At any given concentration of hormone, more A is
bound to receptor than B.
Goiter results from dysfunction of the thyroid gland.
Recall that thyroid hormone potentiates the effects
of epinephrine and the SNS.
Recall that there exists a large store of iodinated
thyroglobulin in thyroid follicles.
Low plasma calcium decreases the ﬁ ltered load of
calcium. It also stimulates parathyroid hormone,
which increases calcium reabsorption from the
distal tubule. This helps to prevent the further loss
of calcium in the urine.
Parathyroid hormone is a potent stimulator of
calcium resorption from bone.
is the chief circulating form, but T
is more active.
Acromegaly is associated with hyperglycemia and
Quantitative and Thought Questions
Epinephrine falls to very low levels during rest and
fails to increase during stress. The sympathetic
preganglionics provide the only major control of the
The increased concentration of binding protein
causes more TH to be bound, thereby lowering
the plasma concentration of
TH. This causes
less negative-feedback inhibition of TSH secretion
by the anterior pituitary, and the increased TSH
causes the thyroid to secrete more TH until the
free concentration has returned to normal. The
end result is an increased
bound to the protein—but a normal free TH. There
is no hyperthyroidism because it is only the free
concentration that exerts effects on TH’s target cells.
Destruction of the anterior pituitary or
hypothalamus. These symptoms reﬂ ect the absence
of, in order, growth hormone, the gonadotropins,
and ACTH (the symptom is due to the resulting
decrease in cortisol secretion). The problem is
either primary hyposecretion of anterior pituitary
hormones or secondary hyposecretion because the
hypothalamus is not secreting hypophysiotropic
Vasopressin and oxytocin (the posterior pituitary
hormones) secretion would decrease. The anterior
pituitary hormones would not be affected because
the inﬂ uence of the hypothalamus on these
hormones is exerted not by connecting nerves but via
the hypophysiotropic hormones in the portal vascular
The secretion of GH increases. Somatostatin,
coming from the hypothalamus, normally exerts an
inhibitory effect on the secretion of this hormone.
Norepinephrine and many other neurotransmitters
are released by neurons that terminate on
the hypothalamic neurons that secrete the
hypophysiotropic hormones. Therefore,
manipulation of these neurotransmitters will alter
secretion of the hypophysiotropic hormones and
thereby the anterior pituitary hormones.
The high dose of the cortisol-like substance inhibits
the secretion of ACTH by feedback inhibition of
(1) hypothalamic corticotropin releasing hormone
and (2) the response of the anterior pituitary to
this hypophysiotropic hormone. The lack of ACTH
causes the adrenal to atrophy and decrease its
secretion of cortisol.
The hypothalamus. The low basal TSH indicates
either that the pituitary is defective or that it is
receiving inadequate stimulation (TRH) from
the hypothalamus. If the thyroid itself were
defective, basal TSH would be elevated because
of less negative-feedback inhibition by TH. The
TSH increase in response to TRH shows that the
pituitary is capable of responding to a stimulus
and so is unlikely to be defective. Therefore, the
problem is that the hypothalamus is secreting too
little TRH (in reality, this is very rare).
malnutrition. Neither growth hormone nor
the thyroid hormones inﬂ uence
Androgens stimulate growth but also cause
the ultimate cessation of growth by closing the
epiphyseal plates. Therefore, there might be a
rapid growth spurt in response to the androgens
but a subsequent premature cessation of growth.
Estrogens exert similar effects.
Blood in the right ventricle is relatively
deoxygenated after returning from the tissues.
Resistance decreases as the fourth power of an
increase in radius, and in direct proportion to a
decrease in vessel length.
See Figure 12–19.
The large total cross-section of capillaries results in
very slow blood velocity.
Increasing colloid osmotic pressure would decrease
ﬁ ltration of ﬂ uid from capillaries into the tissues.
Pressures are higher in the systemic circuit, but
because the cardiovascular system is a closed loop,
the ﬂ ow must be the same in both.
The AV node is the only conduction point between
atria and ventricles, and the slow propagation
through it delays the beginning of ventricular
The diastolic pressure in this example is 85;
adding 1/3 of the pulse pressure gives a MAP of
Reduced ﬁ ring to arterioles would reduce total
peripheral resistance and thus reduce mean arterial
pressure toward normal.