1.5
Homeostasis
Maintaining homeostasis involves continuous monitoring of internal conditions crucial for survival.
Key physiological conditions include body temperature, blood pressure, and nutrient levels.
Each physiological condition has a set point, which is the physiological value around which the normal range fluctuates.
A normal range is a restricted set of values that are healthful and stable.
Example: Normal human body temperature set point is approximately 37°C (98.6°F).
Physiological parameters tend to fluctuate within a normal range around the set point, typically just a few degrees above and below.
Control centers in the brain and other body areas monitor deviations and react to restore homeostasis via negative feedback.
Negative Feedback
Negative feedback is a mechanism that reverses deviations from the set point to maintain homeostasis.
Understanding negative feedback is fundamental for comprehending human physiology.
A negative feedback system has three basic components:
Sensor (Receptor): Monitors a physiological value and reports this to the control center.
Control Center: Compares the sensor's reported value to the normal range. Activates an effector if the value deviates significantly from the set point.
Effector: Causes a change to reverse the deviation and return the value back to the normal range.
Example of Negative Feedback System
Components of a Negative Feedback Loop:
Stimulus: Body temperature exceeds 37°C.
Sensor: Nerve cells in the skin and brain detect the temperature rise.
Control Center: Temperature regulatory center in the brain.
Effector: Sweat glands throughout the body.
Response: Increased heat loss from the body.
Body Temperature Regulation
When body temperature rises:
Skin blood vessels dilate, allowing more blood to flow to the skin surface.
Sweat glands increase output; sweat evaporation takes heat away.
Respiration depth increases, potentially shifting to open-mouth breathing to enhance heat loss from the lungs.
Conversely, activation of the heat-gain center in response to cold:
Reduces blood flow to skin and diverts blood from limbs to deep veins, trapping heat closer to the core.
If heat loss is excessive, the brain stimulates skeletal muscle contractions, causing shivering, which generates heat.
Activation of the thyroid gland leads to the release of thyroid hormone, increasing metabolism and heat production.
Breakdown of glycogen into glucose enhances metabolic processes and heat production.
Importance of Water Regulation
Water concentration in the body is crucial for normal physiological operations.
The body maintains tight control over water levels without conscious effort from the individual.
Key organ regulating water concentration is the kidney.
Positive Feedback
Positive feedback amplifies changes in physiological conditions, moving the system further from the normal range.
Activates changes that are normal only when there is a definite endpoint.
Two primary examples: Childbirth and response to blood loss.
Childbirth Example
During childbirth, the body aims for extreme changes to expel the baby:
First contractions (the stimulus) push the baby toward the cervix.
Cervix has stretch-sensitive nerve cells (sensors) that notify the brain of stretching.
Brain stimulates the pituitary gland to secrete oxytocin:
Oxytocin leads to stronger uterine contractions (effectors).
Cycle of stretching and oxytocin release continues until childbirth.
Blood Loss Example
Positive feedback mechanism in response to a penetrating wound:
Immediate threat is excessive blood loss, leading to reduced blood pressure and perfusion to vital organs.
Body reacts by initiating blood clotting through the release of substances that trigger the clotting cascade.
Each step of clotting accelerates the release of more clotting factors, sealing the damaged area and stopping blood loss effectively without widespread activation of clotting proteins.
This life-saving cascade exemplifies the adaptive nature of positive feedback under critical conditions.