Homeostasis Flashcards
Define homeostasis?
The state of steady internal physical and chemical conditions maintained by living systems.
Define cellular homeostasis?
The processes involved in the maintenance of an internal steady state at the cellular level.
Living systems are considered “open dynamic systems”, what does this mean (2)?
Not at thermodynamic equilibrium.
Continual throughput of matter and energy.
(yet remain remarkably constant in many aspects).
What is the general rule for a steady state?
Sum of inputs = sum of outputs.
(If not, ‘pool size’ rises or falls until they’re equal).
How is the relation between inputs and outputs affected by growth and development?
During growth and development, inputs>outputs.
What is an example of transient imbalances at a cellular level?
Na+ in = Na+ out.
Transient imbalance of this steady state is crucial for action potentials.
What is meant by the term “transient imbalance”?
When a state is unbalanced/unequal or varies over time.
What is an example of transient imbalance at the whole body level (3)?
H2O in = H2O out:
Varies from changes in hydration status.
Ca ions in = Ca ions out:
Except during bone growth/resorption.
O2 in = CO2 out:
Difference reflects stoichiometry of respiration.
Give an example of transient imbalance at the organ level?
Fatty acid uptake = fatty acid oxidation + triglyceride synthesis + triglyceride export:
Insulin resistance causes reduced FA oxidation in muscle/liver = fatty liver.
What is the aim of feedback pathways?
Maintenance at a particular value of the control variable in the face of perturbations.
What happens if a variable leaves its target range with reference to feedback pathways?
Sensor activated.
Feedback signal sent to effector.
Effector opposes unwanted change.
(Negative feedback loop).
What is an example of positive feedback?
Nerve action potential.
What happens, in feedback pathways, if signals are sent to the brain centre?
The brain issues neuronal/hormonal signals to effector organs.
What is the name given for failures in homeostasis (in disease)?
Decompensation.
Name the 4 types of passive transport across a membrane?
Simple diffusion:
Through lipid bilayer.
Facilitated diffusion:
Through non-specific transporter
Facilitated diffusion:
Through specific transporter.
Osmosis:
Through lipid bilayer or aquaporins.
How does the osmolality of extracellular fluid (ECF) affect cell volume?
ECF osmolality up (hypertonic) = Cell volume down.
ECF osmolality down (hypotonic) = cell volume up.
What are two ways the body can regulate extra cellular fluid (ECF) osmolality?
Control of body Na+ content:
Mainly renal Na+ handling.
Control of body water:
Thirst, sweating, renal water handling.
What are considered osmoles in cells?
Ions and metabolites.
How can osmoles be regulated in cells?
By cerebral osmoregulation.
Cellular [Ca2+] is naturally high due to negative membrane potential, how is this combatted in cells?
The energy of Na+ gradient (and ATP directly) is used to pump it out (secondary active transporters?).
What are the sensors for body temperature regulation (2)?
Hypothalamic thermoregulatory centres.
Peripheral temperature sensors.
What are effectors for the regulation of body temperature (3)?
Heat losing:
Behavioural (avoid heat).
Sweating.
Vasodilation.
Heat conserving:
Behavioural (avoid cold).
Vasoconstriction.
Heat generating:
Non-shivering thermogenesis (metabolic).
Shivering.
What occurs if body core temperature exceeds 40C?
Heat stroke leading to cerebral dysfunction.
What is hyperthermia and what are the cascading effects of it?
When heat challenge exceeds regulatory capacity.
Causes organ dysfunction due to heat which impairs regulatory mechanisms.
Results in low cardiac output.
This reduces skin blood flow, causing unwanted positive feedback.
What are the mechanisms for pH regulation in the body?
Buffering dominated by CO2/bicarbonate system.
Renal mechanisms:
Bicarbonate reabsorption/generation.
Acid secretion.
Where are chemoreceptors for pH regulation found?
Central chemoreceptors:
Respiratory centre in medulla oblongata (senses cerebrospinal fluid pH).
Peripheral CO2 sensors in aorta and carotid artery walls.
What are the effectors for pH regulation and the sensing of which chemicals do they react to?
Increased respiratory rate in response to pH fall:
Works by sensing the pressure of CO2 in blood.
Renal acid excretion increases in response to pH fall:
Works by sensing bicarbonate levels.
What are the glucose sensors when regulating blood glucose?
pancreatic alpha and beta cells.
What are the effectors for hyperglycaemia and what effects do they have?
Effector:
Insulin.
Effects:
Stimulates muscle and hepatic glucose uptake.
Stimulates glycogenesis.
Inhibits adipose tissue lipolysis.
What are the effectors for hypoglycaemia and what effects do they have?
Effector:
Glucagon.
Effects:
Stimulates hepatic glycogenolysis and gluconeogenesis.
Release of glucose.
Which forces control water movement across capillary endothelium and thus fluid metabolism?
Starling forces:
Hydrostatic pressure favours egress.
Oncotic gradient favours re-entry.
Where within the hypothalamus would you find osmoreceptors responsible for osmolality regulation?
Supraoptic and paraventricular nuclei:
Control ADH secretion by posterior pituitary.
Lateral preoptic area:
Controls thirst.
What are the effectors and signalling relevant to an increase in osmolality?
Effectors:
Vasopressin (ADH)
Signalling:
Increased thirst.
Increased vasopressin leads to urine water retention.
Renin-angiotensin system (RAS) inhibition.
Where are the sensors for arterial blood pressure located (2)?
Baroreceptors in aortic arch and carotid sinus.
Integrating centres in rostral ventrolateral medulla (autonomic control of cardiac output).
What are the effectors used for decreased blood pressure and what effects to they have?
Effectors:
Angiotensin II.
Aldosterone.
Effects:
Renin-angiotensin system stimulated (synthesises angiotensin II).
This causes vasoconstriction.
Secretion of aldosterone increases renal distal tubular Na+ and H2O resorption.
Where are the sensors for blood volume?
Atria and pulmonary arteries.
How does your body deal with increased blood volume?
Decreased heart rate.
Decreased renin-angiotensin system activity.
Increased secretion of atrial natriuretic peptide (ANP).
Inhibits renal distal tubular Na+ and H2O resorption.
This increases urine salt and water loss.
Decreased ADH secretion.
Leads to increased urine water loss.
