The Structure and Function of the Heart U2

Deoxygenated blood in the Vena Cava carries blood into the right atrium which flows into the right ventricle via the AV valve. Blood leaves the right ventricle via the Pulmonary Artery, through the SL valve, which carries deoxygenated blood to the lungs. Here blood becomes oxygenated. The Pulmonary Vein carries oxygenated blood into the left atrium which flows into the left ventricle via the AV valve. Blood leaves the ventricle via the Aorta, through the SL valve, which carries oxygenated blood to the rest of the body.

The volume of blood pumped through each ventricle per minute, and is determined by two factors. Heart rate (BPM) Stroke volume (volume of blood ejected by each ventricle during each contraction)

CO = HR x SV

The opening and closing of the AV and SL valves are responsible for the heart sounds heard with a stethoscope.

Pattern of contraction and relaxation of the heart in one complete heart beat. Relaxation of the heart muscles is called diastole. Contraction of the heart muscles is called systole. On average the length of one cardiac cycle is 0.8 seconds based on a heart rate of 75 beats per minute.

Atrial Diastole - Both atrial of the heart relax allowing blood to enter the atria. - Volume of blood increases within the atrium which opens the AV valve. - Blood flows down to the ventricles Ventricular Diastole - Both ventricles of the heart relax allowing blood to flow from the atrium to the ventricle.

Atrial Systole - Both atria of the heart contract. - AV valves open allowing remaining blood to flow into the ventricles which are relaxed (Ventricular Diastole). - SL valves are closed. Ventricular Systole - Both ventricles of the heart contract. - AV valves close preventing the blood flowing back into the atrium. - SL valves open allowing blood to flow out of the heart via the pulmonary artery to the aorta. Each systole phase is followed by diastole.

The heartbeat originates within the heart. The sino-atrial node or pacemaker is found within the wall of the right atrium and it sets the rate at which the heart contracts, controlling the cardiac cycle. SAN/Pacemaker is made of auto-rhymic cells. The timing of cardiac muscle contraction is controlled by impulses from the SAN spreading through the atria causing atrial systole. They then travel to the atrio-ventricular node (AVN), located in the centre of the heart. Impulses from the AVN travel down fibres in the central wall of the heart and then up through the walls of the ventricles, causing ventricular systole.

Impulses in the heart generate electrical currents that can be detected by an electrocardiogram.

The wave of electrical impulses spreading over the atria from the SAN (Atrial Systole).

The electrical impulses through the ventricles (Ventricular Systole).

Electrical recovery of the ventricles (Diastole).

The medulla regulates the rate of the SAN through antagonistic action of the autonomic nervous system (ANS). The ANS consists of 2 antagonistic branches: - Sympathetic pathway - Parasympathetic pathway

In increase in the number of nerve impulses at the SAN via the sympathetic nerve increases heart rate through the release of nor-adrenaline.

An increase in the number of nerve impulses at the SAN via the parasympathetic nerve decreases heart rate through the release of acetylcholine.

Blood pressure is the force excreted by blood against the walls of the blood vessels. It is measured in millimetres of mercury (mmHg). A typical blood pressure reading is 120/80 mm/Hg. The top blood pressure reading is the systolic pressure. The bottom blood pressure reading is the diastolic pressure. Blood pressure increases during ventricular systole and decreases during diastole.

Blood pressure is measured using a sphygmomanometer. - An inflatable cuff stops blood flow in the artery, and deflates gradually. - The blood starts to flow (detected by a pulse) at systolic pressure. - The blood flows freely through the artery (and a pulse is not detected) at diastolic pressure.

Major risk factor for many diseases such as coronary heart disease. Commonly found in people that are: - Overweight - Not taking enough exercise - Eating a fatty diet - Consuming too much salt - Drinking alcohol to excess - Stresses