGMP in a cell?
ANP
Nitric oxide