Homeostasis & Negative Feedback – Lecture Review

Homeostasis

• Homeostasis is the process that maintains adequate levels or conditions of specific variables in the body.

• Examples of these variables include:

  • Heart rate

  • Blood pressure

  • Ion concentrations

  • pH

  • Body temperature

  • Glucose levels

Body pH

• pH is a measurement of the hydrogen ion concentration.

• Hydrogen ion is also referred to as a proton.

• Maintaining proper concentrations of the hydrogen ion in the body is important.

• Body pH is often described as being between 7.38 to 7.42.

• There is a scale from 0 to 14.

• Body pH needs to be roughly around 7.4.

Homeostasis: A Dynamic Equilibrium

• Homeostasis is not "homoasis."

  • Homo means same.

  • Stasis means state.

• The body doesn't need to remain in the same state constantly; it can fluctuate within an acceptable range.

  • Acceptable range for heart rate: roughly 60 to 80 beats per minute.

  • Acceptable range for blood pressure: approximately 120/80 (systolic/diastolic).

    • Systolic number (120) represents when the heart is contracting.

    • Diastolic number (80) represents when the heart is in the state of diastole or relaxation.

    • Measured in millimeters of mercury.

  • Acceptable range for glucose levels: approximately 75 to 95 mg per deciliter.

  • Body temperature: Oftentimes we hear the number 98.6 degrees Fahrenheit, which is equivalent to 37 degrees Celsius.

• Homeo means similar, so homeostasis means similar state rather than same state.

• Homeostasis is best described as a dynamic equilibrium.

  • Dynamic is suggesting some flexibility, some change, some movement, some fluidity, and stasis.

  • Equilibrium is suggesting balance.

  • It's like a teeter-totter going up and down within an acceptable range.

• When our body gets out of homeostasis, that's when we get sick or ill, or in the worst-case scenario, we die.

• A lot of our body systems are aimed at maintaining homeostasis.

• When those body systems aren't working anymore, then our body is out of homeostasis.

Negative Feedback

• Negative feedback is the process that maintains homeostasis.

• There is nothing bad or wrong with negative feedback.

• Positive feedback is super important in the body, but positive feedback does not maintain homeostasis.

• If glucose concentrations decrease to 60 mg per deciliter (below the acceptable range of 75-95 mg/dL), negative feedback will bring it back up to the acceptable range.

• When negative feedback increases glucose levels, it only increases it enough to bring it back up to the acceptable range.

• Negative feedback reverses the direction the homeostatic variable is going and brings it back to the acceptable range.

• If we have 135 mg per deciliter, that is elevated blood glucose levels, negative feedback is going to decrease blood glucose levels.

• When we talk about glucose levels, we're talking about glucose concentration in the blood plasma, which should remain around 75 to 95 milligrams per deciliter.

• If we have elevated blood glucose levels, the pancreas releases insulin.

• Insulin binds to insulin-dependent cells like muscle cells, liver cells, and fat cells.

• It will open up the glucose channels via that exocytosis process and facilitate glucose uptake (movement of glucose from the blood into body cells).

• Glucose uptake will lower or diminish the glucose levels in the blood, and the negative feedback process is going to maintain homeostasis of glucose.

• The stimulus is when the homeostatic variable is out of its acceptable range.

• It's negative feedback when the body's response goes in the opposite direction, when the body's response returns that homeostatic variable back to its acceptable rate.

• If we have elevated body temperature (higher than the acceptable range), our body is going to do something to bring it back down, like sweating.

• If our body temperature is really cold (lower than the acceptable range), our body's response is going to start shivering to increase our body temperature.

Control System

• Negative feedback happens at two levels:

  • Systemic level

  • Local control systems

• Systemic is also called a long-distance control system for negative feedback.

• Both systems incorporate these three main features:

  • Input signal

  • Integrating center

  • Output signal

• The input signal is some sort of signal going to a processing center, which we refer to as the integrating center, indicating that something is off

• The integrating center is going to process that and send an output signal to some area in the body to elicit some sort of response.

• The integrating center is generally, when we're talking systemically, either the nervous system, i.e., the brain, or the endocrine system, probably the hypothalamus or the pituitary gland, which is also in the brain.

• If we're talking about elevated blood glucose levels, the signal is going to go to the pancreas saying, "Hey, release insulin to diminish glucose levels in the blood."

• If we're talking about body temperature, system-wide, there's going to be a signal that goes all the way up to the brain, up to a region known as the hypothalamus where our body's thermostat resides.

Local Control System

• Example: Hypoxia (diminished oxygen concentration at the tissues).

• If there is hypoxia, that means oxygen concentration that's lower than the acceptable range.

• We need to increase the oxygen concentration.

• Lining blood vessels are simple squamous epithelial cells referred to as endothelial cells that release a signaling molecule (a gas known as nitric oxide).

• Nitric oxide is going to act upon the smooth muscle making up the wall of this blood vessel.

• Lining the inside of the blood vessel facing the lumen are these endothelial cells.

• On the wall of the blood vessel is smooth muscle, and the smooth muscle regulates the size or lumen of the blood vessel.

• Nitric oxide is going to cause vasodilation of the blood vessel, and what that means is the lumen of the blood vessel gets larger.

• If the lumen gets larger due to vasodilation, that is going to increase blood flow.

• By increasing the volume of this blood vessel, we're going to decrease the pressure within the blood vessel, increase blood flow, and by increasing blood flow, we're going to increase oxygen content at the tissues.

• Local control systems deal with it on its own independently without incorporating the nervous system or the endocrine system.

• This is negative feedback because we started with diminished oxygen levels, which is known as hypoxia, and the body's response, this negative feedback process, returned oxygen back to the acceptable levels.

Systemic Negative Feedback Loop

• Includes:

  • Input signal

  • Control center

  • Output signal

  • Stimulus

  • Sensor

  • Integrating center

  • Output signal

  • Target organ

  • Body's response

• If we have diminished body temperature, that is going to be the stimulus.

• There are going to be sensors throughout the body that are going to detect diminished body temperature.

• The sensor is going to detect that; it's going to send that input signal to the integrating center.

• The integrating center, in the case of body temperature, is going to be the hypothalamus.

• It's going to process that information and determine that because there is diminished body temperature, the temperature needs to increase.

• An output signal is going to be sent to a target organ, and the target organ is going to be the muscles, and the response, the shivering of the muscles, is going to increase body temperature.

• These arrows are going in opposite directions, and that makes this negative feedback.