Understanding Homeostasis: Dynamic Equilibrium

• Health relates to balance of life sustaining forces ("humours" i.e. fluid/liquid).
• The human body consists of trillions of cells.
• Each cell allows movement of certain substances across the cell membrane.

Interstitial Fluid

• Interstitial fluid is water and solutes like ions and gases.
• It moves between cell interiors and blood in nearby capillaries.
• Total body water is about 42 liters (L), which makes up about 55%-60% of body weight.
• Intracellular fluid makes up about 67% of total-body water.
• Interstitial fluid makes up about 26% of total-body water.
• Plasma makes up about 7% of total-body water.

Homeostasis

• Homeostasis is derived from Greek, "Homeo-" means similar and "-stasis" means standing still.
• Homeostasis means staying the same, but is a dynamic, not static, process.
• It ensures conditions are stabilized above and below a set point.
• A deviation from a set point initiates a series of events to restore the set point.
• The restoration of set points is achieved by feedback systems.

Feedback System

• A feedback system is a cycle of events.
• The cycle involves monitoring, evaluating, changing, re-monitoring, and re-evaluating a body condition.
• Maintaining homeostasis requires constant monitoring of internal conditions.
• Physiological conditions each have a set point, examples include body temperature and blood pressure.
• A set point is the physiological value around which the normal range fluctuates.
• Normal range is the restricted set of values that is optimally healthful and stable.

Tight Control System

• A tight control system demonstrates very little variability or scatter around an average value.
• If oxygen and carbon dioxide levels are measured in the arterial blood of a healthy person, they barely change over time, even if the person exercises.
• Glucose concentration increases after meals, and more after larger meals.
• Glucose concentration then returns to the premeal level.

Controlled Condition

• The monitored body condition is called a controlled condition.
• Examples of controlled conditions include body temperature, blood pressure, and blood glucose level.
• A disturbance to a controlled condition is called a stimulus.

Feedback System Components

• A feedback system has 3 main parts:
• Sensor/Receptor: a body structure that monitors changes in a controlled condition and sends input to a control center.
• Control Center: evaluates the input from receptors and produces command outputs when needed.
• Effector: a body structure that receives output from the control center and produces a response or effect that changes the controlled condition.

Feedback System Types

• Feedback system types include negative and positive.
• Negative feedback system: reverses a change in a controlled condition.
• Positive feedback system: strengthens a change in a controlled condition.

Negative Feedback Mechanisms

• Negative feedback mechanisms is inhibitory.
• The response counteracts the input
• This is the most common feedback mechanism.
• The mechanism reverses a deviation from a set point.
• It maintains body parameters within their normal range.
• Examples: blood pressure, blood sugar regulation, cardiac output, and temperature regulation.

Process of Negative Feedback

• In a negative feedback loop, a stimulus—a deviation from a set point—is resisted through a physiological process that returns the body to homeostasis.
• When body temperature exceeds 37°C, nerve cells in the skin and brain act as sensors, triggering the temperature regulatory center in the brain, which sends commands to sweat glands throughout the body to act as effectors.

Temperature Regulation

• Humans have a similar temperature regulation feedback system by promoting either heat loss or heat gain.
• When the brain's temperature regulation center receives data from sensors, it stimulates a "heat-loss center" if it is too hot.
• This stimulation has three major effects:
• Blood vessels dilate to allow more blood flow to the skin surface, which causes heat radiation.
• Sweat glands activate to increase output and heat evaporates from the skin.
• The depth of respiration increases, and the person may breath through an open mouth.
• In contrast, the brain's heat-gain center activates when cold.
• Activation by cold reduces blood flow to the skin, and blood is diverted to deep veins.

Severe Heat Loss

• Severe heat loss triggers an increase in random signals to skeletal muscles.
• The muscle contractions release heat.
• The brain triggers the thyroid gland to release thyroid hormone.
• This increases metabolic activity and heat production throughout the body.
• The brain signals the adrenal glands to release epinephrine (adrenaline).
• The epinephrine causes breakdown of glycogen into glucose for energy.

Blood Pressure Negative Feedback

• An increase in blood pressure (BP) is considered a change in a controlled condition.
• The disturbance is detected by baroreceptors in blood vessels.
• The signal is then sent to the brain (control center).
• The brain responds by sending a signal to the heart and blood vessels (effector) to reduce heart rate and dilate blood vessels.
• These events helps restores normal blood pressure and homeostasis.

Positive Feedback

• Positive feedback intensifies a change instead of reversing it.
• A deviation from the normal range results in the system moving farther away from it.
• Positive feedback in the body is normal only when there is a definite end point.
• Childbirth and response to blood loss are the two examples of positive feedback loops.

Childbirth Positive Feedback

• The first labor contractions push the feutus into the cervix therefore stretching it.
• Increased stretching of the cervix is sensed by stretch-sensitive nerve cells.
• The brain responds by sending a signal to the uterus to contract more forcefully.
• The more the fetus is pushed, the more stretching occurs, and therefore the more forcefully the uterine muscles contract.
• Once the baby is born, then absence of cervix stretching eventually terminates cycle.