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Homeostasis is the constant maintenance of an internal environment through regulatory processes.
Positive feedback:
Where a change made to the equilibrium causes an amplification of that same change.
Negative feedback:
Where a change made to the equilibrium causes a restoration to the same equilibrium, working against the effect of the change.
Alpha cells:
Alpha cells secrete glucagon, which inhibits the action of B-cells in the pancreas.
Beta-Cells:
Secrete insulin, which inhibits the action of alpha cells in the pancreas. B-cells are found in small clusters known as islets of Langerhans.
Alpha (blue) and Beta (Black) cell clustering in the pancreas.
Glucose regulation:
-If the glucose concentration in the blood is too high, it will lower the water potential of the blood and cause water to move into the blood from surrounding cells by osmosis, causing the cells to crenate and die.
-If the blood glucose level is too low, there will not be enough glucose available as a respiratory substrate, so little/no ATP will be able to be formed through respiration.
Temperature and pH regulation:
-High temperatures and high/low pH can cause important enzymes to denature and lose enzyme-substrate complementarity. This means no ES complexes can form and metabolic reactions lose efficiency.
-Low temperatures mean there is often not enough kinetic energy for successful frequent collisions between particles, and therefore reducing metabolic activity again.
Glucagon:
In low blood sugar concentrations, alpha cells are activated and secrete a hormone called glucagon. This stimulates glycogen stored in cells to be broken down to release glucose into the blood (for respiration). This process is known as glycogenolysis.
Insulin:
In high blood sugar concentrations, beta cells are activated and secrete a protein called insulin. This causes glucose transporters to fuse with cell surface membranes (in the liver/gut/fat) and therefore causes glucose to be taken into cells from the blood. Once taken in, the glucose monomers are polymerised to form glycogen. This is known as glycogenesis.
Gluconeogenesis describes the synthesis of glucose from derivatives such as amino acids or lipids.
B-Cell activation:
-High [glucose] in blood means B-cells respire more (producing more ATP by oxidative phosphorylation)
-ATP produced binds to ATP-gated K+ channels in membrane, which close upon binding, building membrane potential.
-Membrane resting potential initially -70mV as net K+ out. When depolarisation occur, membrane becomes more positive (K+ efflux reduced, Ca2+ influx)
-This depolarisation opens voltage-gated Ca2+ channels. When an influx of calcium occurs, vesicles containing insulin are exocytosed to the cell membrane and move insulin into the blood.
Insulin mechanism of action:
-Insulin is carried in the blood and binds to receptors on cells. Upon binding, it activates adenylate cyclase, which catalyses the formation of cyclic AMP (cAMP) from ATP.
-cAMP can act as a secondary messenger (activating protein kinase A), but the main output from cAMP action is exocytosis of vesicles containing GLUT transporters to the membrane.
-Increased presence of GLUT transporters at membrane = more glucose absorbed by cells (more glycogenesis)
Ultrafiltration in the Bowman's capsule:
-High pressure generated by pumping of heart and difference in size between afferent and efferent arteriole. Efferent is narrower.
-Pressure squeezes small molecules through fenestrations in glomerulus (network of capillaries).
-Podocytes and basement membrane act as filtration and rescue mechanisms for incorrect substances passing out of the blood.
-What is left in the efferent arteriole is red blood cells, white blood cells and plasma proteins)
-Glucose, water, ions, sugars, amino acids and vitamins are passed through into the nephron.
Afferent
Efferent
Bowman's capsule
Glomerulus
H2O/Ions/Glucose/AA
Reabsorption of water in the nephron:
-PCT (proximal convoluted tubule) permeable to AA/vitamins, H2O, sugars, so all of these reabsorbed here.
-Ascending limb impermeable to water, but actively transports Na+/Cl- ions out of nephron into medulla, lowering WP of surrounding tissue.
-Descending limb permeable to water. Lowered WP of medullary tissue generates gradient, water reabsorbed by osmosis.
-The loop of Henle is incredibly concentrated, and the ion gradient present means Na+ and Cl- diffuse passively out of loop.
-DCT (distal convoluted tubule) where fine tuning of water potential occurs again through the gradient setup by ion pumping.
-ADH influences water reabsorption in the collecting duct, where water is absorbed through channels in cell membranes known as aquaporins, which are highly permeable to water.
ADH Mechanism of action:
ADH (anti-diuretic hormone) binds to cells on the collecting duct, causing their intracellular cAMP to increase. This cAMP stimulates the exocytosis of vesicles containing aquaporins to migrate to the collecting duct membrane, increasing its permeability to water. Coffee is an example of a diuretic.
Thermoregulation:
Thermoregulation is important because maintaining heat energy allows sufficient energy for frequent successful collisions between particles.
Temperature too high: Active sites of enzymes denature. Complementarity to substrate lost. Decrease in ROR of important enzymatic reactions.
Temperature too low: Not enough KE for frequent successful collisions, so ROR decreases due to lack of energy.
Thermoregulation serves to maintain the temperature (such as in endoderms) around a narrow range.
Coordination and regulation:
These processes are brought about by the thermoregulatory centre in the brain. It has thermoreceptors to detect changes in temperature of the blood, and elicits an appropriate response. Sensory receptors in the skin also send signals to the thermoregulatory centre.
Humans maintain a body temperature of 37.5 degrees Celsius in this way.
Responses to heat:
Sweating: Water has a high specific heat capacity, therefore sweating removes a lot of heat energy and therefore acts as a cooling mechanism.
Hairs life flat: No warm layer of air trapped, so more heat radiated from blood through skin.
Vasodilation: Widening of blood vessels/capillaries closer to skin surface, increases radiation of heat from blood through skin.
Responses to cold:
Shivering: Increases mean KE of particles, brings up average energy through movement.
Hairs stand on end: Warm layer of air trapped, so less heat radiated from blood through skin.
Vasoconstriction: Constriction of blood vessels/capillaries close to skin surface, decreases radiation of heat from blood through skin and blood flow.
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