sl homeostasis Flashcards
Homeostasis as maintenance of the internal environment of an organism
- Variables are kept within preset limits, despite fluctuations in external environment.
- Homeostasis is the ability to maintain a constant internal environment at preset values despite fluctuations from the external environment
- It works by monitoring levels of variables and making corrections by negative feedback mechanisms.
Examples of homeostasis as maintenance of internal environment
- body temp
- glucose level of blood
- water content of blood
- concentration of essential ions
- pH of blood
- heart rate
- pressure of blood in arteries
- concentration of respiratory gases
positive vs. negative feedback
Positive feedback amplifies their initiating stimuli - they move the system away from its starting state.
- Only few examples exist in the human body (e.g. During the menstrual cycle release of FSH stimulates follicle growth which in turn stimulates FSH release)
Negative feedback counteracts changes of various properties from their target value (set points). They form the basis of homeostatic control systems used to keep internal conditions within narrow limits. They require energy but keep the body at stable conditions.
- examples: body temperature regulation and control of blood glucose.
Negative feedback loops in homeostasis
- Students should understand the reason for use of negative rather than positive feedback control in homeostasis and also that negative feedback returns homeostatic variables to the set point from values above and below the set point.
- Homeostasis is controlled by negative feedback. This is the type of control in which conditions are brought back to a set value as soon as it is detected that they have deviated from it.
- see image on slide 6
Regulation of blood glucose as an example of the role of hormones in homeostasis.
- What situations cause the blood sugar level to drop above or below that value?
- Transport of glucose to all cells is a key function of the blood circulation. In human blood a set value of around 90mg/100ml blood is normal.
- Rise above: eating, stress
- Dropping: excess insulin, not eating enough carbs
Regulation of blood glucose as an example of the role of hormones in homeostasis
- Include control of secretion of insulin and glucagon by pancreatic endocrine cells, transport in blood and the effects on target cells.
- Blood sugar levels are adjusted by two hormones: Insulin and Glucagon, which are produced in pancreatic cells (Langerhans islets), which are hormone-secreting glands (endocrine glands). The hormones are transported in the body by the blood to reach their target cells. The Langerhans islets contain two types of cell, α cells (releases glucagon) and β cells (releases insulin).
- Insulin and glucagon are released by endocrine glands. Because the pancreas has multiple functions, it is important to distinguish between endocrine and exocrine glands.
What’s an exocrine glands?
What’s and endocrine gland?
- An exocrine gland is a gland which releases its products into a duct (e.g. digestive enzymes from the pancreas or in the stomach wall, sweat glands, salivary glands,…)
- An endocrine gland is a gland which secretes products (e.g. hormones from ovaries/testes, hypothalamus, pituitary gland) into the bloodstream.
Regulation of blood glucose diagram
Blood glucose level high:
- Stimulation of β cells , which secrete the hormone insulin into the blood stream.
- Insulin causes skeletal muscle fibre and liver cells to absorb the insulin.
- Insulin causes the conversion of glucose to glycogen in the cells (glycogenesis), and to fatty acids and fats (adipose tissue).
- Excess insulin is removed via the kidneys from the blood.
Blood glucose level low:
- Stimulation of α cells of the pancreas to secrete the hormone glucagon.
- Glucagon activates enzymes to convert glycogen and amino acids to glucose (gluconeogenesis).
- It also reduces the rate of respiration. Excess glucagon is removed from the blood via kidneys.
Target cells of insulin and glucagon
Insulin causes migration of membrane proteins and absorption into the cell
Glucagon causes breakdown of polymer storage products in the liver
Physiological changes that form the basis of type 1 and type 2 diabetes
- Students should understand the physiological changes, together with risk factors and methods of prevention and treatment.
- Diabetes is the condition in which a person has consistently elevated blood
glucose levels even during prolonged fasting. This leads to the presence of
glucose in the urine. - see causes and risk factors on slideshow
Thermoregulation as an example of negative feedback control
- Include the roles of peripheral thermoreceptors, the hypothalamus and pituitary gland, thyroxin and also examples of muscle and adipose tissue that act as effectors of temperature change.
- Thermoregulation is control of core body temperature to keep it close to a set point (which might differ at different times of the day, year, organism, etc.) Negative feedback is the basis of thermoregulation.
- thermoreceptors in the skin and by central thermoreceptors in the core of the body and hypothalamus. The hypothalamus is also the regulatory part, taking in information from thermoreceptors and initiating responses.
How is heat generated…
Heat is generated by metabolism in cells. Heat is then distributed by the blood circulation.
Some organs are more metabolically active (like the heart and kidneys, lungs and brain, from which over 70% of the body heat emerges)
The metabolic rate
- The metabolic rate can be decreased or lowered to adjust the amount of heat generated. To increase the metabolic rate, the hypothalamus secretes thyrotropin releasing hormone (TRH), which activates the pituitary gland to release thyroid stimulating hormone (TSH), which in turn stimulates thyroxin (T4) production by the thyroid gland.
- Thyroxin in the blood circulation stimulates oxygen consumption and increases the metabolic reactions that generate heat as a waste product.
Target cells for tyroxin
- The target cells for tyroxin are muscles, brain and liver which respond accordingly.
- Adipose tissue also acts as an insulator and reduces heat loss. Brown adipose tissue can generate heat at a rapid rate – small mammals and newborn babies have larger quantities of this type of adipose tissue, as they are particularly prone to heat loss.
diagram explaining thermoregulation
