Transcript The Cardiovascular System

The Cardiovascular System
Overview
The system
consists of two
separate loops
The pulmonary loop is fed by
the right heart and is the site of
exchange of respiratory gases
with the atmosphere.
The systemic loop is fed by the
left heart and serves blood to
all the rest of the body,
including the heart tissue itself.
Systemic capillaries are the
sites of exchange of respiratory
gases, nutrients and wastes
with the tissues.
Obviously, the volume flow of
blood through one loop must be
equal to that of the other, as
averaged over times longer
than a few heartbeats.
Systemic circulatory beds are arranged in parallel
Arterial blood flows through
only one set of capillaries
before entering the venous
side of the system - with a few
exceptions: these exceptions
involve what are called portal
circulations. The two main
ones that you will meet with in
the postnatal circulation are
the hepatic portal system that
carries blood from the
intestine to the liver, and the
hypothalamic-hypophyseal
portal system that carries
blood from the hypothalamus
of the brain to the anterior
pituitary. These will be
addressed in detail later.
Some suggestions for how to think about this system
•
Think of the systemic arteries and systemic veins as two tanks. We will speak
of them as ‘the venous side’ and ‘the arterial side’. The arterial side holds blood
under high pressure – the venous side is a low-pressure system.
•
Arterioles are the sites of highest flow resistance in the system, so individually
they determine the rate of blood flow to individual tissues and together they
determine how rapidly blood ‘runs off’ from the arterial side to the venous side.
•
The arterial tank is small. Arteries are narrow and stiff. A single drop of blood
spends only a few tens of seconds on the arterial side. At any instant, the volume of
blood on the arterial side is determined by the interaction of the heart’s pumping
action versus the ease of runoff. The mean arterial pressure is a direct function of
the volume of blood that is on the arterial side.
•
The venous tank is large. Veins are large and compliant. At any instant, most of the
blood volume is on the venous side of the circulation.. It may take an individual drop
of blood quite a while (minutes) to pass through this part of the loop and return to the
heart.
The Heart
Electrical Basis of Heart Activity
Essential Features of Vertebrate
Cardiac Muscle
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Striated
Cells connected by gap junctions
Dually innervated by ANS
Spontaneously active – driven by
specialized pacemaker cells.
• In postnatal mammals: highly dependent
on oxidative metabolism
Differentiation of Functional Cell Types
in the Heart
• Nodal fibers- spontaneously active
pacemakers that can initiate heartbeat
• Conducting fibers - rapidly carry
excitation from one part of the heart to
another
• Myocardial fibers - compose most of the
mass of the heart and provide essentially
all of the force.
Nodal and conducting fibers in the heart
Sequence of electromechanical
events in a heartbeat
• Spontaneous AP in SA nodal pacemakers
• Excitation spreads throughout atria, followed
by atrial contraction
• Excitation reaches AV node, enters bundle of
His, is conducted into both ventricles by
branch bundles, and is rapidly spread
throughout the ventricular myocardium by
Purkinjie fibers - followed by ventricular
contraction or systole.
Agenda of topics to consider
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Nature of pacemaker action potentials
Control of heart rate by the ANS
Nature of myocardial action potentials
Relationship between myocardial action
potentials and the electrocardiogram
• Myocardial excitation-contraction
coupling
• Control of myocardial contractile force
by stretch and by autonomic inputs
Action potentials
recorded from the
ventricular
myocardium, SA
node and atrial
myocardium.
The total time in
each window is
about 300 msec
for the
myocardial cells
and about 150
msec for the SA
nodal cell.
Pacemaker potentials
• Nodal cells (pacemakers) do not have
stable resting potentials.
• Instead, the cells undergo a
spontaneous, slow depolarization
(prepotential) until threshold is reached.
• The upstroke of the AP is slow
compared to nerve and skeletal muscle.
• Each action potential leads to a
temporary afterhyperpolarization that
leads into the next prepotential.
Prepotential
Action potential
The rate of pacemaker potentials is
modulated by the autonomic NS
• In the absence of any autonomic input, the
natural rate of pacemaker potentials is
about 100/min in human heart.
• Cholinergic input slows the heart rate by
slowing depolarization during the
prepotential; adrenergic input increases
the heart rate by increasing the rate of
depolarization.
This slide shows the effects of isoproterenol (a beta agonist;
A), stimulation of the vagus nerve (B), and two concentrations
of Ach. Before and after traces are overlain; c indicates
control beats and * experimental beats.
Key features of the myocardial action potential
• Rapid upstroke
• LONG plateau – Ca++ entry occurs during the plateau
and triggers Ca++ induced Ca++ release
• Potential relatively stable during the interbeat interval,
except in disease.
The electrocardiogram is an extracellular
recording of the myocardial AP
• Voltage is measured at several spots on
the body surface - because body fluid is
a conductor of electricity, these spots
could be thought of as wires connected
directly to the heart surface.
• A voltage difference will exist only when
some parts of the heart are depolarized
while others are not. During the
interbeat interval, OR when the ventricle
is all depolarized, the EKG trace will
return to baseline.
If we measure the voltage difference
between the right arm and left leg
over a heart cycle, we will watch
excitation start in the atria, spread
through the ventricles, and end with
ventricular repolarization
Relationship between atrial and ventricular AP s and EKG
waves
Atrial
Ventricular