Homeostasis and Endocrine

Homeostasis is the maintenance of a constant internal environment. It involves control by negative feedback (a change that sets of a response that cancels out the change).

Several substances need to be maintained at a constant level:

Substance	Too High?	Too Low?
Temperature (Thermoregulation) Carried out by the hypothalamus	Body temperature ↑… one sweats and when the sweat evaporates… the body cools down.	If the body temperature ↓… one shivers (uses glucose to create heat in muscles) … the body warms up.
Water (Osmoregulation) Carried out by the pituitary gland	Water level ↑… less ADH secreted so more water is flushed out… water level decreases.	Water level ↓… More ADH secreted so more water is reabsorbed… water level increases.
O₂ and CO₂ Carried out by medulla oblongata	If there is any change to the optimum level of these gases, the medulla oblongata detects it and sends signals to the heart and lungs to work faster (too hight) /slower (too low).
Glucose Carried out by hypothalamus	Glucose ↑… Insulin produced by pancreas and will stimulate the liver to convert excess glucose to glycogen and it will also stimulate the body to use up the glucose… glucose level returns.	Glucose ↓… Glucagon produced by pancreas and will stimulate liver to convert glycogen to glucose… glucose level returns.

Diabetes Mellitus (Hyperglycaemia)

Due to a mutation (random change in DNA) for which the gene that produces insulin is affected and insulin cannot be produced, the glucose level will always remain high. This is known as diabetes mellitus (type 1).
To deal with this, one must get regular insulin injections.
Some of the symptoms include:
- Person is thin and weak and will lack energy
- Excessive thirst, hunger and sugary urine production
- Sweet smelling breath
- Cataract formation

The Skin

Mechanisms to Conserve Heat:

The skin reduces heat loss by reducing:
- Convection – erector muscle contracts to pull hair up and trap air for insulation
- Evaporation – sweat gland doesn’t secrete sweat
- Radiation – vasoconstriction of arterioles (as they have muscles) leading to surface capillaries, shunts blood away from skin
To generate heat – muscles generate more heat by shivering
Controlling external environment:
- Addition of internal heating
- Wearing a jacket

Mechanisms to Increase Heat Loss:

The skin increases heat loss by increasing:
- Convection – erector muscles relax so hair lies flat and no air is trapped
- Evaporation – sweat is produced by sweat glands and when it evaporates a cooling effect is created
- Radiation – vasodilation of arterioles leading to surface capillaries
Controlling the external environment:
- AC/Fan can be switched on
- Wearing thin clothes

Endocrine System

An endocrine gland (one without any ducts) secretes a substance into the bloodstream. This substance is known as a hormone which carries messages for target organs. The message instructs the organs to speed up or slow down a process. Hormones are always carried by a network of blood vessels.

A hormone is a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Hormone Action

Negative Feedback Control

Adrenaline

Also known as the emergency or ‘fight or flight’ hormone, adrenaline is produced when one experiences an extreme emotion (such as nervousness, fear or anger)
It is secreted by adrenal glands
The purpose of adrenaline is to help get more energy (through respiration)

What Happens Due to Adrenaline?

Faster breathing and heart rate
Liver converts glycogen to glucose
Blood supply to some organs, which don’t require blood at that moment, is cut down (for example the digestive system [feeling of butterflies in the stomach] and the skin [the skin will pale]) so that more oxygen and glucose can go to muscles
Shivering muscles because of the high amount of energy produced
Dilated pupil to allow as much light as possible to enter the eye so that the brain can receive as much information as possible
Urinary bladder contracts