CIRCULATION
I. Circulatory system
A. Open versus closed systems. Arthropods
and most mollusks have open systems and a peristaltic tube heart. An open
system is where the blood comes in direct contact with the cells. "White"
blood cells exist, and also cells looking much like red blood cells exist,
but the function of the latter is unknown.
II. Closed system - anatomy
A. Closed system circulation (where blood is always
contained in vessels):
1. Artery carries blood from heart , vein is carrying
blood to heart.
2. Path: Aorta---> arteries---> arterioles ----->
capillaries---> venules---> veins---> vena cava---> right atrium--->
right ventricle---> pulmonary artery---> lung capillary bed---> pulmonary
vein---> left atrium---> left ventricle---> aorta--->
3. Portal system: where a capillary bed is
encountered, blood is gathered into veins and then again distributed to capillary
bed without going through the heart. The second capillary bed is the portal
system.
4. The largest number of networks are the capillaries.
5. The most amount of blood at any given time is found
in the veins.(show transparency) Mall's work with dogs in 1888.
6. The highest blood pressure is in the arteries (aorta
would have the most). The least blood pressure would be in the veins ( vena
cava would have the least). (Actually, in a standing animal the arteries of
the feet may have the highest pressure and the veins of the head the lowest.)
B. Evolution apparently had available to it early on
tubes of endothelial cells with smooth muscle wrapped around these tubes
, as we find these both in the digestive system and circulatory system. Elastin
is laid down over these tubes to help maintain shape and elasticity.
C. Capillaries - at any given time, a high percentage
of capillaries do not have blood flowing through them.
1. The rate, and even direction, of blood flow through
these are always changing constantly. Yet capillaries are unique in that they
have no muscle cells. Often a single layer of 1-4 µm thick endothelial
cells.
2. The capillary diameter is small, even sometimes
smaller than the 7 µm diameter of the red blood cells.
3. Highly permeable to gases, nutrients and even water.
Only erythrocytes and proteins tend to be held back.
a. Blood pressure is always forcing water out.
b. Osmotic pressure is always pulling water back
in.
c. The lymphatic system picks up any water that
is not drawn back in.
D. Arterioles
1. Often a sphincter muscle at the junction
of arterioles and capillaries.
2. The sphincter muscle is a smooth muscle that controls
flow of blood into the capillaries.
3. If all of these muscles were to relax, shock would
result. (Shock : sudden loss of blood pressure) (Show how capillaries
may have back flow).
E. Venules, veins and arteries.
1. All have muscle cells which control blood flow
and pressure..
2. Veins
a. Longitudinal smooth muscle increases tension
and blood pressure; circumventing muscle controls volume and flow.
b. Have valves to insure unidirectional flow of
blood, since at this point blood pressure is much lower after having come
out of the capillaries. The additional effect of gravity can cause negative
pressure.
c. Oblong nature of veins helps in volume control
d. Striated muscular action is often important in
getting blood back to the heart. (Soldiers standing at attention often faint).
F. Heart - must not only provide force but also
direction.
1. Peristaltic pump
a. Annelid dorsal vessel pumps blood through a closed
system.
b. Tube hearts of insects. Peristalsis always important
although valves are found in some.
2. Hagfish hearts - found frequently and always prior
to pumping into a capillary bed.
3. Valved heart of mammals:
a. Atria receive blood and pump it into the ventricles.
b. Ventricles must pump blood into a capillary bed
and therefore are stronger.
c. Because of valves controlling the direction,
only a single uniform contraction is needed.
d. Mammals essentially have two hearts found in
the same location. The heart pumps blood to the body, the right heart pumps
blood to the lungs.
e. Made of cardiac muscle.
f. See diagram for probable evolutionary development
of the heart.
G. Muscle-vein pumping mechanism. Veins with valves
arrange themselves in between skeletal muscles such that when the muscles
contract blood is squeezed toward the heart. This helps facilitate movement
of blood and increases venous blood pressure. (A similar system works for
the lymphatic system.)
III. Heart rate control.
A. All heart cells inherently can generate their own
rhythmic beat. The fastest of these are located in the sino-atrial node
(SA node) on the right atrium.
1. Once the SA node "fires" (initiates an action potential
leading to a contraction), this will stimulate the adjoining cells to fire
before their own signal tells them to do so. Therefore the SA node is often
referred to as the "pacemaker".
2. A wave of contraction emulates over the atria from
the SA node.
B. This wave like response of one cell stimulating the
next does not flow into the ventricular cells because their is an insulating
layer of lipid material between the atria and ventricles. However, at this
lipid barrier between the atria and ventricles is a small structure known
as the atrio-ventricular node (AV node).
1. This node does receive the wave like stimulus started
at the SA node.
2. Upon receiving this signal, the AV node delays
for just a moment and then sends its own signal out to the ventricles via
a special muscle-nerve type tissues called the BUNDLE OF HIS or bundle
fibers.
3. The bundle of His terminates at the base of the
ventricles and innate action potentials in another group of cells known as
the PURKINJE FIBERS.
4. As the wave of action potentials travel from the
base of the ventricles up, these action potentials stimulate the ventricular
cells to contract.
C. This sequence allows for a coordinated heart beat:
the SA node initiates the whole process---> the atria contract filling
the ventricles ---> the AV node receives the signal and delays a moment
thus allowing the ventricles to fill ---> the signal travels to the base
of the ventricles from the AV node and initiates a wave of contraction of
the ventricles from the base up ----> this forces the blood in the ventricles
out into the arteries (aorta and pulmonary artery) located at the top of
the ventricles.
D. Valves prevent back flow of blood into undesired
areas.
IV. Adjustments to the inherent heart rate and strength.
(Note that increased strength or increased rate does not necessarily translate
to mean more blood to the tissues. Often considered is the concept of
cardiac output = heart rate times the amount of blood pumped per beat
(stroke volume). Thus the cardiac output has units of volume/time).
A. Medulla sends speeding or slowing signals
to the heart via the sympathetic or parasympathetic nervous system respectively.
1. SYMPATHETIC NERVOUS SYSTEM - most of the
nerves coming to the heart from the medulla terminate at the SA node. When
stimulated they release norepinephrine (noradrenalin) from the nerve
endings. Norepinephrine reduces the membrane potential (depolarizes)
of the SA node cells and thus causes them to fire faster than their normal
rate (strength per beat also increased). The norepinephrine depolarizes the
cells by increasing the Na+ and Ca++
permeability via cyclic AMP.
2. Parasympathetic nerves (vagus nerve) follow
the same scenario with the exceptions that acetylcholine is released
which increases the membrane potential (hyperpolarizes because of increased
facilitated diffusion of K+) thus leading to slower
heart rate.
B. The medulla will send out these signals according
to information it receives from many sources via nerves:
1. Carotid bodies and aortic bodies detecting
oxygen levels.
2. Carbon dioxide-pH detectors located in the medulla
itself which is indirectly sensing the CO2-pH levels in the blood around the
brain.
3. Baroreceptors in the carotid arteries, aorta,
and a few other minor places which detect blood pressure.
4. Hypothalmus detecting body temperature.
(a. Nicotinic receptors are one kind of acetylcholine
receptor. These are the receptors that are predominant in skelatal muscle
and lead to depolarization when acetylcholine binds and thus can stimulate
an action potential. The drug nicotine can also bind to these receptors and
elicit a response, hence their name. Some of these receptors exist in the
heart.)
(b. Muscarinic receptors are another kind of acetylcholine receptor,
and are predominant in smooth muscle. When acetylcholine binds to this type
of receptor, there is an ion leakage such that there is usually a hyperpolarization,
thus making it harder to generate an action potential and slowing down the
heart rate. Muscarine is a drug from mushrooms that can mimic acetylcholine
and hence bind to this type of receptor.)
c. The heart contains BOTH nicotinic and muscarinic receptors,
with a predominance of muscarinic receptors. Acetylcholine generally slows
down the heart, but not always given that both nicotinic and muscarinic receptors
exist there, and because the effects of muscarinic receptors are not always
so straight forward. Atropine is a drug that blocks the effect of an muscarinic
receptor, and is often used to reactivate the heart when it stops.
C. Medulla (brain) can also stimulate the medulla
of the adrenal glands via the sympathetic nervous system which will cause
the release of epinephrine (adrenaline) which in turn will reach the heart
via the blood and cause an increased heart rate. In this type of response
the medulla often receives its signal from higher brain centers which are
anticipating some event that may require physical action (fight or flight
response).
1. Carotid bodies and aortic bodies detecting oxygen
levels. Send out action potentials to the medulla of the brain when CO2 is
high and O2 is low.
2. Carbon dioxide-pH detectors located in the medulla itself,
which is indirectly sensing the CO2-pH levels in the blood around the brain.
3. Baroreceptors in the carotid arteries, aorta, and a few
other minor places which detect blood pressure.
4. Hypothalmus detecting body temperature
D. STARLING EFFECT is a direct influence
on the heart. More physical activity causes the muscle-vein pump system to
come into action causing increased venous pressure.
a. Thus more blood enters the right atrium
leading to it being stretched more than usual; in response the atrial muscle
will contract harder.
b. The stretching of the right atrium also
stretches the sinoatrial node making its membranes more leaky to ions making
it easier to fire an action potential. The result is an increased rate.
V. Systemic control of the capillary beds.
A. Since capillaries have no smooth muscle,
they themselves can not contract or dilate. However a small circular muscle
around the arteriole just before entering the capillary bed can contract
and dilate. It is these PRECAPILLARY SPHINCTERS (ARTERIOLE SPHINCTERS)
that control blood flow in the capillaries of the body.
B. Because of the contraction of these arteriole
sphincters, approximately 90% of capillaries in an animal body have no flowing
blood at any given time. If all of these capillaries where open then there
would be a catastrophic loss of body blood pressure.
C. Arteriole sphincters are highly responsive
to epinephrine and norepinephrine released from the adrenals (which would
be released on a systemic basis for the reasons mentioned above). However,
the way in which the arterial sphincters react depends on where they are
located in the body.
1. Skeletal muscle arteriole sphincters
will relax in the presence of epinephrine and norepinephrine, thus increasing
the blood supply to these skeletal muscles.
2. Smooth muscles (particularly of the digestive
system) arteriole sphincters will contract in the presence of these same
hormones thus leading to a decreased blood supply to the digestive system.
3. These are again responses which are part
of the flight or fight response.
VI. Localized control - always mediated via
the arteriole sphincters.
A. Increased carbon dioxide, decreased pH,
and decreased oxygen can be directly detected by arteriole sphincters which
will cause relaxation of these muscles and thus increased blood supply.
B. Histamine relaxes arteriole sphincters
and leads to increased blood supply. Histamine is a localized hormone released
by many tissues in response to injury. (this includes injury to overactive
phagocytitic cells attacking the invasion of foreign molecules).
VII. Water loss from capillaries. Water (and
all components of plasma except cells and proteins) is typically lost from
capillaries given that they have thin walls and are under high pressure.
A. Most of this plasma like fluid is retrieved
farther along in the capillary due to the osmotic pressure left in the blood
because of the proteins left there (albumins).
B. Lymphatic System
1. Structure:
a. Lymphatic capillaries pick up
fluids that have leaked out of blood capillaries due to high pressure. Theses
lymphatic capillaries have blind endings (i.e. they do not complete a circuit).
b. Lymphatic vessels collect fluid from
the lymphatic capillaries.
1a. Have valves.
2b. very similar to veins.
3c. require skeletal muscle action for
movement of the lymphatic fluid.
c. Lymphatic vessels all collect and dump
into the brachiocephalic veins via two ducts.
d. Lymph nodes are specialized areas of
the lymphatic system primarily for agranulocyte formation.
e. Lacteals are lymphatic capillaries
of the villi of the small intestine used to pick up fat droplets.
f. Lymphatic fluid is very much like plasma.
2. Disease