Part I: Anatomy and Physiology Update Con't.
Conduction System of the Heart
- SA (Sino-atrial node)
- Atrial preferential pathways; anterior intermodal, middle, posterior intermodal.
- AV (Atrio-ventricular node)
- Bundle of HIS
- Left Bundle Branch
- Right Bundle Branch
- Purkinje Fibres
Contractility of Heart Muscle
Electrical conduction in the heart is unique and remarkable.
Heart muscle possesses the following properties:
- Automaticity – pacemaker ability
- Conductivity – each cell has ability to conduct impulses to the next cell.
- Contractility – ability to contract (make each cell shorter or longer)
- Irritability – each call has ability to contract on its own, to send out impulses to other cells without first being stimulated from another source.
These properties make the myocardium different from other muscle cells in the body. The normal activity of the heart conducts impulses from one point (SA node) to another point (individual muscle cells) thus stimulating a uniform and effective contraction f the heart.
Various factors affect the activity of the cardiac muscles. The availability of oxygen, after load, nervous control, muscle condition, and other factors can affect the force of the contraction of the heart. Drugs can also affect the contraction of the heat. Certain drugs depress the heart actively and others can cause excitation. The nurse should be aware of all the factors that influence heart activity.
Blood Flow Through the Heart (refer back to fig. 1)
Blood flow through the heart is shown in the illustration. Blood is shown as it enters the heart, circulates, and then leaves the heart. In relation to the psychical assessment performed by most nurses, keep in mind the changes in circulation which will be assessed. Impeded flow my cause extra heart sounds and/or physical changes. Also, reduced flow will usually cause changes that can be assessed by the nurse.
Physical Characteristics Important to Blood Flow:
- Diameter of the blood vessels.
- Cross-section areas of the chambers and vessels.
- Length of the vessels.
Qualities of blood:
|
18% |
|
12^ |
|
8% |
|
5% |
|
2% |
|
5% |
|
25% |
|
25% |
Velocities of Blood Flow:
The velocity of blood flow is directly related to the amount of circulating blood volume and the area of the vessels.
Blood returns to the heart from the general circulation. Almost 50% of all blood in the body is in the systemic veins of the body. This system includes small veins and venules and blood in the pulmonary circulation. The small veins usually offer little resistance to blood flow. The large veins do offer much resistance to the flow of blood to the heart. This is an important nursing implication, as the patient who is more active will have better flow of blood back to the heart. With reduced activity, the blood tends to pool I the large vessels and can lead to severe venous stasis. Blood returns to the heart via the superior and inferior vena cavae, and into the right atrium.
From the right atrium blood flows to the right ventricle and is then propelled into pulmonary circulation. After blood is aerated with fresh oxygen, it is returned to the left side of the heart into the left atrium.
From the left atrium the blood is ejected into the left ventricle. The left ventricle then pumps the blood out of the heart into the general circulation. The aorta is the first vessel to carry blood, and, at that same time, coronary arteries are fed oxygenated blood to circulate though the heart.
The above is only a brief outline of the circulation of blood. Be sure you can trace the blood through the heart. Be sure that you can name all the valves and chambers of the heart as blood flows through. You should also be able to list the major arteries of the body. When you perform the assessment, it will be necessary for you to know these vessels and their location.
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