ARTERIAL RECEPTORS
The aorta and carotid arteries contain receptors that
detect changes in the blood. The aortic arch, which
receives blood pumped by the left ventricle of the
heart, curves over the top of the heart. The left and
right carotid arteriesare branches of the aortic arch
that take blood through the neck on the way to the
brain. In each of these vessels are pressoreceptors and
chemoreceptors (see Fig. 12–7).
Pressoreceptorsin the carotid sinuses and aortic
sinus detect changes in blood pressure. Chemorecep-
torsin the carotid bodies and the aortic body detect
changes in the oxygen and carbon dioxide content and
the pH of blood. The impulses generated by these
receptors do not give rise to sensations that we feel but
rather are information used to make any necessary
changes in respiration or circulation. We will return
to this in later chapters, so one example will suffice
for now.
If the blood level of oxygen decreases significantly,
this change (hypoxia) is detected by carotid and aortic
chemoreceptors. The sensory impulses are carried by
the glossopharyngeal (9th cranial) and vagus (10th
cranial) nerves to the medulla. Centers in the medulla
may then increase the respiratory rate and the heart
rate to obtain and circulate more oxygen. These are
the respiratory and cardiac reflexes that were men-
tioned in Chapter 8 as functions of the glossopha-
ryngeal and vagus nerves. The importance of these
reflexes is readily apparent: to maintain normal blood
levels of oxygen and carbon dioxide and to maintain
normal blood pressure.
AGING AND THE SENSES
All of the senses may be diminished in old age. In the
eye, cataracts may make the lens opaque. The lens
also loses its elasticity and the eye becomes more
farsighted, a condition called presbyopia. The risk
of glaucoma increases, and elderly people should be
tested for it because treatment is available that can
prevent blindness. Macular degeneration, in which
central vision becomes impaired first, is a major cause
of vision loss for people over 65. Reading and close
work of any kind become difficult.
In the ear, cumulative damage to the hair cells in
the organ of Corti usually becomes apparent some
time after the age of 60. Hair cells that have been
damaged in a lifetime of noise cannot be replaced
(regrowth of cochlear hair cells has been stimulated in
guinea pigs, but not yet in people). The deafness of
old age ranges from slight to profound; very often
high-pitched sounds are lost first, while hearing
may still be adequate for low-pitched sounds. The
sense of equilibrium may be diminished; the body is
slower to react to tilting, and falls may become more
frequent.
Both taste and smell become less acute with age,
which may contribute to poor nutrition in elderly
people.SUMMARY
Changes take place all around us as well as within
us. If the body could not respond appropriately to
environmental and internal changes, homeostasis
would soon be disrupted, resulting in injury, illness, or
even death. To respond appropriately to changes, the
brain must know what they are. Conveying this infor-
mation to our brains is the function of our senses.
Although we may sometimes take our senses for
granted, we could not survive for very long without
them.
You have just read about the great variety of inter-
nal and external changes that are detected by the sense
organs. You are also familiar with the role of the nerv-
ous system in the regulation of the body’s responses.
In the next chapter we will discuss the other regula-
tory system, the endocrine system. The hormones
of the endocrine glands are produced in response to
changes, and their regulatory effects all contribute to
homeostasis.216 The Senses
BOX9–6 MOTION SICKNESS
Motion sickness is characterized by cold sweats,
hyperventilation, nausea, and vomiting when the
person is exposed to repetitive motion that is
unexpected or unfamiliar, or that cannot be con-
trolled. Seasickness is a type of motion sickness,
as is carsickness (why children are carsick more
often than adults is not known).
Some people are simply not affected by the
rolling of a ship or train; for others, the constant
stimulation of the receptors for position first
becomes uncomfortable, then nauseating. For
those who know they are susceptible to motion
sickness, medications are available for use before
traveling by plane, train, boat, or car.