What are the veins?
Once the nutrients and oxygen are delivered to an organ, the blood needs to return to the heart to start the cycle over again. The capillary branches rejoin, forming larger, flexible vessels capable of holding blood. These collecting blood vessels are called veins. The small veins link up to become larger veins.The larger veins eventually lead back to the atria of the heart via the inferior vena cava (the main vein carrying blood to the heart from the lower body) or the superior vena cava (the main vein carrying blood from the upper body, especially the head). Unlike the artery,the vein has very little muscle layer. Also,1-way valves help the blood to flow in 1 direction toward the heart through the veins. Incompetence of these valves can lead to varicose veins and accumulation of fluid in the legs upon sitting or standing for long periods of time.
What are the capillaries?
In the organs, the vessels become small and their walls very thin. These tiny vessels are called capillaries. Across the capillaries, oxygen is exchanged between the blood and the organs. Also, these capillaries facilitate nutrient delivery and waste pickup. The capillaries are so thin and small that blood elements are forced to line up single file to pass though this vessel. At this scale, the individual blood cells pass into the capillaries like cars passing through a tollgate. Having regulators at the openings of the capillaries means that blood can be diverted and delivered to the organs in quantities proportionate to the needs of these organs. For instance, when someone is jogging, more blood is delivered to the muscles of the legs to meet the higher oxygen demands.
Describe the electrical system of the heart
The electrical system of the heart (see Figure 4) is made of a natural pacemaker called the sino-atrial node (SA node), which generates electrical impulses at the normal rate of 60-100 beats per minute. The SA node is located in the right atrium. Once the SA node generates electricity, it is conducted across a wiring system from the atria down to the ventricles.
At the junction of the atrium and ventricle, the atriventricular node (AV node) is present and acts as a filter, allowing only a certain number of beats to reach the lower part of the heart. It protects the heart from excessive beats generated by the atria in certain disease states. The AV node also acts as a backup pacemaker in case the SA node fails.The AV node, however, generates lesser number of electrical impulses than the SA node at 5060 beats per minute.
Once electricity crosses beyond the AV node, it is conducted to the bottom part of the ventricles via 2 major wires called the right (to the right ventricle) and left (to the left ventricle) bundles. Problems in these bundles lead to heart blocks that quite often require insertion of a permanent pacemaker. Heart blocks prevent electricity from reaching the ventricles adequately, slowing the heartbeats considerably and leading to fainting or near-fainting spells.
What is an ECG?
An electrocardiogram or ECG is a recording of the electrical impulse of the heart from the surface of the body.The ECG can tell a doctor whether the electricity of the heart is conducting normally or if there is an abnormality that warrants immediate attention.The ECG can also tell (but not all the time) whether a heart attack is acutely happening in a patient with chest symptoms.The ECG also can give clues to other structural problems in the heart and is an integral part of the evaluation of a potential heart patient.
What is the pericardium?
The pericardium is a sac in which the heart sits. It is not essential to the life of a person. However, it has several useful functions, including limiting short-term distention of the heart, and helps maintain the shape of the heart. It also facilitates the interaction between the chambers of the heart and serves as a mechanical barrier for infection. Furthermore, the pericardium helps lubricate the heart and minimizes friction. Other functions of the pericardium are beyond the scope of this book.
The pericardium can be affected by numerous disease states. The cavity between the heart and the pericardium is called the pericardial cavity. This cavity can become filled with fluids after an infection, bleeding, trauma, tumors, or other causes. The pressure generated from this fluid can compress the heart. Also, thickening of the pericardium can limit the heart expansion and causes constrictive disease. This can cause shortness of breath and worsening fluid in the lower legs.
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