Heart Structure and Cardiac Cycle

Large ctive organisms need a circulatory system because:

• Small surface area to volume ratio
• Diffusion too slow and diffusion distance too great
• To supply enough oxygen / glucose
• To prevent CO2 building up
• Active so high oxygen / energy demands

Open -The blood is not maintained inside vessels, it bathes the tissues.

Insects have an “open” circulatory system.  The hemocoel (body cavity) is filled with hemolymph (blood) that bathes the organs.  A tubular heart features ostia through which hemolymph enters.  The tubular heart contracts which maintains flow.

Closed Circulation- The blood is maintained inside vessels.

Single Circulatory system- Blood passes through the heart once, for each circuit of the body.

Double circulatory system- Blood passes through the heart twice, for each circuit of the body.

The cardiac cycle is a sequence of events, which make up one heartbeat. At rest each cycle lasts 0.8s and happens approximately 75 times every minute to make sure the respiring tissues get sufficient blood flow. There are two main sections to the cardiac cycle:

Systole: Contracton can be atrial or ventricular

Dystole: Relaxation

Pressure changes in the heart- Fluids move from an area of high pressure to an area of low pressure.

When a chamber relaxes (Diastole) it:

• Increases in volume
• decreases in pressure
• causing blood to flow in (from an area of higher pressure)

When a chamber contracts (Systole) it:

• decreases in volume
• increases in pressure
• forces blood out (to an area of lower pressure)

The sino-atrial node is responsible for initiating the electrical impulse, and is located in the posterior wall of the right atrium. It is often referred to as the pacemaker. Both atria contract as the electrical impulse passes through the atria walls. This causes atrial systole.

The electrical impulse is received by the atrio-ventricular node at the base of the right atria. The AV node delays the passing of the electrical impulse to allow the atria to finish contracting before the ventricles can begin. The AV node sends the impulse down into the bundle of His located in the septum of the heart. The bundle of His splits into two branches and spread the electrical impulse through the ventricular walls via the purkyne fibres. This causes ventricular systole.

The electrical impulse fades and the normal charge of the heart is restored allowing the ventricles to relax. This causes diastole.

Controlling the cardiac cycle- The heart is mixture of nerve, muscle and connective tissue. The muscle tissue is different from that of our skeleton, it is Myogenic . These muscle cells contract and relax  naturally, without any stimulation by the nerve tissue .

This natural contraction does need to be controlled to produce the co-ordinated   stages of the cardiac cycle, this is achieved using specialised nerve tissue in the muscular  wall   of the heart.

The Electrocardiogram (ECG)- An ECG is used to monitor heart function. The trace recorded represents the electrical activity in the heart during each cardiac cycle. A cardiology technician will ask the patient to remove clothes from the waist up. Electrodes will be placed on the arms, legs and chest. ECG cream is used between the electrodes and skin. The patient lies down and stays relaxed for 5 minutes. The trace produced can be analysed to monitor heart function.