Homeostasis (6.6) Flashcards
Define homeostasis
The process of maintaining a constant internal environment despite large variations in the external environment.
How do homeostatic mechanisms operate?
Via a feedback loop which may involve either the nervous or endocrine systems (or both).
Describe a feedback loop. Use overheating as an example.
Stimulus - Sensor/receptor - Effector - Response.
Body gets hot - Heat sensed by thermoreceptors - Sweating initiated - Body cools down.
Negative feedback systems return back to optimum. Positive feedback systems move away from the optimum and can be fatal.
Explain the homeostatic control of body temperature.
The temperature of the blood is monitored by the hypothalamus in the brain. Thermoreceptors sense the temperature and control is effected by neurons and thyroxine.
What are the body’s response to overheating and chilling?
Overheating:
- Skin arterioles will dilate so more blood flows through the skin. As more heat is transferred from the core to the skin, the temperature of the skin will rise so more heat will be lost to the environment.
- Skeletal muscles will remain relaxed and at rest so no extra heat is generated.
- Sweat glands will secrete large amounts of sweat to dampen the skin. Water will evaporate from this damp skin which has a cooling effect.
- The muscles of the hair follicles will be relaxed.
- Behaviours are initiated to seek cool eg. drinking water
Chilling:
- Skin arterioles will contract so less blood flows to the skin. The temperature of the skin will fall so less heat is lost to the environment.
- Skeletal muscles will rapidly and repetitively contract to generate heat (shivering).
- Sweat glands will not secrete sweat.
- Muscles of hair follicles will contract, trapping air as insulation.
- Behaviours are initiated to seek warmth eg. find shelter
What is the body’s response when blood glucose levels are high?
β cells in the pancreas will produce insulin.
Insulin stimulates the liver and muscle cells to absorb glucose from the blood and convert it into glycogen. This process lowers blood glucose levels. Example of negative feedback.
What is the body’s response when blood glucose levels are low?
α cells in the pancreas will produce glucagon.
Glucagon stimulates liver cells to break glycogen down into glucose and release the glucose into the blood stream. Example of negative feedback.
Outline Type I and Type II diabetes.
Type I diabetes is early onset and is caused by insufficient insulin production (the body destroys its β cells in an autoimmune reaction). Its treatment is insulin shots.
Type II diabetes is late onset and is caused by insulin resistance (often triggered by obesity). Its treatment is weight loss.
What is leptin?
Leptin is a hormone produced by adipose fat cells that regulates energy balances and fat stores by inhibiting hunger. It does this by binding to receptors on the hypothalamus.
What is ghrelin?
Ghrelin is a hormone produced by the gastrointestinal tract (mainly the stomach) that increases appetite. It is sometimes called the ‘hunger hormone’.
Outline the experiments carried out to test leptin as a treatment for obesity.
The blood circulation of obese and healthy mice were fused together parabiotically.
When the obese mice with no leptin were fused to healthy mice they lost weight as leptin from the healthy mice was transferred to the obese mice. This indicated the potential of leptin as a treatment for obesity.
However, when the obese mice with defective leptin receptors were fused to healthy mice they remained obese. This was because its body could not respond to leptin. The obese mouse actually overproduced leptin to compensate for low receptor sensitivity so the healthy mice became emaciated as significant levels of leptin were transferred into its bloodstream.
Why is leptin not an effective treatment in humans for obesity?
Because, in most cases their obesity is caused by a decreased sensitivity to leptin and not a deficiency. So treating with leptin had no effect on their weight. The patients remained obese.
What is thyroxine?
Thyroxine is a hormone secreted into the blood by the thyroid gland in the neck. Its role is to regulate the body’s metabolic rate (including temperature).
Describe the feedback loop system that produces/releases thyroxine.
The hypothalamus in the brain monitors the activity of the body and stimulates the pituitary gland in the brain which releases TSH (thyroid stimulating hormone). TSH stimulates the thyroid gland and produces thyroxine.
*Note: When the body is cold, thyroxine is produced as it increases the metabolic rate of the body which generates more heat (and vice versa).
What can iodine deficiency cause in the body?
The hormone thyroxin is partially composed of iodine so a deficiency of iodine leads to a decreased production of thyroxin (potentially causing hypothyroidism).
