MCAT Anatomy > Homeostasis



Homeostasis is the ability of an organism to maintain a relatively stable internal environment. This involves maintaining different many variables such as oxygen, electrolytes, pH, hormone levels and much more through the process of feedback loops. Through both negative feedback loops the body is able to respond to both internal and external changes in order to keep the body within the optimal normal range. These include for functions like body temperature, fluid balance and electrolyte concentrations.

In homeostasis various opposing forces are working together in a state of dynamic equilibrium or are in balance together. Think of the Chinese philosophical symbol of Ying and Yang. Other types of equilibrium are the complex equilibrium which is the regulation of one parameter such as heat which affects other variables such as hydration.

One of the important features of homeostasis is the level of systems integration that occurs to make it possible. Different functions such as body temperature, hydration and more are controlled by multiple organ systems that work together through coordinated efforts.

If the body is unable to maintain homeostasis this is known as a pathological condition. This can be either due to internal failure such as a tumor or renal failure or an external failure such as injury or infection.


See Regulation

Homeostasis is maintained through different methods of control and regulation. These include both local control and reflex control systems.

Local Control

Local Control means that the cell, tissue or organ has an automatic response some environmental change. One example of this is the local control of blood flow in order to get more oxygen.

Reflex Control Systems

Reflex control systems are responses that are coordinated by the nervous and endocrine systems and act far away from the targeted tissue. An example of this is cardiovascular responses to exercise which include increased heart rate, and reduced blood flow to inactive areas of the body.

Feedback Loops

See Feedback Loops

Feedback loops are essential in maintaining the homeostasis of the body. Feedback loops come in two different types, positive feedback loops and negative feedback loops. Only negative feedback loops are involved in maintaining homeostasis. There are several components to feedback loops which include a receptor, a control center and an effector.

The receptor receives the stimulus and has a sensor that measures the controlled variable. Usually each receptor is involved in a different task with a different compound.

The control center processes and integrates the signal and compares it to the set-point and then sends instructions via the output signal.

This process occurs within the Nervous System. Following this the effector carries out the instructions to adjust the controlled variable.

Important Concentrations

You should do your best to memorize this table as it is essential to understand the concentrations of various important molecules and compounds within the body. In addition to medical school this is usually required knowledge for your anatomy and physiology class in college.

Na+10 mM134-145 mM
K+140 mM3.5-5.5 mM
Ca2+0.1 uM2-3 mM
Glucose (fasting)-70-100 mg/dL
Protein-7 g/dL

Controlled Variables

Controlled, also known as regulated variables include the following;

Core Temperature37o C
Ionic Fluid Composition300 mOsm/L
Plasma Glucose Concentration100 mg/dL
Blood Oxygen Levels
Blood Pressure70-110 mmHg
Blood Flow (exercise)

MCAT Anatomy

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