192 ❯ STEP 4. Review the Knowledge You Need to Score HighHomeostasis
Homeostasis is the maintenance of balance. Hormones can work antagonistically to main-
tain homeostasis in the body. Two examples we will talk about are insulin/glucagon and
calcitonin/PTH:- Insulin/glucagon.Both are hormones of the pancreas and have opposing effects on blood
glucose. Let’s say that you eat a nice sugary snack that pushes the blood glucose above its
desired level. This results in the release of insulin from the pancreas to stimulate the uptake
of glucose from the blood to the liver to be stored as glycogen. It also causes other cells of
the body to take up glucose to be used for energy. Sometimes if you go a long time between
meals, your blood glucose can dip belowthe desired level. This sets glucagon into action
and causes its release from the pancreas. Glucagon acts on the liver to stimulate the removal
of glycogen from storage to produce glucose to pump into the bloodstream. When the glu-
cose level gets back to the appropriate level, glucagon release ceases. This back-and-forth
dance works to keep the glucose concentration in our bodies relatively stable over time. - Calcitonin/PTH.Like glucose, the body has a desired blood calcium (Ca^2 +) level it tries
to maintain. If it drops below this level, PTH is released by the parathyroid gland and
works to increase the amount of Ca^2 +in circulation in three major ways: it (a) releases
of Ca^2 +from bones, (b) increases absorption of Ca^2 +by the intestines, and (c) increases
reabsorption of Ca^2 +by the kidneys. If the blood Ca^2 +level gets too high, the thyroid
gland releases calcitonin, which pretty much performs the three oppositeresponses to
PTH’s work: it (a) puts Ca^2 +intobone, (b) decreases absorption of Ca^2 +by the intestines,
and (c) decreases reabsorption of Ca^2 +by the kidneys.
One last distinction we want to make before we move on is to touch on the difference
between protein hormones and steroid hormones.
Protein hormonesare too large to move into cells and thus bind to receptors on the
surface of cells. In response to the binding of a protein hormone, a change occurs in the
receptor that leads to the activation of molecules inside the cell, called second messengers,
which serve as intermediaries, activating other proteins and enzymes that carry out the mis-
sion. The second messenger to know for this exam is cyclic adenosine monophasphate
(cAMP), involved in numeroussignal cascade pathways. Protein hormones activate cAMP
through a multi-step process that begins with protein–hormone activation of relay proteins
such as G proteins.These proteins are able to directly activate a compound known as
adenyl cyclase,which in turn produces cAMP.
Since we discussed regulatory mechanisms earlier, it is important to point out that there
are G proteins that function to inhibitcAMP and work antagonistically to hormones that
activate cAMP.
Steroid hormonesare lipid-soluble molecules that pass through the cell membrane and
combine with cytoplasmic proteins. These complexes pass through to the nucleus to inter-
act with chromosomal proteins and directly affect transcription in the nucleus of cells.
Immune System
What we are about to witness is an absolute treat. We just got word from the central office
that the body we are touring has just received a vaccination.A vaccine is given to a patient
in an effort to prime the immune system for a fight against a specific invader. This truly is
a rare opportunity for us to see the immune system in action.
We have reentered the general bloodstream circulation of the body in an attempt to find
some activity. While we are in transit, we will explain some basic immune system terms to you.NYC teacher:
“This could make
a nice subques-
tion to an essay.
Understand these
relationships.”CT teacher:
“Concentrate on
the various cell
types and the dif-
ference between
specific and non-
specific defense.”KEY IDEABIG IDEA 2.C.1
Animals use feed-
back to maintain
homeostasis.