2B notes (homeostasis)
Describe the roles of major components of a response pathway from stimulus to response.
Explain how the body uses homeostasis to keep internal variables near a set point.
Identify major variables regulated by homeostasis.
Define hormone, endocrine gland, endocrine tissue, and target cell.
Explain how disruption of homeostasis relates to disease.
Compare and contrast positive and negative feedback.
Response Pathway Components
Pathway Sequence: The body responds to changes through a structured pathway:
Stimulus: A detectable change in the internal or external environment.
Receptor: A component that detects the stimulus.
Control Center: Integrates the information received from the receptor and sends out instructions.
Effector: Carries out the instructions from the control center.
Response: The body's physiological reaction to the stimulus.
Reflex Example (Heat Damage):
Stimulus: Extreme heat that damages tissue.
Receptors: Thermoreceptors (detect temperature) and nociceptors (detect pain) sense the extreme temperature.
Control Center: The spinal cord integrates this information.
Effectors: Muscles receive instructions.
Response: The hand retracts rapidly from the heat source.
Feedback Mechanisms and Homeostasis
Homeostasis: The process of maintaining a stable internal environment despite external fluctuations.
Internal variables typically fluctuate around a specific set point.
Negative Feedback: A mechanism that reduces or opposes the original stimulus, thereby helping to maintain balance and stability.
It brings a variable back towards its set point.
Positive Feedback: A mechanism that amplifies or intensifies the original stimulus.
This process continues until a specific endpoint or event is reached.
Examples of Homeostatic Variables
Body Temperature: Maintained within a narrow range of 36.5^{\circ}\text{C} - 37.5^{\circ}\text{C} (97.7^{\circ}\text{F} - 99.5^{\circ}\text{F}).
Blood Glucose: Fasting levels are typically between 70 - 110 \text{ mg/dL}.
Blood pH: Kept strictly between 7.35 - 7.45 to ensure proper enzyme function.
Blood Pressure: A common set point is approximately \sim 120/80 \text{ mmHg}.
Respiratory Gases (PaO2/PaCO2): Partial pressure of carbon dioxide (\text{PaCO}_2) is regulated, typically around 35 - 45 \text{ mmHg}.
Electrolytes: Various ions (e.g., sodium, potassium, calcium) are maintained at specific, variable concentrations.
Blood Osmolarity: Maintained within the range of 280 - 300 \text{ mOsm/kg}.
Specific Feedback Examples
Negative Feedback Example (Body Temperature Regulation):
The body temperature is maintained around a set point of 98.6^{\circ}\text{F}.
If body temperature rises, sweating is activated to cool the body.
If body temperature drops, shivering is activated to generate heat and warm the body.
Both responses counteract the initial change, bringing temperature back to the set point.
Positive Feedback Example (Childbirth):
Stimulus: The stretching of the cervix during labor.
Receptors: Mechanoreceptors in the cervix detect the stretch.
Control Center: Information is sent to the hypothalamus.
Hormone Release: The hypothalamus signals the posterior pituitary to release oxytocin.
Effector: Oxytocin causes stronger uterine contractions.
Amplification: Uterine contractions lead to even more cervical stretch, further increasing oxytocin release and stronger contractions.
Endpoint: This loop continues to amplify until the baby is delivered.
Homeostatic Failure
When feedback loops fail to function properly, physiological dysregulation occurs.
This disruption of homeostasis can lead to various diseases and conditions.
Examples of Homeostatic Failure:
Fever: An elevated body temperature beyond the set point, often due to infection.
Hypothermia: A dangerously low body temperature, where the body's warming mechanisms are insufficient.
Diabetes: Chronic hyperglycemia (high blood glucose) due to failed glucose regulation. This can lead to long-term damage to blood vessels, nerves, and organs.
Hormonal Signaling
Endocrine Signals: Hormones secreted by endocrine glands that travel long distances through the bloodstream to act on distant target cells (cells with specific receptors for the hormone).
Hormone: A signaling molecule transported in the blood to target cells.
Endocrine Gland: An organ that secretes hormones directly into the bloodstream.
Endocrine Tissue: Groups of cells within an organ that secrete hormones.
Target Cell: A cell that possesses specific receptors for a particular hormone and responds to it.
Paracrine Signals: Signaling molecules that act on nearby cells without entering the bloodstream.
Autocrine Signals: Signaling molecules that act on the same cell that secreted them.
Cytokines: Hormone-like signaling molecules primarily secreted by immune cells, playing roles in immune responses and inflammation.
Blood Clotting Pathway
The blood clotting process is an example of a positive feedback loop that contributes to overall homeostasis by preventing excessive blood loss.
Pathway:
Stimulus: Damage occurs to a blood vessel.
Detection: Platelets detect signals released from the damaged vessel.
Activation and Release: Activated platelets release chemicals that attract more platelets.
Recruitment: More platelets are recruited to the site of injury.
Endpoint: A blood clot forms, sealing the vessel and stopping bleeding.
This positive feedback amplifies the platelet aggregation process until the formation of a stable clot.
Comparison of Feedback Types
Negative feedback works to stabilize a physiological variable, keeping it within a narrow range around a set point.
Positive feedback accelerates or intensifies a physiological process, driving it towards a specific endpoint or completion.