Background+Reading7

Unit 6: The Cardio-Vascular and Respiratory Systems //How exercise affects these systems

////Source// The heart is a fist sized organ that lies behind the sternum (breast bone) In adults it is about the size of a clenched fist. The heart is composed of four chambers: 1. The Atria (Atrium) which is the superior part of the heart at the top 2. The Ventricles (ventricle) which is the inferior or lower part of the heart at the bottom

The ventricles are more muscular than the atria as they are responsible for the pumping action The heart is also divided lengthways as well as in half. There are four quarters to the heart.

The left side of the heart is responsible for moving blood carrying oxygen throughout the body (systemic function or circulation) The right side of the heart is responsible for the blood that is not carrying oxygen is returned to the lungs (pulminary function)

Source

There are several **valves** within the heart (bicuspid, tricuspid, pulmonary, aortic) which prevent the backflow of blood. There is a thick layer of muscle called the **myocardium** situated between the **endocardium** and **pericardium** on the outside.
 * Coronary arteries** cover the exterior of the heart. This feeds the heart with blood. Blockages to these arteries cause most of the problems associated with heart attacks.

The heart can be thought of as a two pump system. The cardiac cycle explains what takes place during one complete heart beat.

Diastole phase - the filling of the heart Systole phase - the emptying of the heart

Each cycle takes approximately 0.8 seconds and occurs on average 72 times per minute.

The first stage of the cycle is when the upper chambers of the heart (relaxing) are filled with blood returning from: 1. the body via the venae cavae to the right atrium 2. the lungs via the pulmonary vein to the left atrium **Remember this is a double pump!!**

The blood is pushed from an area of high pressure to an area of lower pressure. The ventricles now fill with blood (as they are relaxing). The atria contract (systole) and empty the last of the blood out.

The second stage is ventricular systole. The last set of valves open upon the contraction of the ventricles (systole) and force blood into either the pulmonary artery (right) or the aorta (left)

Trained athletes have bradycardia which accounts for a decreased resting heart rate.


 * How does the heart work?**

The heart produces its own impulses (myogenic). The S-A node on the heart (sino-atrial node) is located in the right atrial wall. The S-A node emits an impulse that determines heart rate. It spreads through the heart fibres and causes the atria to contract. It then passes to the A-V node (atrioventricular node) which then acts as a distributer and passes the action throughout the ventricles. There is a slight delay to allow for the contraction of the atria.


 * Heart Regulation and Control**

The heart is regulated automatically. It has an ANS (autonomic nervous system). We don't have to think about it.

The brain through the sympathetic nervous system (see previous lesson) releases adrenaline and noradrenaline which speeds up the action of the heart. Adrenaline increases the strength of the ventricular contraction and therefore stroke volume. Noradrenaline aids the spread of the impulse throughout the heart and therefore increases heart rate.

The brain through the parasympathetic nervous system releases acetylcholine which slows the spread of impulses and reduces the heart to resting rate.

This is an antagonistic relationship in this case between the sympatheic and parasympatheic systems.