IBDP Biology - Cardiac Cycle

In this IBDP Biology topic, you can learn about heart structure.

• The heart is a muscular organ found in all vertebrates that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions.
• It is divided into 4 chambers; the right atrium, the left atrium, the right ventricle and the left ventricle. The ventricles and atria are separated by a wall of muscle called the septum
• One-way valves between the chambers keep blood flowing through your heart in the right direction. As blood flows through a valve from one chamber into another the valve closes, preventing blood flowing backwards. These are attached to tendinous cords which attach to the ventricles which prevent the valves from turning inside out thus allow backwards blood flow

• Your heart is a single organ, but it acts as a double pump. The first pump carries oxygen-poor blood to your lungs, where it unloads carbon dioxide and picks up oxygen. It then delivers oxygen-rich blood back to your heart. The second pump delivers oxygen-rich blood to every part of your body. 
• The coronary arteries lie upon the surface of the heart. They feed the organ with much needed oxygenated blood. If these become blocked they may cause angina or a heart attack
• Your right ventricle pumps blood to your lungs and your left ventricle pumps blood all around your body. The muscular walls of the left ventricle are thicker than those of the right ventricle, making it a much more powerful pump. For this reason, it is easiest to feel your heart beating on the left side of your chest.

• The heart is made of cardiac muscle, composed of cells called myocytes. When myocytes receive an electrical impulse they contract together, causing a heartbeat. Since myocytes are constantly active, they have a great requirement for oxygen, so are fed by numerous capillaries from two coronary arteries.
• Cardiac muscle is myogenic, which means that it can contract on its own, without needing nerve impulses. Contractions are initiated within the heart by the sino-atrial node (SAN, or pacemaker) in the right atrium. This extraordinary tissue acts as a clock, and contracts spontaneously and rhythmically about once a second, even when surgically removed from the heart.

• Both ventricles relax simultaneously. This results in lower pressure in each ventricle compared to each atrium above. The atrioventricular valves open partially. This is followed by the atria contracting which forces blood through the atrioventricular valves. 
• It also closes the valves in the vena cava and pulmonary vein. This prevents backflow of blood.

• Both atria then relax. Both ventricles contract simultaneously. This results in higher pressure in the ventricles compared to the atria above. The difference in pressure closes each atrioventricular valve creating the hearts lub sound. This prevents backflow of blood into each atrium. 
• Higher pressure in the ventricles compared to the arota and pulmonary artery opens the semi lunar valve and blood is ejected into these arteries. So blood flows though the systemic circulatory system via the aorta and vena cava and through the lungs via the pulmonary vessels 

• Immediately following ventricular systole, both ventricles and atria relax for a short time. Higher pressure in the aorta and pulmonary artery than in the ventricles closes the semi lunar valves creating the hearts lub sound. This prevents backflow of blood. Higher pressure I the vena cava and pulmonary vein than in the atria in the refilling of the atria

• At the top of the right atrium neat the point the vena cava empties blood into the atrium is the sinoatrial node (SAN)
• This small patch of tissue creates electrical activity.
• The SAN initiates a wave of excitation at regular intervals that occur approximate between 55-80 bpm.
• The SAN is known as the pacemaker

• The wave of excitation spreads quickly over the walls of both atria and along the membranes of the muscle tissue.
• It causes the cardiac muscle cells to contract and is atrial systole
• At the base of the atria there is a section of tissue that can not conduct the wave so the wave cannot spread to the ventricles.

• The atrioventricular node is the only route through the section of non conducting tissue. As a result a delay occurs. This allows time fro the blood to move from the atria to the ventricles before they contract
• After the delay  the wave of excitation is conducted down specialized tissues purkyne tissue. This runs down the inter ventricular septum
• At the base of the septum the wave spreads up the ventricular walls and cases them to contract. Thus forcing blood upwards and out of the ventricles

The rate at which the heart beats and the volume of blood pumped at each beat (the stroke volume) can both be controlled. The product of these two is called the cardiac output – the amount of blood flowing in a given time:

The cardiac output can increase dramatically when the body exercises. There are several benefits from this:

1. To get oxygen to the muscles faster
2. To get glucose to the muscles faster
3. To get carbon dioxide away from the muscles faster
4. To get lactate away from the muscles faster
5. To get heat away from the muscles faster

This is the end of the topic

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Drafted by Eva (Biology)

Photo references:

1. https://ib.bioninja.com.au/standard-level/topic-6-human-physiology/62-the-blood-system/heart-structure.html
2. http://www.nataliescasebook.com/tag/cardiac-action-potentials
3. https://biologydictionary.net/cardiac-cycle/
4. https://www.researchgate.net/figure/Diagram-of-electrical-conduction-in-the-heart-A-Normal-rhythm-Showing-the_fig2_327558169
5. https://en.wikipedia.org/wiki/Cardiac_output