Heart structure

AIM: By the end of this unit of work you should be able to label a heart and describe its function.

If you are confused by any of the terms used look at the glossary at http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookTOC.html

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

 

  1. An epithelium
  2. 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).
  3. The layer that contains the cardiac muscle and elastic tissue.

 

Look at the information at

http://biology.about.com/science/biology/library/organs/heart/blheart.htm or http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookTOC.html and your text book to answer these questions.

  1. What is the function of the coronary artery?
  2. Why do the ventricles have thicker walls than the atria?
  3. Why is the left ventricle wall thicker than the right?
  4. What is the function of the semi-lunar valves.
  5. What is the function of the chords attached to the valve flaps between the atria and the ventricles?

Check your answers by looking at heart answers on the intranet.

You should now be able to label the simplified diagram of the heart shown below.

Check your answers by looking at heart diagram answers on the intranet.

 

Heart Function

'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.

Follow this process by looking at the animations at http://www.innerbody.com/htm/body.html.

Pressure and volume changes during the cardiac cycle.

As the blood flows into the atria at the start of the cardiac cycle their thin walls distend under the pressure. When they contract blood is pushed through into the ventricles, which expand. When the ventricles contract the blood pressure rises, when it is greater then that in the atria the cuspid valves (atrioventricular valves) are forced shut preventing back flow into the atria. The blood is forced out of the ventricles into the arteries. As the ventricles relax the blood pressure in the arteries will become greater than that in the ventricles. The semi-lunar valves are then forced shut preventing the blood from flowing back into the ventricles.

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. The larger sound ('lub') is caused by the closing of the cuspid valve and the quieter sound ('dub') by the semi-lunar valves.

Try the exam question on heart function.

 

 

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).

Now try the summary questions on heart function.