MEDI211 – CONTROL MECHANISMS EXAM NOTES

Control Theory - Homeostasis = regulation of the physical, chemical and thermal status of all cells within an organism. - Control vs. Regulation o Control = modifying physiological functions to support life. § E.g. Heart rate, breathing o Regulation = regulating a variable through ‘control’ functions § E.g. Vessel diameter controlled to regulate blood pressure. § More stable thresholds = more tightly maintained than controlled variables. - Types of control o On/off control = effector is either maximally turned on or completely turned off. § Outcome: fluctuating levels, rarely near target. o Proportional control = output signal by effector is in direct proportion to the stimulus applied. § Outcome: stable levels, mostly near target – better maintenance of homeostasis. - Types of Feedback o Negative feedback = output reduces original effect of stimulus o Positive feedback = output enhances original effect of stimulus. § Control infrequent events that do not require regulation. § E.g. Oxytocin release during childbirth causes increased contractions. o Feedforward = anticipatory signal that primes the control mechanism. § Initiated in the brain via autonomic nervous system - Other considerations for homeostasis o Redundancy § The more critical the parameter, the more systems dedicated to its preservation § Failure of one system compensated by increased use of other systems – the parameter is still regulated. o Equilibrium = parameter is well-regulated o Acclimatisation = set a new equilibrium (adaptation) § E.g. changes in breathing due to high altitude.

Metabolism - Oxidation/reduction o Reactions tightly controlled to avoid dangerous rates of heat production § Oxidation in cellular metabolism results in ATP formation only 30% efficient, 70% is wasted energy as heat. - Energy balance: o 1st law of thermodynamics = energy cannot be created nor destroyed. o 2nd law of thermodynamics = chemical transformations always result in loss of free energy available to drive metabolic processes. § Total internal energy (E) = G + TS • Gibbs free energy (G) = energy available for use • Temperature (T) & Entropy (S) = wasted energy - Energy intake (1 kcal = 4.2 kJ): o Food & beverage composition: § 1g CHO = 4 kcal or 17 kJ • 70kg body can store 700g glycogen = 2800 kcal § 1g fat = 9 kcal or 38 kJ • Adipose tissue comprised of 87% lipids • 1kg of body fat = 870g lipids § 1g protein = 4 kcal or 17 kJ • Structural/functional roles, no energy provision except during starvation = 15%. § 1g alcohol = 7 kcal or 30 kJ o Calorimetry = measuring heat production § Direct calorimetry • Burns food, & heat provides measure of food energy composition as kcal. § Indirect calorimetry • Measure O2 consumption & CO2 production • O2 consumption is directly proportional to heat production as heat produced by cellular respiration. - Respiratory Quotient (RQ) o Ratio of moles CO2 produced per mole O2 consumed (= CO2 / O2) § CHO = 1.0 RQ, protein 0.8, fat 0.7. - Metabolic rate o Resting metabolic rate (RMR) = estimate of energy required while at rest. § Calories required can rise in response to heavy exercise, cold exposure, illness/fever. § Healthy, young 70kg human requires 2100 kcal/day. o Basal metabolic rate (BMR) = clinical calculation of energy expended to sustain vital functions when awake, measured under several standardised conditions. § Conditions allow continuity between results collected across the world = comparisons between cohorts. • Taken in the morning after a good sleep. • Fasting for 12 hours • 1 hour quite rest • 25°C air temperature. - Energy storage – necessary because energy intake is intermittent but energy expenditure is continuous. o ATP: adenosine triphosphate – energy stored in its bonds, which is released to drive other reactions. § ATP à ADP + P + energy - Carbohydrates o Absorption: § Oligo- & polysaccharide digestion • Salivary amylase in mouth à pancreatic amylase in small intestine. • Enzymes on brush border of small intestine lumen degrade oligosaccharides into monomers. § Transporters move monosaccharides across intestinal cell, into the interstitial space & into the bloodstream. • GLUT2 – on luminal membrane, transports monosaccharides into cell cytoplasm. o Na+/Glucose Transporter 1 • GLUT5 – basolateral membrane, transports monosaccharides into laminapropria - Lipids – triglycerides (dietary lipid) o Absorption § Undergo mechanical breakdown in the GI tract • Bile salts emulsify fats into micelles • Micelles can diffuse through thick mucous (acidic) layer. § Glycerol & short/medium chain fatty acids diffuse straight through to the blood. • Diffuse through luminal enterocyte membrane • Break out of micelle & become part of the membrane or diffuse out basolateral membrane. § Long-chain fatty acids are converted into chylomicrons • Enter enterocytes, converted to chylomicrons by ER & released in Golgi vesicle to interstitial space. • Chylomicrons picked up by lacteals for transport in lymphatic system. o Enters cisterna chyli > thoracic duct > junction of internal jugular & subclavian vein § From bloodstream to storage: • Lipoprotein Lipase (LPL) o Exported from adipocyte/hepatocyte cell to blood vessel endothelium, held within plasma membrane (stimulated by insulin). o Hydrolyses triglycerides in chylomicrons & lipoproteins into fatty acids & glycerol. o Insulin induces LPL synthesis = promotes lipid uptake for storage. • Glycerol taken up by liver & stored as glucose, via gluconeogenesis, or converted to pyruvate for glycolysis.