Understanding Blood Vessels, Heart Function, and ECG
Blood Vessels: Structure and Function
There are three main classes of blood vessels: arteries, veins, and capillaries. Their primary function is to transport blood throughout the body. They are distinguished by the amount of muscle and elastic tissue in their walls.
Arteries
Arteries have stronger and thicker walls and are formed of three layers:
- Endothelium: Formed by epithelial tissue, it contains smooth muscle and elastic tissue.
- Outer layer: Composed of elastic tissue.
- Lumen: Fairly small but increases due to its elasticity.
Veins
Veins also have three layers, but they are less elastic and flexible. They have a well-developed middle layer. Veins have a larger lumen than arteries, and their walls are thinner. They have valves at certain sections that prevent the backflow of blood.
Capillaries
Capillaries are very fine vessels of smaller caliber and shorter length. Their lumen is very small. They have no muscle or elastic tissue and are composed of endothelium with only one cell layer.
Cardiac Cycle: Systole and Diastole
The heart works with cycles of contraction and relaxation. The contraction movement is called systole, and it boosts blood to the body. The relaxation movement is called diastole, and it facilitates the inflow of blood to the heart. The cycle lasts approximately 0.8 seconds between cardiac contraction and the following one.
Heartbeat: A Two-Phase Pumping Action
The heartbeat is a two-phase pumping action that takes about a second. As blood collects in the upper chambers, the heart’s natural pacemaker sends an electrical signal that stimulates the contraction of the atria. This contraction pushes blood through the tricuspid and bicuspid valves into the ventricles, which are at rest. This phase is the longer one and is called diastole. The second phase begins when the ventricles are full of blood. The electrical signals generated by the pacemaker travel along a pathway of cells to the ventricles, causing them to contract; this is called systole. The heart does not act independently but regulates the cardiovascular system to meet the needs of the organism under various conditions. The heart is a muscle designed to remain strong and reliable for 100 years.
Electrocardiogram (ECG)
An electrocardiogram (ECG) measures the heart’s electrical activity. Electrodes are placed on the chest, arms, and legs, allowing for a graphical representation of the heart’s electrical activity.
- P wave: Indicates that the atria (the two upper chambers of the heart) are contracting to pump blood out.
- QRS complex: Indicates that the ventricles (the two lower chambers of the heart) are contracting to pump blood out.
- ST segment: Indicates the amount of time that elapses from the end of contraction.
- T wave: Indicates the resting period of the ventricles.
Cardiac Automaticity
Specialized centers on the heart wall maintain the heartbeat automatically. Cardiac contraction originates in the sinoatrial node, located in the right atrial wall near the mouths of the vena cava. It is composed of special heart muscle cells that can generate their own momentum and contract. It is also called the pacemaker because it initiates the stimulation of the heart’s automaticity. The atria contract almost simultaneously due to the speed of the drive, which is very fast, lasting 100 milliseconds. The impulses are carried quickly by special muscle fibers called the bundle of His.