Biology 2: Homeostasis

Homeostasis

• Homeostasis is the maintenance of a constant internal environment for life functions despite changes in the external environment.
• Physiological factors controlled in mammals include:
  • Core body temperature
  • Metabolic wastes (e.g. carbon dioxide and urea)
  • Blood pH
  • Blood glucose concentration
  • Water potential of the blood
  • Concentrations of respiratory gases (oxygen and carbon dioxide) in the blood
• Homeostasis involves corrective actions that correct or reverse changes in these factors.

Homeostatic Control

• There are two coordination systems in mammals that are involved in homeostatic control:
  • Nervous system (electrical impulses)
  • Endocrine system (chemical messengers)

Structure of Homeostasis

• The hypothalamus receives information from nerves, processes it, and sends signals to the pituitary gland.
• The pituitary gland is located at the base of the hypothalamus and synthesizes and secretes hormones in response to hormones from the hypothalamus.
• The posterior pituitary is an extension of the hypothalamus and secretes neurohormones synthesized in the hypothalamus.
• The anterior pituitary is an endocrine gland that synthesizes and secretes hormones in response to hormones from the hypothalamus.

Posterior Pituitary Hormones

• Two posterior pituitary hormones are:
  • Antidiuretic hormone (ADH)
  • Oxytocin
• These neurohormones are stored and released in response to nerve impulses transmitted by the hypothalamus.

Anterior Pituitary Hormones

• Hormones secreted by the anterior pituitary control diverse processes in the human body, including:
  • Metabolism
  • Osmoregulation
  • Reproduction
• Hypothalamus regulates each set of hormones by using releasing or inhibiting hormones on the anterior pituitary gland.

Thyroid Regulation

• Thyroid regulation involves the HPT axis, which includes the hypothalamus, pituitary gland, and thyroid gland.
• The hypothalamus produces thyroid-releasing hormone (TRH), which stimulates the anterior pituitary gland to produce thyroid-stimulating hormone (TSH).
• TSH stimulates the thyroid gland to produce thyroid hormones (T3 and T4).
• Thyroid hormones regulate metabolism, growth, and development.

Thermoregulation

• Thermoregulation is the control of body temperature, which involves both the nervous and endocrine systems.
• The hypothalamus is the central control for body temperature.
• When the body temperature is too low, the thyroid gland increases the production of thyroid hormones, which leads to an increase in metabolism and heat production.

Blood Glucose Regulation

• Blood glucose regulation involves the balance between insulin and glucagon.
• Insulin:
  • Converts glucose to glycogen
  • Increases the rate of glucose conversion to glycogen
  • "Traps" glucose into cells
  • Increases the rate of glucose conversion to fat and stored in adipose tissue
• Glucagon:
  • Converts glycogen to glucose
  • Increases the rate of gluconeogenesis

Diabetes

• Diabetes mellitus is a metabolic disease characterized by high blood glucose levels.
• There are two types of diabetes:
  • Type 1 diabetes (insulin-dependent diabetes):
    • Beta cells do not produce insulin
    • Viral infection, autoimmune response, and genetics are risk factors
  • Type 2 diabetes (non-insulin-dependent diabetes):
    • Insulin is produced, but target cells do not respond to it adequately
    • Obesity, genetic link, and a diet high in sugars are risk factors
• Symptoms of diabetes include:
  • High glucose levels in the blood and urine
  • Increased hunger and thirst
  • Frequent urination