The heart is a muscular pump which pushes blood around the body. It is roughly about the size of a fist, and is madeup of cardiac muscle. The heart is divided ito four chambers:
Right Atrium
Left Atrium
Right Ventricle
Left Ventricle
The heart is also divided into upper and lower compartments by the interventricular septum. The heart tissue is called endocardium , and the muscle is called myocardium. The heart is covered by two layers of fibrous tissue called pericardium.
The heart contains a series of valves which ensure the correct direction of blood flow. The tricuspid valve - which separates the Right Atrium from the Right Ventricle. The pulmonary valve - which separates the Right Ventricle from the pulmonary artery. The bicuspid (or mitral) valve - which separates the Left Atrium from the Left Ventricle, and the aortic valve - which separates the Right Ventricle from the ascending aorta.
There are four main blood routes into and out of the heart, these are:
Vena Cava (Vein)
Pulmonary artery
Aorta (Artery)
Pulmonary Vein
The heart wall structure consists of three layers:
Epicardium - thin outer layer. This gives the surface of the heart a smooth texture.
Endocardium - smooth inner lining, which is continuous with the large blood vessels to which the heart connects.
Myocardium - makes up the bulk of the heart, and is responsible for the pumping action. It is made up of strong cardiac muscle fibres, connected by electrical synapses.
The Cardiac Cycle
The Cardiac cycle describes the complete round of cardiac systole and diastole with the intervals between or commencing with any events in the heart's action to the moment when the same event is repeated.
Systole - contraction (working)
Diastole - ventricular relaxation (resting)
The blood flows from area of high pressure to an area of low pressure, unless the flow is blocked by a valve. The events on the left and right of the heart are the same, but pressures are lower on the right hand side. Contraction of the myocardium generates pressure changes which result in orderly movement of blood.
The Atrial Systole
- The heart is full of blood and ventricles are relaxed.
- Both atria contract, and blood passes down to the ventricles.
- Atrio-Ventricular valves open due to blood pressure.
- 70% of blood flows passively down to the ventricles, so the atria don't have to contract much.
Ventricular Systole
- Atria relax
- Ventricular walls contract, and blood is forced out.
- Pressure of blood forces the Atrio-Ventricular valves to close
- The pressure of blood opens the semi-lunar valves.
- Blood passes into the aorta and pulmonary arteries.
Diastole
- Ventricles relax.
- The pressure in the ventricles falls below that in the artery.
- Blood under high pressure in arteries causes the semi-lunar valve to close.
- During diastole, all heart muscles relax.
Regulation of the Cardiac Cycle:
- Cardiac muscle is myogenic (self exciting).
- The heart's contractions occur spontaneously though frequency can be affected by exercise/danger.
- These affected contractions are coordinated by the sinoatrial (S.A) and Atrio-Ventriuar (A.V) nodes.
- S.A node is located in the upper wall of the right atrium, and is responsible for wave of electrical stimulation that initiates atrial contraction.
- Once the wave reaches the A.V node in the lower right atrium, it's delayed there before being conducted through various structures leading to a contraction of ventricles.
- A delay at the A.V node allows time for all blood in the atria to fill respective ventricles
The bundle of His are the muscle fibres responsible for conducting the waves of electrical activity to the Purkyne fibres.
Purkyne fibres are finer muscle fibers found in the right and left ventricle walls. They carry waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract simultaneously, from the bottom up.
Click on the following link to see a video of a beating heart. This may help you understand this post more easily:http://www.youtube.com/watch?v=rguztY8aqpk
Bibliography:
http://www.10b3circulation.wordpress.com/
cgpartnership handout, week 22
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