PHYSIOLOGY OF THE CIRCULATORY ORGANS.
352. The walls of all the cavities of the heart are composed of
muscular fibres, which are endowed with the property of contracting
and relaxing, like the muscles of the extremities. The contraction and
relaxation of the muscular tissue of the heart, produce a diminution
and enlargement of both auricular and ventricular cavities. The
auricles contract and dilate simultaneously, and so do the ventricles;
yet the contraction and dilatation of the auricles do not alternate
with the contraction and dilatation of the ventricles, as the
dilatation of the one is not completed before the contraction of the
other commences. The dilatation of the ventricles is termed the
_di-as'to-le_ of the heart; their contraction, its _sys'to-le_.
353. The ventricles contract quicker and more forcibly than the
auricles, and they are three times longer in dilating than contracting.
The walls of the right ventricle, being thinner than the left, are more
distensible, and thus this cavity will contain a greater amount of
blood. This arrangement adapts it to the venous system, which is
more capacious than the arterial. The thicker and more powerful walls of
the left ventricle adapt it to expel the blood to a greater distance.
354. The valves in the heart permit the blood to flow from the
auricles to the ventricles, but prevent its reflowing. The valves at
the commencement of the aorta and pulmonary artery, permit the blood
to flow from the ventricles into these vessels, but prevent its
returning.
-=-=-=-=-=-=-=-=-=-=-=-=
352-366. _Give the physiology of the circulatory organs._ 352. What do
the contraction and relaxation of the muscular walls of the heart
produce? How do the auricles and ventricles contract and dilate? 353.
What is said of the contraction and dilatation of the ventricles in
the heart? How is the right ventricle adapted to its function? How the
left? 354. What is the use of the valves in the heart? Those of the
aorta and pulmonary artery?
-=-=-=-=-=-=-=-=-=-=-=-=
355. The function of the different parts of the heart will be given,
by aid of fig. 74. The blood passes from the right auricle (3) into
the right ventricle, (5,) and the tricuspid valves (6) prevent its
reflux; from the right ventricle the blood is forced into the
pulmonary artery, (7,) through which it passes to the lungs. The
semilunar valves (9) prevent this circulating fluid returning to the
ventricle. The blood, while passing over the air-cells in the lungs,
in the minute divisions of the pulmonary artery, is changed from a
bluish color to a bright red. It is then returned to the left auricle
of the heart by the pulmonary veins, (11, 11.)
[Illustration: Fig. 74. 1, The descending vena cava, (vein.) 2, The
ascending vena cava, (vein.) 3, The right auricle. 4, The opening between
the right auricle and the right ventricle. 5, The right ventricle. 6, The
tricuspid valves. 7, The pulmonary artery. 8, 8, The branches of the
pulmonary artery that pass to the right and left lung. 9, The semilunar
valves of the pulmonary artery. 10, The septum between the two ventricles
of the heart. 11, 11, The pulmonary veins. 12, The left auricle. 13, The
opening between the left auricle and ventricle. 14, The left ventricle.
15, The mitral valves. 16, 16, The aorta. 17, The semilunar valves of the
aorta.]
-=-=-=-=-=-=-=-=-=-=-=-=
355. Describe the course of the blood from the right auricle in the
heart to the lungs.
-=-=-=-=-=-=-=-=-=-=-=-=
_Observation._ If the blood is not changed in the lungs, it will not
flow to the pulmonary veins. This phenomenon is seen in instances of
death from drowning, strangling, carbonic acid, &c. The same is true,
but in a less degree, of individuals whose apparel is tight, as well
as of those who breathe impure air, or have diseased lungs.
356. The left auricle, (12,) by its contraction, forces the blood into
the left ventricle, (14.) The mitral valves (15) prevent its
reflowing. From the left ventricle the blood is forced into the aorta,
(16,) through which, and its subdivisions, it is distributed to every
part of the system. The semilunar valves (17) prevent its returning.
_Observation._ The parts of the circulatory organs most liable to
disease are the valves of the heart, particularly the mitral. When
these membranous folds become ossified or ruptured, the blood
regurgitates, and causes great distress in breathing. The operations
of the system are thus disturbed as the movements of the steam engine
would be if its valves were injured, or did not play freely.
357. The difference between the functions of the pulmonary artery and
aorta is, the former communicates with the right ventricle of the
heart, and distributes only impure blood to the lungs; the other
connects with the left ventricle of the heart, and distributes pure
blood to the whole body, the lungs not excepted. At the extremity of
the divisions of the aorta, as well as the pulmonary artery, are found
capillary vessels. This curious net-work of vessels connects with the
minute veins of the body, which return the blood to the heart.
_Observation._ The function of the veins of the systemic circulation
is similar to the office of the arteries in the lungs, and that the
veins of the pulmonic circulation transmit to the heart the pure, or
nutrient blood, and thus supply the arteries of the general system
with assimilating fluid.
-=-=-=-=-=-=-=-=-=-=-=-=
What is the effect when the blood is not changed in the lungs? 356.
Describe the circulation of the blood from the left auricle to the
general system. What part of the circulatory organs is most liable to
disease? What is the effect when the valves are diseased? 357. Give
the difference in the functions of the pulmonary artery and aorta.
Show the relation between the functions of the arteries and veins both
of the pulmonic and systemic circulation.
-=-=-=-=-=-=-=-=-=-=-=-=
358. The veins that receive the blood from all parts of the body,
follow nearly the same course as the arteries. The myriads of these
small vessels beneath the skin, and others that accompany the
arteries, at last unite and form two large trunks, called _ve'na ca'va
as-cend'ens_, and _de-scend'ens_.
_Observation._ A peculiarity is presented in the veins which come from
the stomach, spleen, pancreas, and intestines. After forming a large
trunk, they enter the liver, and ramify like the arteries, and in this
organ they again unite into a trunk, and enter the ascending vein, or
cava, near the heart. This is called the portal circulation.
359. The ventricles of the heart contract, or the "pulse" beats, about
seventy-five times every minute; in adults; in infants, more than a
hundred times every minute; in old persons, less than seventy-five
times every minute. The energy of the contraction of this organ varies
in different individuals of the same age. It is likewise modified by
the health and tone of the system. It is difficult to estimate the
muscular power of the heart; but, comparing it with other muscles, and
judging from the force with which blood is ejected from a severed
artery, it must be very great.
_Observation._ The phenomenon known under the name of pulse, is the
motion caused by the pressure of the blood against the coats of the
arteries at each contraction of the ventricles.
360. The following experiment will demonstrate that the blood flows
from the heart. Apply the fingers upon the artery at the wrist, at
two different points, about two inches apart; if the pressure be
moderately made, the "pulse" will be felt at both points. Let the
point nearest the heart be pressed firmly, and there will be no
pulsation at the lower point; but make strong pressure upon the lower
point only, and the pulsation will continue at the upper point,
proving that the blood flows from the heart, in the arteries, to
different parts of the system.
-=-=-=-=-=-=-=-=-=-=-=-=
358. What is the course of the veins? What peculiarity is observable
in the veins of the liver? 359. How often does the heart contract, or
the pulse beat, in adults? In infants? In old persons? What is said of
the energy of its contraction in different persons? How is the pulse
produced? 360. Demonstrate by experiment that the blood flows from the
heart.
-=-=-=-=-=-=-=-=-=-=-=-=
361. There are several influences, either separately or combined that
propel the blood from the heart through the arteries, among which may
be named,--1st. The contraction of the muscular walls of the heart.
2d. The contractile and elastic middle coat of the arteries aids the
heart in impelling the blood to the minute vessels of the system. 3d.
The peculiar action of the minute capillary vessels is considered, by
some physiologists, as a motive power in the arterial circulation.
4th. The pressure of the muscles upon the arteries, when in a state of
contraction, is a powerful agent, particularly when they are in active
exercise.
362. The following experiments will demonstrate that the blood from
every part of the system flows to the heart by the agency of the
veins. 1st. Press firmly on one of the veins upon the back of the
hand, carrying the pressure toward the fingers; for a moment, the vein
will disappear. On removing the pressure of the finger, it will
reappear, from the blood rushing in from below.
2d. If a tape be tied around the arm above the elbow, the veins below
will become larger and more prominent, and also a greater number will
be brought in view, while the veins above the tape are less distended.
At this time, apply the finger at the wrist, and the pulsation of the
arteries still continues, showing that the blood is constantly flowing
from the heart through the arteries, into the veins; and the
increased size of the veins shows that the pressure of the tape
prevents its flowing back to the heart.
-=-=-=-=-=-=-=-=-=-=-=-=
361. State the influences that propel the blood from the heart. 362.
Demonstrate by the first experiment that the blood flows to the heart.
By the second experiment.
-=-=-=-=-=-=-=-=-=-=-=-=
363. The influences that return the blood to the heart through the
veins, are not so easily understood as those that act on the blood in
the arteries. Some physiologists have imputed an active propulsive
power to the capillary vessels in carrying the blood through the
veins. This is not easily explained, and perhaps it is as difficult to
understand. An influence upon which others have dwelt, is the suction
power of the heart in active dilatation, acting as a _vis a fronte_
(power in front) in drawing blood to it.
364. Another influence that aids the venous circulation is attributed
to the propulsive power of the heart. It is not easy to comprehend how
this power of the heart can be extended through the capillary vessels
to the blood in the veins. Again, an important agency has been found,
by some physiologists, in the inspiratory movements, which are
supposed to draw the blood of the veins into the chest, in order to
supply the vacuum which is created there by the elevation of the ribs
and the descent of the diaphragm.
365. One of the most powerful causes which influence the venous
circulation, is the frequently-recurring action of the muscles upon
the venous trunks. When the muscles are contracted, they compress that
portion of the veins which lie beneath the swell, and thus force the
blood from one valve to the other, toward the heart. When they are
relaxed, the veins refill, and are compressed by the recurring action
of the muscles.
_Observation._ The physician, in opening a vein, relies on the
energetic contractions and sudden relaxations of the muscles, when he
directs the patient to clasp the head of a cane, or the arm of a
chair; these alternate motions of the muscles cause an increased flow
of blood to the veins of the ligated arm.
-=-=-=-=-=-=-=-=-=-=-=-=
363. What is said of the influences that return the blood to the
heart? What is said of the propulsive power of the capillaries? Of the
suction power of the heart? 364. Give another influence. State another
agency. 365. What is one of the most powerful causes which influence
venous circulation? Give practical observation.
-=-=-=-=-=-=-=-=-=-=-=-=
[Illustration: Fig. 75. An ideal view of the circulation in the lungs and
system. From the right ventricle of the heart, (2,) the dark, impure
blood is forced into the pulmonary artery, (3,) and its branches (4, 5)
carry the blood to the left and right lung. In the capillary vessels (6,
6) of the lungs, the blood becomes pure, or of a red color, and is
returned to the left auricle of the heart, (9,) by the veins, (7, 8.)
From the left auricle the pure blood passes into the left ventricle,
(10.) By a forcible contraction of the left ventricle of the heart, the
blood is thrown into the aorta, (11.) Its branches (12, 13, 13) carry the
pure blood to every organ or part of the body. The divisions and
subdivisions of the aorta terminate in capillary vessels, represented by
14, 14. In these hair-like vessels the blood becomes dark colored, and is
returned to the right auricle of the heart (1) by the vena cava
descendens, (15,) and vena cava ascendens, (16.) The tricuspid valves
(17) prevent the reflow of the blood from the right ventricle to the
right auricle. The semilunar valves (18) prevent the blood passing from
the pulmonary artery to the right ventricle. The mitral valves (19)
prevent the reflow of blood from the left ventricle to the left auricle.
The semilunar valves (20) prevent the reflow of blood from the aorta to
the left ventricle.]
366. The muscles exercise an agency in maintaining the venous
circulation at a point above what the heart could perform. As the
pulsations are diminished by rest, so they are accelerated by
exercise, and very much quickened by violent effort. There can be
little doubt that the increased rapidity of the return of blood
through the veins, is, of itself, a sufficient cause for the
accelerated movements of the heart during active exercise.
_Observation._ The quantity of blood in different individuals varies.
From twenty-five to thirty-five pounds may be considered an average
estimate in a healthy adult of medium size. The time in which the
blood courses through the body and returns to the heart, is different
in different individuals. Many writers on physiology unconditionally
limit the period to three minutes. It is undeniable that the size and
health of a person, the condition of the heart, lungs, and brain, the
quantity of the circulating fluid, the amount and character of the
inspired air, and the amount of muscular action, exert a modifying
influence. The time probably varies from three to eight minutes.
-=-=-=-=-=-=-=-=-=-=-=-=
366. What causes the accelerated movements of the heart during active
exercise?
-=-=-=-=-=-=-=-=-=-=-=-=
_Note._ Let the pupil review the anatomy and physiology of the
circulatory organs from fig. 75, or from anatomical outline plates,
No. 6 and 7.