--- module: 001-01 language: en chapter: 1 title: "Homeostasis, Cellular Energetics, and Physiological Reserve" module_title: "Foundations" source_sha256: fe5ea4d8c344dede686abfd056e32cf3670291da3ab1703f8d31c9a9498e68eb --- # Homeostasis and physiological foundations ## Regulated stability ### Cells need a stable surrounding environment ### Homeostasis is dynamic, not fixed #### Variables fluctuate around useful ranges ### Receptor, control centre, effector ### Negative feedback opposes the disturbance #### Rising temperature increases heat loss #### Rising glucose: insulin promotes uptake and storage #### Dominant pattern because it stabilises ### Positive feedback amplifies to an endpoint #### Cervical stretch, oxytocin, stronger contraction #### Amplifying steps in blood clotting #### Needs a stopping condition ## Membrane transport ### Bilayer separates intracellular and extracellular fluid #### Oxygen and carbon dioxide cross the lipid region #### Ions and polar molecules need proteins ### Diffusion and facilitated diffusion follow gradients #### No direct cellular energy required ### Osmosis toward more non-penetrating solute ### Tonicity predicts long-term cell volume #### Hypotonic: cell swells; hypertonic: cell shrinks ### Active transport moves against the gradient #### Sodium-potassium pump: 3 sodium out, 2 potassium in #### Gradients support voltage, volume, signalling ### Endocytosis and exocytosis move larger material ## Compartments and concentration ### Plasma and interstitial fluid form extracellular fluid ### Capillary walls and cell membranes as barriers #### Plasma proteins more restricted than water ### Sodium outside, potassium inside #### Maintained dynamically, not by a sealed wall ### Concentration is amount divided by volume #### Water loss concentrates, water gain dilutes #### Cannot reveal compartment size or total content ## Tissues as functional systems ### Epithelium covers, lines, forms glands #### Tight packing and junctions make barriers ### Connective tissue supports, binds, stores, transports #### Cells separated by extracellular matrix #### Bone, cartilage, fat, tendon, blood ### Muscle converts chemical energy into force #### Skeletal, cardiac, smooth roles differ ### Nervous tissue detects, processes, communicates #### Neurons signal, glia support and insulate ## Integration across levels ### Cells, tissues, organs, organ systems cooperate ### Failure at one level propagates #### Low oxygen delivery limits respiration #### Less ATP weakens transport and ion gradients #### Eventually organ function is impaired ## Cellular energy and protein traffic ### Glycolysis in cytosol yields pyruvate and little ATP ### Citric acid cycle feeds the electron transport chain #### Proton gradient drives ATP synthase #### Oxygen is the final electron acceptor ### Oxygen deprivation hits excitable tissue fast #### Pumps fail, voltage and calcium destabilise ### Rough ER, Golgi, then sorting to destination ### Lysosomes digest, peroxisomes oxidise fatty acids ### Cytoskeleton shapes, transports, separates chromosomes ### Protein function controlled at many levels #### Fast modification or slow gene expression ## Cell communication and adaptation ### Contact, local, synaptic, hormonal signals ### Receptor binding changes intracellular pathways #### Amplification enlarges small signals #### Termination stops activity after the stimulus ### Down-regulation with persistent high stimulation #### Explains tolerance and altered sensitivity ### Up-regulation with low stimulation ### Gap junctions couple cardiac and smooth muscle ### Tight junctions preserve compartments ### Anchoring junctions distribute mechanical stress ## Tissue repair and regeneration ### Haemostasis, inflammation, proliferation, remodelling #### Collagen reorganises, tensile strength improves ### Epithelia and blood-forming tissue renew rapidly ### Liver has substantial regenerative capacity ### Cardiac muscle and central neurons replace poorly #### Fibrosis keeps structure but loses function ## Physiological reserve and ageing ### Reserve is maximum capacity minus baseline ### Much reserve lost before resting values change #### Exercise, illness, ageing, stress expose limits #### Failure appears first under demand ### Ageing narrows the workable range #### Water, concentrating ability, vessels, immunity, muscle #### Same disturbance may exceed compensation ### Interpret measurements with function and context