Question

Describe the cardiac cycle, using the terms systole and diastole. What is the role of the SA node and the AV node in the cardiac cycle? (pages \(97-99\) )

Short answer

The cardiac cycle consists of systole and diastole, during which the heart contracts and relaxes, respectively, to pump blood throughout the body. Systole has two events: atrial systole, when the atria contract and push blood into the ventricles, and ventricular systole, when the ventricles contract and push blood out of the heart. Diastole is also divided into ventricular diastole, when the ventricles relax and fill with blood, and atrial diastole, when the atria relax and fill with blood from the veins. The SA node, located in the right atrium, generates electrical impulses to set the heart's rhythm and regulate the atrial contraction. The AV node, located between the atria and ventricles, slows down the electrical impulse from the SA node, allowing for the atria to complete their contraction and the ventricles to fill with blood before the ventricles contract synchronously.

Step-by-step solution

Describe the cardiac cycle

The cardiac cycle is the sequence of events that occur when the heart contracts and relaxes to pump blood throughout the body. It can be divided into two main phases: systole and diastole. Systole is the phase when the heart contracts and pumps blood out of its chambers. Diastole is the phase when the heart relaxes and fills with blood.

Explain systole

Systole consists of two distinct events: atrial systole and ventricular systole. Atrial systole starts when the atria contract, pushing the blood into the ventricles. This corresponds to the "lub" sound in the familiar "lub-dub" heartbeat. Ventricular systole follows, during which the ventricles contract and propel the blood out of the heart and into the arteries. This accounts for the "dub" sound of the heartbeat.

Explain diastole

Diastole also consists of two complementary events: ventricular diastole and atrial diastole. During ventricular diastole, the ventricles relax, and the pressure inside them decreases, allowing them to fill with blood. Atrial diastole occurs simultaneously with ventricular diastole, the atria relax and start filling with blood from the veins. Diastole ends when the atria contract, marking the beginning of the next cardiac cycle.

Describe the role of the SA node

The sinoatrial (SA) node, often called the "pacemaker" of the heart, is a group of specialized cells located in the right atrium. It generates an electrical impulse, setting the heart's rhythm and ensuring that the atria contract in a coordinated manner. The electrical impulses from the SA node spread through the atria and stimulate both atria to contract, initiating the atrial systole. It plays a crucial role in regulating the timing of the cardiac cycle and managing the overall heart rate.

Describe the role of the AV node

The atrioventricular (AV) node is another group of specialized cells located between the atria and ventricles. It receives the electrical impulse from the SA node and slows it down slightly, allowing time for the atria to complete their contraction and the ventricles to fill up with blood entirely before the impulse is transmitted to the ventricles. The ventricles then contract synchronously, initiating the ventricular systole. The AV node plays a vital role in coordinating the heart's contractions and ensuring that the ventricles receive the impulse in a staggered fashion, resulting in optimal blood flow. In summary, the cardiac cycle consists of two primary phases, systole and diastole, with the contraction and relaxation of the heart's chambers. The SA node sets the heart rate by generating electrical impulses that regulate the atrial contraction, while the AV node coordinates the contraction of the ventricles by slowing down the impulse from the SA node and ensuring optimal blood flow.

Key concepts

Systole and Diastole

Understanding the cardiac cycle begins with familiarizing oneself with systole and diastole, the two critical phases that keep our heart pumping and blood flowing. Let's break them down:

Systole is when the heart's muscle contracts to propel blood out of the chambers. This powerhouse movement occurs first in the atria, then in the ventricles, and is associated with the first 'lub' sound of the heartbeat. This sound signifies that your heart is actively working to push the blood to all corners of your body.

Diastole, on the other hand, is the quieter phase of the heartbeat. It is when the heart relaxes and the chambers fill with blood, preparing for the next bout of action. It is during diastole that the ventricles fill up with the blood that just entered the atria, and it corresponds to the softer 'dub' sound. This rhythmical alternation between systole and diastole keeps blood circulating efficiently throughout the body.

SA Node

The Sinoatrial (SA) node is akin to the conductor of an orchestra, but for your heart's rhythm. It dictates the beat that the rest of the heart follows. Located in the right atrium, the SA node sparks an electrical impulse that travels through the heart, prompting the muscle to contract.

How does it do it? This 'pacemaker' generates an electrical signal that swiftly passes through the atria, causing them to contract and pump blood into the ventricles. The SA node's natural rhythm sets the cadence for the entire cardiac cycle, thus influencing your heart rate. It's a delicate system that ensures your heart beats in a coordinated and timely fashion.

AV Node

Following closely after the SA node's cue is the Atrioventricular (AV) node. This cluster of cells plays a pivotal role in the timing of heart contractions. As the electrical impulse from the SA node reaches the AV node, this little gatekeeper slows it down for just a moment.

But why the delay? It's essential for the atria to finish squeezing the blood into the ventricles before the ventricles start their contraction. This brief pause granted by the AV node makes sure your heart's lower chambers are fully filled with blood before they contract and send the blood surging into your arteries. Thanks to the AV node, your heart's pumping action is both smooth and staggered for maximum efficiency.

Heart Contractions

Heart contractions are the muscular actions that literally make your heart beat. They can be subdivided into atrial and ventricular contractions, corresponding to the upper and lower heart chambers, respectively.

During the systolic phase, the atria contract first, squeezing blood into the ventricles, followed by the powerful contractions of the ventricles, which pump blood out to the body and lungs. During diastole, both chambers relax, allowing the heart to fill with blood again. This consistent sequence of contractions is crucial for maintaining a steady and uninterrupted blood flow, ensuring that each cell in your body receives the oxygen and nutrients it needs to thrive.

Blood Circulation

The cardiac cycle is just one piece of the larger mosaic of blood circulation in the body.

Blood circulation involves not only the heart but a vast network of vessels that transport blood to every nook and cranny of your body. As the heart contracts and relaxes, blood is pushed out into arteries, meanders through tissues via capillaries, and then returns to the heart through veins. This loop, which includes both the systemic and pulmonary circuits, is critical for delivering oxygen and nutrients while whisking away waste products. Without the orchestrated events of the cardiac cycle and the intricate pathways of blood vessels, the vital functions of blood circulation could simply not take place.