Cardiovascular System - BLOCK QUIZ ANSWERS
Describe
the chambers of the heart and their associated structures. Explain the direction
of blood flow through these chambers and indicate the major differences in
function between each of them.
The
heart is divided into four separate hollow chambers. The superior chambers are
referred to as atria and the inferior chambers are called ventricles. Each
atrium has a flap like appendage called an auricle and this helps in storing
additional blood in the atrium.
The
ventricles are surrounded by a relatively thick myocardium when compared to the
atria. This is because their main function is to act as pumps. The point to note
here is that the left ventricle has a myocardial mass which is 3 times that of
the right ventricle because of the huge amount of resistance offered by the
systemic circulation with respect to the pulmonary circulation.
The
myocardium displays a specific internal structure in both the atria and the
ventricles. The irregular spikes seen in the atria are called pectinate muscles
and in the ventricles these spikes are a little more prominent and are known as
trabeculae carnea.
The
atria and ventricles are separated from each other by thin layer of connective
tissue and the two ventricles are separated by the interventricular septum. The
two atria are separated by the interatrial septum. The functional significance
of these is to separate the actions of the two structures, so that they act in
synchrony with one another.
There
are valves present in these chambers separating the atria and the ventricles.
These valves are known as atrioventricular valves and the left valve is a
bicuspid valve whereas the right valve has three cusps associated with it.
Tendon like structures, chordae tendonae extend from these valves and attach to
the papillary muscles that project out from the ventricles.
Blood
flow through the heart begins when the venous return from the inferior and
superior venae cava enter the right atrium. Passing through the tricuspid valve,
the blood is then pumped through the pulmonary trunk and arteries towards the
lungs where the blood is oxygenated. Then the blood enters the heart once more
through the pulmonary veins, and into the left atrium, through the bicuspid
valve and into the left ventricle. The contraction of this ventricle results in
blood being ejected through the aorta into the systemic circulation.
Major
points: Atria, ventricles, Myocardial mass & associated function
What
causes the first and second heart sounds?
The first and second heart sounds are the normal sounds heard through the stethoscope during auscultation of the heart. The first heart sound is due to the closure of the atrioventricular valves. The synchronous shutting of these valves means that ventricular contraction is in progress. The second heart sound is produced due to the closure of the semi-lunar valves. This means that diastole is in progress after ventricular systole.
Define
the term cardiac output. Describe how an increase in activity of sympathetic
nerves innervating the heart would increase cardiac output.
Cardiac
output is defined as the amount of blood ejected per minute by each ventricle.
This is largely determined by the no. of times the ventricles contract per
minute and the amount of blood which is ejected each time.
The
no. of ventricular contractions per minute is defined as the Heart Rate and the
volume of blood ejected during each contraction is known as the stroke volume.
Thus CO can be represented by the following equation: CO = HR * SV and is
measured in L/min. We note that CO is directly proportional to both HR and SV
and therefore a rise in one would ultimately result in a rise in Cardiac Output.
If
there is an increased in activity of sympathetic nerves innervating the heart,
then the heart rate would increase. The heart rate is normally in control of the
parasympathetic fibres of the autonomic nervous system. This is due to the vagal
influence which has a tonic breaking effect. But cardiac accelerator nerves
arising from the sympathetic division act via noradrenergic B1 receptors in
order to increase the heart rate. A significant rise in sympathetic nerve
activity will result in increased levels of heart and this condition is
clinically known as tachycardia.
The mechanism of increase firing of the myocardial fibres resulting in more contractions is due to the nerves reducing the threshold of the plasma membrane such that threshold is easily reached thus resulting in more excitation.
Major Points: Cardiac Output = HR * SV, Heart rate, Stroke Volume, L/min, Proportionality, Sympathetic innervation and consequences, Parasympathetic innervation and consequences, Vagal tone, Noradrenergic B1 receptors, Tachycardia, SA node threshold and its influence in intrinsic heart rate.
Name
the three layers of the blood vessel wall and list the major components of each.
Describe how the make up of these layers defines functional differences between
the aorta, an arteriole and a main vein.
The
three layers of the blood
vessel are the tunica intima, tunica media and tunica adventitia. The
tunica intima is the innermost layer comprising of the endothelial layer, simple
squamous epithelium and some lamina propria. The main function of this layer is
to provide a smooth low friction surface for the blood to flow through the blood
vessel. The intermediate layer is comprised of a circular layer of smooth muscle
and some elastic tissue. This acts to change the diameter of the blood vessel in
the event of increased blood supply and other situations. The smooth muscle is
richly innervated by sympathetic nerve fibres of the autonomic nervous system.
The outer most layer is the tunica adventitia and this is largely composed of
collagen fibres and irregular dense connective tissue. This supports the hollow
structure of the blood vessel and protects against internal and external
stressed on the vessel.
The
aorta is the largest vessel of the body leading away from the heart supplying
systemic blood to the surrounding tissues. This type of blood vessel has a large
lumen, thick elastic walls and the tunica adventitia will contains its own blood
supply, termed vasa vasorum. The elasticity of the aorta is required because the
diameter of the blood vessel will change during systole and diastole. This is
functionally different to an arteriole which has relatively less elastic fibres
in its tunica media and has a high ratio of muscle. This type of blood vessel is
richly innervated with nerves because it is relatively unelastic and required
muscular contractions to change the luminal size in the event of
vasoconstriction
or vasodilation. A main vein also contains the three layers mentioned above, but
these layers are much thinner and the lumen is large. This means that the vein
is more easily collapsed, also assisted by low luminal pressures. To assist
blood flow, due to low pressure gradient, the veins have valves similar to the
semilunar valves of the heart and these are folds of the tunica intima, which
prevent the backflow of the blood.
Major
Points: Three layers &
functional significance of each layer
Discuss
the intrinsic control of blood flow to the tissues.
Control
of blood flow can either be intrinsic or extrinsic. Intrinsic mechanisms
involves myogenic regulation and metabolic effects. Extrinsic mechanisms
involves hormonal, neural regulation.
Myogenic
regulation of blood flow normally occurs at the arteriole level. When increased
blood flow occurs to a blood vessel, the muscle fibers which form the tunica
media get distended and stretched. As a result the blood vessel lumen becomes
enlarged. As a reflex to this event, the smooth muscle surrounding the blood
vessels contracts, therefore constricting the lumen. This consequently reduces
luminal size and blood flow is decreased.
The
other type of intrinsic mechanism used in order to control the blood flow to
tissues and capillary beds is by the metabolic effects. The rata of metabolism
affects the rate at which blood is flowing to that particular tissue. Normally,
an increasing metabolic rate will mean that blood flow to that tissue involved
will increase.
For
example: increased exercise levels will require increased oxygen to the skeletal
muscles and as a result blood flow to this region will increase while blood flow
to other tissues not involved actively in the exercise routine will be
decreased. Also build up of metabolites and other waste products such as CO2,
K+, H+ will also induce an increase in blood flow. In situations where the blood
flow is increased as a result of increased
demand by tissues is called active hyperemia. In cases where a decrease
in oxygen supply or an increase in metabolite build up, it is called reactive
hyperemia.
Also
Nitric Oxide, produced by some endothelial cells is a potent vasodilator. Thus
in situations
where increased blood flow is required, as is the case during inflammation, NO
is released to increase blood flow to that region.
Major
points: Intrinsic versus Extrinsic mechanisms &
Neural and hormonal
Discuss the forces involved in the exchange of fluid between blood in a non-fenestrated capillary and the extracellular fluid in the surrounding tissues.
The
exchange of fluid between blood and the extracellular fluid is due to two main
forces acting opposite to each other. The hydrostatic pressure exerted by the
blood is the force exerted on the walls of the blood vessel by the blood
travelling in it. This force tends to push fluid out of the capillary and into
the extracellular fluid and surrounding tissues. On the other hand, the presence
of plasma proteins in the extracellular fluid results in an increase in plasma
oncotic pressure, or colloid osmotic pressure. If more plasma proteins are
present then the tendency of fluid to reenter the capillaries from the
extracellular fluid is greater.
Normally,
the capillary hydrostatic forces and the plasma oncotic forces are imbalanced in
such a way that the former is larger than the latter. This results in a net
filtration pressure, which drives fluid into the extracellular fluid. In the
case of fluid reentering the venous circulation, the plasma oncotic pressure
exceeds that of the venule hydrostatic pressure (due to lower blood flow) and
therefore fluid tends to enter the veins. This is a net reabsorption pressure.
But the net reabsorption pressure is less than the net filtration pressure and
some fluid remains in the extracellular area to be taken away by the lymphatic
vessels.
In
the event of odema, the plasma oncotic pressure is lower that the venule
hydrostatic pressure and therefore, fluid tends to remain in the extracellular
fluid. This causes accumulation of fluid as the fluid still leaves the
capillaries due to the different in pressures here. Odema results as a result of
increased capillary hydrostatic pressure, decreased plasma oncotic pressure or
the inability of the lymphatic vessels to take away the excess fluid.
Major
Points: Hydrostatic Pressure