What is atrial depolarization

Blood does not flow back into the vena cava because of inertial effects of the venous return and because the wave of contraction through the atria moves toward the AV valve thereby having a "milking effect. Just following the peak of the a-wave is the x-descent. This is sometimes referred to as the "atrial kick.

After atrial contraction is complete, the atrial pressure begins to fall causing a pressure gradient reversal across the AV valves. This causes the valves to float upward pre-position before closure. When one of the variables change, CO as a whole will change as a result. This can also be used to predict other regulated variables, such as blood pressure and blood volume. The heart rate is determined by spontaneous action potential generation in the sinoatrial SA node and conduction in the atrioventricular AV node.

It refers to the number of heartbeats over the course of a minute. Sympathetic nervous system activation will stimulate the SA and AV nodes to increase the heart rate, which will increase cardiac output. Parasympathetic nervous system activation will conversely act on the SA and AV nodes to decrease the heart rate, which will decrease cardiac output. Stroke volume refers to the amount of blood ejected from the heart during a single beat.

Contractility of the heart refers to the variability in how much blood the heart ejects based on changes in stroke volume rather than than changes in heart rate. Additionally, another indicator known as the ejection fraction EF is used to evaluate stroke volume and contractility. A higher EF suggests more efficient heart activity. Cardiac output is an indicator of mean arterial blood pressure MAP , the average measure of blood pressure within the body.

TPR is a measure of resistance in the blood vessels, which acts as the force by which blood must overcome to flow through the arteries determined by the diameter of the blood vessels.

The exact relationship is such that a twofold increase in blood vessel diameter doubling the diameter would decrease resistance by fold, and the opposite is true as well. CO can also predict blood pressure based on blood volume.

Essentially, this means that higher venous blood return to the heart also called the preload will increase SV, which will in turn increase CO. This is because sarcomeres are stretched further when EDV increases, allowing the heart to eject more blood and keep the same ESV if no other factors change. The main implication of this law is that increases in blood volume or blood return to the heart will increase cardiac output, which will lead to an increase in MAP.

The opposite scenario is true as well. For example, a dehydrated person will have a low blood volume and lower venous return to the heart, which will decrease cardiac output and blood pressure. Those that stand up quickly after lying down may feel light-headed because their venous return to the heart is momentarily impaired by gravity, temporarily decreasing blood pressure and supply to the brain.

The adjustment for blood pressure is a quick process, while blood volume is slowly altered. Blood volume itself is another regulated variable, regulated slowly through complex processes in the renal system that alter blood pressure based on the Starling mechanism. Privacy Policy. Skip to main content. Cardiovascular System: The Heart.

Search for:. Physiology of the Heart. Electrical Events Cardiac contraction is initiated in the excitable cells of the sinoatrial SA node by both spontaneous depolarization and sympathetic activity. Learning Objectives Describe the electrical events of the heart. Key Takeaways Key Points The sinoatrial SA and atrioventricular AV nodes make up the intrinsic conduction system of the heart by setting the rate at which the heart beats. The SA node generates action potentials spontaneously.

The SA node fires at a normal rate of 60— beats per minute bpm , and causes depolarization in atrial muscle tissue and subsequent atrial contraction. The AV node slows the impulses from the SA node, firing at a normal rate of bpm, and causes depolarization of the ventricular muscle tissue and ventricular contraction. Sympathetic nervous stimulation increases the heart rate, while parasympathetic nervous stimulation decreases the heart rate.

Key Terms pacemaker : A structure that sets the rate at which the heart beats. The first positive deflection in the complex is called an R wave. A negative deflection after an R wave is called an S wave. Some QRS complexes do not have all three deflections. But irrespective of the number of waves present, they are all QRS complexes:.

NB: The first wave of the last complex is a negative deflection. Therefore, it qualifies to be called a Q wave. Since all QRS complexes have an R wave, there must be one in this example as well, although it may be so small that it is not visible.

A negative deflection following an R wave is an S wave. Such an ectopic foci nearly always results in impulses being conducted over slower pathways within the heart, thereby increasing the time for depolarization and the duration of the QRS complex. The shape of the QRS complex in the above figure is idealized. In fact, the shape changes depending on which recording electrodes are being used. The shape also changes when there is abnormal conduction of electrical impulses within the ventricles.

The isoelectric period ST segment following the QRS and ending at the beginning of the T wave is the time at which both ventricles are completely depolarized. This segment roughly corresponds to the plateau phase of the ventricular action potentials. The ST segment is very important in the diagnosis of ventricular ischemia or hypoxia because under those conditions, the ST segment can become either depressed or elevated.

The T wave represents ventricular repolarization. Generally, the T wave exhibits a positive deflection. The reason for this is that the last cells to depolarize in the ventricles are the first to repolarize. This occurs because the last cells to depolarize are located in the subepicardial region of the ventricles and these cells have shorter action potentials than found in the subendocardial regions of the ventricular wall.

So, although the depolarization of the subepicardial cells occurs after the subendocardial cells, the subepicardial cells undergo phase 3 repolarization before the subendocardial cells. Therefore, repolarization waves generally are oriented opposite of depolarization waves green versus red arrows in figure , and repolarization waves moving away from a postive recording electrode produce a positive voltage.