You should be familiar with both the internal and internal
structure of the heart.
External features of the heart.
The whole of the heart is surrounded by a
membrane called the pericardium. The heart wall itself has
three layers
An epithelium
A layer of connective tissue that contains the blood vessels
that supply the heart wall with digested food and oxygen
(this includes the coronary artery).
The layer that contains the cardiac muscle and elastic
tissue.
'To maintain the circulation
of blood within the blood vessels.'
Some definitions.
Septum;
the wall of tissue that divides heart into its two halves.
The mammalian heart is a double pump. This is a unique feature
of mammalian hearts.
Valves; prevent the blood
flowing backwards through the heart. Tricuspid valve separates
the right atrium and the right ventricle. The bicuspid valve
separates the left atrium and the left ventricle. Semi-lunar
valves are simple flap like valves in the arteries that leave
the heart.
Systole; contraction of
the heart (or one of the chambers).
Diastole; relaxation of
the heart (or one of its chambers).
The cardiac cycle
When the atria contract (atrial
systole) the ventricles relax (ventricular diastole), blood
flows from the atria into the ventricles. During ventricular
systole and atrial diastole the cuspid valves close and blood
is forced into the arteries. At the same time blood flows
into the atria from the veins.
Pressure and volume changes during the cardiac cycle.
Blood will flow from high to
low pressure, unless prevented by a valve. The closure of
the valves (cuspid and semi-lunar) is what can be heard using
a stethoscope.
Insert exam question with
links to answers.
How the heart beats.
Cardiac muscle has the unusual
property of being myogenic. This means that the stimulation
for contraction comes from the muscle itself . A heart will
continue to beat even if the nerves leading to it have been
cut. Cardiac muscle is also unusual in that it can keep on
contracting without fatigue and that it has cross connections
between the fibres.
The contraction of the heart
starts at one point then spreads over the whole of the heart.
That point is the sino-atrial node on the right atrium. The
wave of contraction rapidly spreads over both atria. The tissue
between the atria and the ventricles in inexcitable (will
not pass on the contraction) except at the atrio-venticular
node. This passes the wave of excitation on to the Bundle
of His and the Purkyne fibres which spread the excitation
through the ventricle walls. The passage of the excitation
through the atrio-ventricular node introduces a short time
delay that means that the atria contract before the ventricles.
An electrocardiogram measures
the electrical changes in the heart as it contracts.
The P wave is caused by atrial
contraction (systole), QRS complex by ventricular contraction
(systole) and the T wave by ventricular relaxation (diastole).
