By Z. Ortega. Gutenberg College. 2017.
CORONARY CIRCULATION The Work Done by the Heart Determines Its Cardiac Blood Flow Decreases During Systole and Oxygen Use and Blood Flow Requirements Increases During Diastole The coronary circulation provides blood flow to the heart toprol xl 25mg amex. Blood flow through the left ventricle decreases to a minimum During resting conditions cheap toprol xl 100mg mastercard, the heart muscle consumes about when the muscle contracts because the small blood vessels 276 CHAPTER 17 Special Circulations 277 TABLE 17. Blood flow in the left coronary artery during derived from the breakdown of adenosine triphosphate cardiac systole is only 10 to 30% of that during diastole, (ATP) in cardiac cells, is a potent vasodilator, and its release when the heart musculature is relaxed and most of the blood increases whenever cardiac metabolism is increased or flow occurs. The compression effect of systole on blood flow blood flow to the heart is experimentally or pathologically is minimal in the right ventricle, probably as a result of the decreased. Blockade of the vasodilator actions of adenosine lower pressures developed by a smaller muscle mass with theophylline, however, does not prevent coronary va- (Fig. Changes in blood flow during the cardiac cycle sodilation when cardiac work is increased, blood flow is in healthy people have no obvious deleterious effects even suppressed, or the arterial blood is depleted of oxygen. Vasodilatory prostaglandins, H , The heart musculature is perfused from the epicardial CO2, NO, and decreased availability of oxygen, as well as (outside) surface to the endocardial (inside) surface. Mi- myogenic mechanisms, are capable of contributing to coro- crovascular pressures are dissipated by blood flow friction nary vascular regulation. No single mechanism adequately as the vessels pass through the heart tissue. Therefore, the explains the dilation of coronary arterioles and small arter- mechanical compression of systole has more effect on the ies when the metabolic rate of the heart is increased, or blood flow through the endocardial layers where compres- when pathological or experimental means are used to re- sive forces are higher and microvascular pressures are strict blood flow. This problem occurs particularly in heart diseases of dothelial cells—in response to blood flow-mediated dila- all types, and most kinds of tissue impairment affect the tion (see Chapter 16) and in response to ATP, adenosine subendocardial layers. Coronary Vascular Resistance Is Primarily Coronary arteries and arterioles are innervated by the Regulated by Responses to Heart Metabolism sympathetic nervous system and can be constricted by nor- Animal studies indicate that about 75% of total coronary epinephrine, whether released from nerves or carried in the vascular resistance occurs in vessels with inner diameters of arterial blood. This observation is supported by more important in equalizing blood flow through the lay- clinical measurements in humans that show little arterial ers of the heart than in reducing blood flow to the heart pressure dissipation in normal coronary arteries prior to muscle in general. The coronary arteries and larger arteri- their smaller branches entering the heart muscle tissue. The oles predominately have 1 receptors, which induce vascu- majority of the coronary resistance vessels—the small ar- lar constriction when activated by norepinephrine.
When cover the central area of the cornea and are the dye reaches the blood vessels of the eye cheap toprol xl 100mg online, generally more durable purchase 50 mg toprol xl with mastercard. Soft lenses cover special ultraviolet lights enable the phys- the entire cornea and are generally more ician to photograph the vessels for later fragile. Any swelling or leakage of the vessels lenses must be individually prescribed and of the retina is apparent on the photograph. They are helpful for a variety of visual conditions, but they do not cor- TREATMENT AND MANAGEMENT OF rect astigmatism. CONDITIONS OF THE EYE AND Generally, there are no complications BLINDNESS associated with wearing eyeglasses. Con- tact lenses, however, can damage the eye Eyeglasses and Contact Lenses if they are not worn and cared for proper- ly. Not all people can or should wear con- Corrective lenses may be in the form of tact lenses. Because there cause corneal abrasions and associated are so many different types of eye disorders complications. Individuals who do not use that interfere with visual acuity corrective good hygienic practices when inserting lenses must be prescribed individually. They the contact lens may develop an infection are prescribed by an ophthalmologist (a of the eye, in which case contact lenses physician who specializes in the diagno- should not be worn. Refrac- hance the life of the individual, not just to tive surgery may be used to treat myopia enhance the visual system. The types of de- (nearsightedness), hyperopia (farsighted- vices used should be based on individual ness), or astigmatism (irregularity in the needs as well as the willingness and abili- shape of the cornea or lens resulting in dis- ty to use them.
If atrial excitation never reaches the inverted T wave associated with the abnormal QRS com- ventricles cheap 50mg toprol xl with mastercard, as in the example in Figure 13 toprol xl 25 mg low price. This normal wave of atrial excitation does not result gree (complete) atrioventricular block. Ventricular excitation does not occur because, when the impulse arrives, a portion of the The ECG Provides Three Types of Information AV node is still refractory following excitation by the pre- About the Ventricular Myocardium mature complex. A prolonged interval following a The ECG provides information about the pattern of excita- premature ventricular beat is the compensatory pause. In this case, tive ventricular myocardium, and abnormal dipoles result- the P wave is abnormal but the QRS complex is normal. It provides Premature beats are often called extrasystoles, frequently a no direct information about the mechanical effectiveness of misnomer because there is no “extra” beat. However, in the heart; other tests are used to study the efficiency of the some cases, the premature beat is interpolated between two heart as a pump (see Chapter 14). Disease or injury present, but their timing is independent of each other. This can affect the pattern of ventricular depolarization and pro- is complete atrioventricular block in which the AV node duce an abnormality in the QRS complex. In this example, the distance right side of the heart is not conducting (i. The atrial pace- tivity coming from the normally depolarized left side of the maker is probably in the SA node, and the ventricular pace- heart. The resulting QRS complex has an abnormal shape maker is probably in a lower portion of the AV node or because of aberrant electrical conduction and is prolonged bundle of His.