Blood glucose and blood pressure
Blood glucose regulation
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Purpose: to hold blood glucose near a set point of about 90 mg per 100 cm3 (roughly 5 mmol per litre).
- Too low and cells lack a respiratory substrate. The brain is most vulnerable, because it cannot store glycogen and respires glucose almost exclusively, so it depends on a continuous supply from the blood.
- Too high and blood water potential falls, drawing water out of cells by osmosis, and glucose molecules bind to proteins over time and impair their function.
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Components:
- Receptors — the cells of the islets of Langerhans in the pancreas detect blood glucose directly. This system is unusual in that the receptor and the control centre are the same cells.
- Effectors — the liver, skeletal muscle and adipose tissue.
- Hormones — insulin from the β (beta) cells and glucagon from the α (alpha) cells.
When blood glucose rises
- After a meal, glucose absorbed from the small intestine raises blood glucose above the set point.
- β cells detect the rise and secrete insulin into the blood.
- Insulin binds to receptors on liver, muscle and adipose cells, and produces three effects:
- Increased glucose uptake. Vesicles carrying GLUT4 glucose transporter proteins fuse with the cell membrane, so the membrane becomes more permeable to glucose and more enters by facilitated diffusion. Note this is a permeability change, the same kind of mechanism as ADH in the kidney.
- Glycogenesis — glucose is converted to glycogen for storage in liver and muscle.
- Increased respiration and conversion of excess glucose to fat.
- Blood glucose falls toward the set point. As it does, insulin secretion decreases — the response is self-limiting.
When blood glucose falls
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Between meals, or during exercise, respiring cells remove glucose faster than it is absorbed.
- α cells detect the fall and secrete glucagon.
- Glucagon acts mainly on the liver:
- Glycogenolysis — stored glycogen is hydrolysed back to glucose, which is released into the blood.
- Gluconeogenesis — glucose is synthesised from non-carbohydrate sources such as amino acids and glycerol, which matters once glycogen stores are depleted.
- Blood glucose rises toward the set point, and glucagon secretion decreases.
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Adrenaline also raises blood glucose by stimulating glycogenolysis. This is not part of the homeostatic loop but an anticipatory response — it raises glucose availability ahead of demand during a stress response, before any deficit has occurred.
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Insulin and glucagon are antagonistic — they have opposite effects and are the opposing effectors of this system, which is what allows correction in either direction.
Blood pressure regulation
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Purpose: to maintain adequate perfusion of tissues — blood must be delivered to every capillary bed under enough pressure to supply oxygen and glucose and remove waste.
- Too low and tissues are inadequately supplied. The brain again is most vulnerable, because it has no reserve.
- Too high and vessel walls are damaged over time, and delicate capillary beds such as those in the kidney are injured.
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Components:
- Receptors — baroreceptors, which are stretch receptors in the walls of the carotid sinus and the aortic arch. They detect the degree to which the artery wall is stretched, which reflects the pressure inside.
- Control centre — the cardiovascular centre in the medulla oblongata.
- Effectors — the sinoatrial (SA) node, which sets heart rate; the ventricular muscle, affecting stroke volume; and the smooth muscle of the arterioles, setting vasomotor tone.
How blood pressure is corrected
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When blood pressure rises:
- Artery walls stretch more, so baroreceptors increase their firing rate to the medulla.
- The medulla increases parasympathetic output along the vagus nerve to the SA node.
- Heart rate falls and the force of contraction decreases, so cardiac output falls.
- Arterioles vasodilate, reducing resistance to flow.
- Blood pressure falls toward the set point.
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When blood pressure falls:
- Artery walls stretch less, so baroreceptor firing decreases.
- The medulla increases sympathetic output.
- Heart rate and stroke volume increase, raising cardiac output.
- Arterioles vasoconstrict, increasing resistance.
- Blood pressure rises toward the set point.
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Why this system is fast. It is entirely nervous, with no hormone step, so correction begins within a heartbeat or two. This matters because a fall in blood pressure — on standing, for instance — must be corrected in seconds if the brain is not to be under-supplied. Compare glucose regulation, which is hormonal and acts over minutes to hours, matching a disturbance that develops slowly.
Selective advantage
- Glucose regulation allows an animal to feed intermittently while supplying its cells continuously. Without it, cells would be flooded after a meal and starved between meals, and the brain could not function reliably. Storing surplus as glycogen also buffers against periods without food.
- Blood pressure regulation allows an animal to change posture and activity level rapidly without interrupting the supply to its brain — so it can stand, run and stop suddenly. An animal unable to correct pressure within seconds could not sustain the fast, variable activity that escaping a predator requires.
Worked Example
Worked Example
A person eats a meal, then two hours later begins vigorous exercise without eating again.
- Blood glucose rises from 90 to 140 mg per 100 cm3 within 45 minutes, then returns to about 90 over the next hour.
- During exercise, blood glucose stays close to 85 mg per 100 cm3 despite muscle consuming glucose rapidly.
- Blood insulin is high after the meal and low during exercise; blood glucagon shows the reverse pattern.
- Heart rate rises from 70 to 150 beats per minute during exercise, and blood pressure remains within its normal range throughout.
Explain the responses shown, and explain how each control system re-establishes stability.
Answer:
After the meal: correcting a rise in glucose.
Glucose absorbed from the small intestine raises blood glucose to 140 mg per 100 cm3, above the set point.
- β cells in the islets of Langerhans detect the rise directly and secrete insulin — which is why measured insulin is high.
- Insulin acts on liver, muscle and adipose cells:
- Vesicles carrying GLUT4 transporters fuse with the cell membranes, increasing permeability to glucose, so more enters by facilitated diffusion.
- Glycogenesis converts glucose to glycogen for storage in the liver and muscle.
- Glucose is therefore removed from the blood into cells and into storage, and blood glucose falls back to about 90 over the next hour.
- As it falls, the stimulus to the β cells weakens, so insulin secretion decreases — the response is self-limiting, which is why glucose settles at the set point rather than being driven below it.
During exercise: correcting a fall in glucose.
Muscle is now respiring glucose rapidly, removing it from the blood far faster than any is being absorbed. Blood glucose nonetheless stays close to 85 — barely below the set point — which shows the system is actively replacing what is consumed.
- α cells detect the fall and secrete glucagon — which is why glucagon is high and insulin low. The two are antagonistic, so suppressing insulin also prevents further glucose being removed into storage.
- Glucagon acts on the liver:
- Glycogenolysis hydrolyses stored glycogen back to glucose, released into the blood.
- Gluconeogenesis synthesises glucose from amino acids and glycerol, which becomes important as glycogen is depleted.
- Glucose released into the blood therefore replaces what the muscle is consuming, holding the concentration near the set point.
Why the brain matters here. The reason this must be corrected so precisely is that the brain cannot store glycogen and respires glucose almost exclusively. Muscle can draw on its own glycogen, but the brain depends entirely on blood glucose — so the system is protecting the brain's supply, not the muscle's.
Blood pressure during exercise.
Exercise increases cardiac output substantially — heart rate more than doubles — which would raise blood pressure sharply if nothing opposed it. Pressure nonetheless stays within its normal range.
- Baroreceptors in the carotid sinus and aortic arch detect increased stretch of the artery walls as pressure begins to rise.
- The cardiovascular centre in the medulla oblongata responds by promoting vasodilation of arterioles, particularly those supplying active muscle.
- Vasodilation reduces resistance to flow, so the same increase in cardiac output produces much less increase in pressure. Blood is simultaneously redistributed to the muscle that needs it.
Comparing the two systems. Both use negative feedback with opposing effectors, but their signalling differs to match their disturbances:
- Glucose control is hormonal, acting over minutes to hours — appropriate for a disturbance that develops slowly as food is absorbed or consumed.
- Blood pressure control is nervous, correcting within a heartbeat or two — necessary because a fall in pressure would under-supply the brain within seconds.
The speed of each control system is therefore matched to how quickly its disturbance becomes dangerous.