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Category: Medical

Topic: Cellular Physiology

Level: EMR

Next Unit: Aerobic vs Anaerobic Metabolism

16 minute read


The body regulates a balance between extreme states to optimize health. If you get overheated on a warm day, your body will sweat to lose heat in order to maintain a healthy temperature. If you get too cold, your body will shiver to create body heat to maintain a healthy temperature. Homeostasis is the body's Goldilocks in the story trying to find what is "just right." In the body, it works via compensation. Compensation is adding effects to the opposite side of the scale to achieve balance. For example, if calcium is too high, the kidneys will increase excretion of it to lower it back to the "just right" range. Compensation also means reversing an undesired state.


  • If your blood pressure drops, your heart rate will increase to try to restore the adequate vessel pressure to maintain tissue and organ perfusion.
  • As CO2 levels increase, the body’s response is to breathe deeply and depending on the level of CO2, possibly breathe rapidly (the higher the level of CO2, the faster the body tries to rid itself of the overload; Kussmaul respirations).
  • If the CO2 levels in the blood are low, the body does not need to breathe as fast or deep. Low CO2 can result from prolonged tachypneic (Kussmaul) respirations or from decreased CO2 levels (bradypnea) as a result of a disturbance within the chemoreceptors from a medical condition (COPD) or trauma (head injury; Biot's or Cheyne-stokes respirations).

Homeostasis can usually be described in terms of "feedback."


Negative Feedback Loops

In a negative feedback loop, the result of a reaction causes a decrease or a reversal in that reaction. An example of negative feedback is the maintenance of blood pressure. When the blood pressure gets too high from increased cardiac strength and frequency of contractions, baroreceptors in the carotid sinus send a signal up to the glossopharyngeal nerve (Cranial Nerve XI) which, among other functions, causes the brain to decrease stimulation to the heart muscle. (If blood pressure is high, heart rate decreases; if blood pressure is low, heart rate increases.)

Blood pressure homeostasis is much more complicated than this, involving fluid management via the kidneys, osmotic regulation via osmoreceptors in the hypothalamus, arterial wall tone, etc.

Another method of homeostasis based on "negative feedback" is when estrogen falls too low, as at the end of a menstrual cycle. The brain's hypothalamus will sense this and take action to reverse this "negative" value for estrogen with a stimulation of a series of hormones that within their own positive feedback subsystems (see below), produce hormones that will result in estrogen rise again. This loop includes the ovaries, pituitary gland, and hypothalamus.


Positive Feedback Loops

In a positive feedback loop, an action causes a stimulus to create even more of the same action. An example is platelet activity in blood clotting. When tissue is torn, chemicals are released which cause platelets to activate. Once activated, they release another chemical that signals more platelets to activate. Of course, the entire clotting cascade is much more complicated, but within it are several examples of homeostasis via positive feedback loops.

The main difference between positive feedback and negative feedback loops is their response to change. Negative feedback reduces/reverses change; positive feedback amplifies change.


Goal of Homeostasis

The goal of homeostasis is survival. By and large, the body can persist in good health on its own without external intervention by balancing its own systems and provoking the correct behaviors in humans. When the body is damaged or under attack by disease processes, the body may not be able to regulate itself properly via homeostasis.

(Example, AIDS will interfere with both positive and negative feedback loop capabilities within the immune system.)

Chemoreceptors detect high levels of carbon dioxide in the blood triggering the medulla rhythm centers in the brain to stimulate both the external intercostal muscles (intercostal nerve) and diaphragm (phrenic nerve) to increase the rate of breathing and volume of inhaled air. Both central chemoreceptors (located in the medulla) and peripheral chemoreceptors (located in the aorta and carotid arteries) work together to trigger spontaneous breathing.

Stretch receptors are activated as a secondary response to inhalation through the peripheral chemoreceptors, activating the medullary vagal center and thereby slowing the heart rate.


The Bare Necessities

Certain elements are required for the normal function of the cells, tissues, and organs of the human body. Oxygen and glucose are two things the body needs to make energy and perform their functions.

  • Oxygenation is a type of gas exchange between alveoli and capillaries within the lungs where oxygen is loaded onto red blood cells for delivery.

  • Perfusion describes the degree to which tissue or organs are receiving that delivery of oxygenated blood. Poor perfusion means that the tissue or organs are not getting sufficient oxygenated blood.

Hypoperfusion means "inadequate tissue perfusion," also know by its other term, "shock."

Several conditions must remain within certain specific parameters to allow for homeostasis. These conditions include the composition of the ambient air, the patency of the airway, the mechanics of ventilation, the regulation of respiration, the transport of gases, the blood volume, the effectiveness of the heart as a pump, and blood vessel size and resistance.

From the macroscale (drinking water when you're thirsty) to the nanoscale (protein synthesis and molecular receptors), homeostasis represents a continuum in constant search for the "Goldilocks" range for all systems.

NOTE: Of particular significance for EMS responders, pediatrics patients can compensate for longer periods of time than mature adults, but once their bodies have reached exhaustion, they crash fast.

For infants and babies, have a high index of suspicion that circulatory problems may have a respiratory cause. By addressing the respiratory problems first, the heart problems can fix themselves once they have adequate amounts of oxygenated blood.