Understanding Homeostasis

• Homeostasis refers to the automatic processes a living thing uses to maintain a steady internal environment while adjusting to external conditions.
• Most homeostatic control relies on negative feedback.
• Negative feedback triggers a response that reverses change.
• Organisms maintain steady internal conditions via various structures and processes.

Objectives

• Explain the concept of homeostasis.
• Differentiate between negative and positive feedback control mechanisms with examples.
• Understand the importance of homeostatic balance.

Control Systems of Homeostasis

• Receptors are body structures that monitor changes in a controlled condition and send input to a control center.
• The pathway from the receptor to the control center is called the afferent pathway.
• Examples of receptors include mechanoreceptors, thermoreceptors, and photoreceptors.
• The control center receives input from receptors.
• Once receiving input, the control center provides an output or stimulus as a command to the effector.
• The brain is an example of a control center.
• The effector is a body structure that receives the output or stimulus from the control center.
• The effector produces a response or effect that changes the controlled condition.
• The controlled condition returns the body to normal conditions.
• The endocrine and nervous systems closely regulate homeostasis.
• The liver, kidneys, lungs, pancreas, skin, and endocrine glands help maintain homeostasis.

Types of Feedback Mechanisms

Negative Feedback Mechanism

• A negative feedback system reverses a change in a controlled condition.
• Negative feedback mechanisms return systems to a target set point when deviations from normal levels occur.
• When a variable rises, negative feedback brings it down, and vice versa.
• This type of response has a balancing effect on the body.
• Examples include body temperature regulation, glucose regulation, and osmoregulation.

Examples of Negative Feedback

Body Temperature Regulation

• When too hot, thermoreceptors in the skin detect the increase in body temperature.
• The brain, which is the control center, increases stimulation of sweat glands and relaxes blood vessels in the skin.
• Sweat glands and blood vessels are effectors that produce sweat and dilate, respectively.
• When too cold, thermoreceptors in the skin detect the decrease in body temperature.
• The brain decreases stimulation of sweat glands and constricts blood vessels in the skin.
• Sweat glands, blood vessels, and skeletal muscle are effectors that cease sweat production, constrict, and contract (shivering), respectively.

Glucose Regulation

• When blood glucose is too high, pancreatic beta cells and the intestine act as receptors.
• The autonomic nervous system serves as the control center.
• The liver, body muscles, and fats are effectors.
• Insulin stimulates glucose uptake and promotes glycogen storage.
• When blood glucose is too low, pancreatic alpha cells and the intestine act as receptors.
• The autonomic nervous system serves as the control center.
• The liver and skeletal muscle are effectors.
• Decreased insulin results in decreased glucose uptake, increased glycogen breakdown, and increased glucose synthesis.

Positive Feedback Mechanism

• Unlike negative feedback, a positive feedback system amplifies or reinforces a change in the body's controlled conditions.
• Levels move away from a set point during this change.
• This type of effect has an amplifying effect on the stimulus.
• Examples include fruit ripening, normal childbirth, and lactation.

Examples of Positive Feedback

Normal Childbirth

• Stretch receptors in the cervix act as receptors.
• The brain serves as the control center.
• Uterine muscles function as effectors.
• Stretch receptors in the cervix detect pressure from the baby's head.
• The hypothalamus signals the pituitary gland to release oxytocin.
• Oxytocin stimulates stronger contractions to push the baby further down.

Fruit ripening

• Ethylene triggers further release of ethylene.
• Ripening of one fruit encourages the ripening of others.