Physiology - General Principles

Question 1

How is water distributed through the body compartments and how is it affected by age and gender

Answer 1

• total body fluid makes up 60% of total body weight
• total body fluid is 1/3 extracellular and 2/3 intracellular

ECF = 20% of total body weight (15% interstitial fluid, 5% plasma)
ICF = 40% of total body weight

age: total body water decreases with age
gender: total body water is higher in males

Question 2

What are the buffer systems in blood

Answer 2

carbonic acid/bicarbonate
plasma proteins
haemoglobin (deoxygenated is better)

Question 3

How does the carbonic acid/bicarbonate system work

Answer 3

H2O + CO2 ⥧ H2CO3 (carbonic acid) ⥧ H+ + HCO3-

Question 4

Describe the henderson-hasselbalch equation

Answer 4

used to determine the concentration of weak acid in solution

pH = pKa + log([A-] / [HA])

Question 5

What is osmolarity, what contributes to it and what is normal serum osmolarity

Answer 5

definition: the number of osmoles of solute per litre of solution
factors: mainly Na+, K+, Cl-, HCO3-, but also urea, glucose, proteins

normal serum value: 290mOsmol/L

Question 6

How does plasma differ from intracellular fluid in terms of composition

Answer 6

• intracellular: low Na+, high K+, low calcium, low chloride, high proteins
• extracellular: high Na+, low K+, high calcium, high chloride

Question 7

What are the phases of protein synthesis

Answer 7

1) transcription: produces mRNA from DNA

2) translation: produces proteins from mRNA, occurs along the ribosome with post-translation modification in ER

Question 8

Describe the process of secretion of proteins from cells

Answer 8

• polypeptide sequences are cleaved off (prohormone becomes hormone)
• some proteins have leader sequences that target the golgi apparatus and are then secreted by exocytosis
• some proteins are secreted from the cytoplasm via ATP dependant membrane transporters

Question 9

Describe the different ways in which a substance can cross a cell membrane

Answer 9

1) passive
- diffusion = small non-polar molecule passively cross a membrane by diffusion down its electrochemical gradient
- facilitated diffusion = use specific transmembrane proteins, substance moves down its electrochemical gradient

2) active
- endocytosis/exocytosis = substances engulfed or expelled
- ion channels = ligand gated, voltage gated
- carriers = substances bind to molecules that then change configuration and moves substance into the cell

Question 10

Explain the process of secondary active transport and provide examples

Answer 10

-transport of substance down its electrochemical gradient provides energy to transport another against its gradient

examples:

• co-transport = glucose + Na+, Cl- + Na+
• counter-transport = Ca+2 + Na+, H+ + Na+

Question 11

Describe the sodium potassium pump

Answer 11

• antiport, uses ATP to move 3 Na+ out of a cell and 2 K+ into a cell via a carrier protein
• has transmembrane alpha and beta subunits, only the alpha subunit involved in transport
• binding site for Na+ and ATP is intracellular, binding site for K+ is extracellular

Question 12

How do cells communicate with each other

Answer 12

• neuronal (synaptic) = uses neurotransmitters released at synaptic junctions
• endocrine = uses hormones to reach cells via circulating blood
• paracrine = products diffuse in ECF to affect nearby cells
• autocrine = chemical messengers bind to receptors on same cell
• gap junctions = allow transfer of chemical messenger directly from one cell to a neighbouring cell

Question 13

How do receptors respond to variations in messengers

Answer 13

• downregulation = number of receptors decreases when messenger is in excess
• upregulation = number of receptors increases when messenger is deficient

Question 14

How do messengers act

Answer 14

-activate receptors = receptor kinases, nuclear receptors, ion channel receptors, g proteins

• first messengers = act as extracellular ligands that activates a second messenger
  examples: adrenaline, acetylcholine, dopamine, histamine, substance p
• second messengers = intracellular signalling molecules that activate downstream pathways
  examples: cAMP, cGMP, IP3, DAG

Question 15

Describe the synthesis, metabolism and function of cAMP

Answer 15

synthesis: formed inside the membrane, adenylyl cyclase converts ATP to cAMP, which activates protein kinase A
metabolism: cAMP is metabolised by phospho-diesterase
function: intracellular 2nd messenger that stimulates protein synthesis and activates intracellular enzyme systems