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  1. Define homeostasis as the maintenance of a constant internal environment

Homeostasis is the maintenance of a constant internal environment.

 

  1. Explain that homeostasis is the control of internal conditions within set limits

Homeostasis is the control of internal conditions within set limits.

For example, the normal set limit for body temperature in humans is about 37oC. Homeostatic mechanisms control our body temperature so that even when it fluctuates it, it stays very close to the set limit.

Similarly, we have a set limit for blood glucose levels. After a meal, when blood glucose levels shoot up, homeostatic mechanisms cause the blood glucose concentration to fall back down to the set limit.

 

  1. Explain the concept of control by negative feedback

Negative feedback is when a fluctuation in a particular parameter, such as body temperature, is reduced so that it returns to its normal range of functioning. This is done by triggering a sensor that stimulates a response in an effector that reduces the fluctuation. In other words, any change is counteracted so that it returns to its set-point.

Examples of negative feedback systems include thermoregulation (control of body temperature), blood glucose concentration, osmoregulation (control of blood water potential), etc. This is because any changes in any of these parameters results in the body acting so that the change is minimised and is brought back to its normal range.

For example, if body temperature rises, the body will act to decrease the temperature back to 37oC (which is the set-point of body temperature). If body temperature falls, the body will act to increase its temperature back to 37oC.

 

  1. Describe the control of the glucose content of the blood by the liver and the roles of insulin and glucagon from the pancreas

The hormones insulin and glucagon, secreted by the pancreas, control blood glucose concentration.

Insulin is produced by the beta cells of the islets of Langerhans in the pancreas. Glucagon is produced by the alpha cells of the islets of Langerhans in the pancreas.

The hormones travel to the liver in the blood, which is the organ that controls blood glucose levels.

Glycogen is a short-term storage molecule. It is a polymer made of glucose molecules.

When the blood glucose level increases above its set point:

The pancreas secretes insulin, which travels to the liver in the bloodstream. Insulin stimulates liver cells to absorb glucose and stimulates the conversion of glucose to glycogen. Insulin also encourages an increase in the rate of respiration – this means more blood glucose is taken up by cells and respired. All of this reduces blood glucose levels.

When the blood glucose level decreases below its set point:

Glucagon is secreted by the pancreas, which then travels to the liver via blood. The hormone glucagon stimulates the conversion of glycogen to glucose (this process is gluconeogenesis), and glucose is released back into the bloodstream. This increases blood glucose levels.

 

  1. Name and identify on a diagram of the skin: hairs, hair erector muscles, sweat glands, receptors, sensory neurones, blood vessels and fatty tissue

Here are two diagrams, seeing as I couldn’t find everything that you should probably know on just one:

The sensory receptors labelled in the first diagram could be a touch receptor, pressure receptor, temperature receptor, etc.

 

  1. Describe the maintenance of constant internal body temperature in humans in terms of insulation, sweating, shivering and the role of the brain (limited to blood temperature receptors and coordination)

Since the next learning objective also covers something very similar, I’m going to put that here, too, before I start explaining.

  1. Describe the maintenance of a constant internal body temperature in humans in terms of vasodilation and vasoconstriction of arterioles supplying skin surface capillaries

Humans maintain our body temperature at 37oC.

Fat is an insulator, so when the external temperature fluctuates, it prevents our internal temperature from similarly fluctuating. This is because it traps heat inside our body and slows down the warming up of our body from an external source.

When the temperature changes, temperature receptors in the skin detect this information and send it as impulses through nerves to a part of our brain called the hypothalamus. This part of our brain is in charge of maintaining constant body temperature – it works like a thermostat.

It sends electrical impulses along nerves to body parts that help regulate body temperature.

When the temperature rises, the hypothalamus stimulates:

  • Hair to lie flat – the hair erector muscle is relaxed, allowing the hair to lie flat against the skin, so no air is trapped close to the skin, so we are insulated less. This is less effective in humans as we do not have as much body hair/ fur as some animals.
  • Vasodilation – the muscles in the walls of the arterioles supplying skin-surface capillaries relax, increasing the size of the arteriole lumen. More blood flows, so more heat can be lost to the environment from the blood at a time.
  • Sweating – sweat is secreted by sweat glands. It evaporates, taking heat from the skin with it, causing the body to cool down.
  • Metabolism slows down – metabolism usually consists of exothermic reactions (reactions that give off heat energy) so slower metabolism means less heat is given off at a time.

When the temperature falls, the hypothalamus stimulates:

  • Hair to stand erect – the erector muscle contracts, pulling the hair to stand up straight. This allows the hair to trap air close to the skin. As air is an insulator, it traps heat close to the skin, warming up the body.
  • Vasoconstriction – muscles in arteriole walls contract, making the lumen smaller, so less blood travels through the skin at a time, reducing the heat loss per unit time.
  • Reduces sweating – so less sweat evaporates, making the body cool down less.
  • Metabolism may increase.
  • Shivering – muscles in some part of the body involuntarily contract and relax very quickly, producing heat as a result.

 

 

 

Notes submitted by Sarah

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