Plasma Membrane -> Separates the cell exterior from the cell interior (cytoplasm).
Nucleus -> Membrane bound structure that contains deoxyribonucleic acid (DNA) which is the set of instructions for the synthesis of all the body’s proteins.
Mitochondria -> Structure bound by a double membrane and the site at which the energy stored in sugars and other organic molecules is transferred to ATP, the chemical which acts as the “currency” for energy in the cell.
Ribosomes -> Not bound by a membrane. Sites of protein synthesis. May be free – floating in the cytoplasm – or bound to the endoplasmic reticulum.
In order to keep the right amount of stuff in the cell, we’ve got to make sure that all the fluid surrounding our cells (i.e., the extracellular fluid ) has the right assortment of nutrients, ions, etc.
We keep both our cells and the fluid surrounding our cells in a dynamically stable environment via a process called HOMEOSTASIS.
BP is a variable that we’ve got to maintain at a certain level
We have sensory receptors that measure the BP in the body. They’re located in the aorta (the big blood vessel coming out of the heart) and in the carotid arteries (the large vessels that bring blood to the brain).
These pressure receptors measure BP and then send the info (we can call this input ) to a control center in the brain – the particular BP control center is in the medulla oblongata of the brain
The movement of a variable in one direction causes the body to enact processes that cause the variable to move in the opposite direction (so as to return the value to the correct level) – we call it negative feedback
Let’s look at BP again:
Increased BP Sensed by pressure receptors in aortic arch and carotid sinus Input sent via afferent pathway to medulla oblongata Current BP compared with set point and error signal generated Output sent along efferent pathway to heart and blood vessels Heart rate & force of contraction decrease Blood vessel diameter increases BP DECREASES
Why is Negative Feedback so common in the body?
Every time a variable starts changing too much, we’ve got to bring it back to normal. We’ve got to counteract its change.
THAT’S NEGATIVE FEEDBACK
Other examples you will encounter:
Maintenance of blood [Ca 2+ ], blood [Glucose], blood pH, and many others
What this means is that the homeostatic variables are NOT kept rigidly fixed upon a single value. They are kept within a certain range, and when they exit that range – that’s when negative feedback loops turn on to bring them back.