Arterial System PDF

# Arterial System ## Learning Objectives On completion of study of this chapter, the student will be able to: - Give a note on the sympathetic innervation of the arterial system. - Understand the importance of sympathetic (vasoconstrictor) tone in maintaining arterial volume and pressure. - Nam...

# Arterial System ## Learning Objectives On completion of study of this chapter, the student will be able to: - Give a note on the sympathetic innervation of the arterial system. - Understand the importance of sympathetic (vasoconstrictor) tone in maintaining arterial volume and pressure. - Name the types of arteries and give their functions. - Appreciate the importance of the Windkessel effect of larger arteries. - Trace the arterial pulse pressure waves. - Learn the principles of direct and indirect methods of BP measurement. The student may also be able to: - Describe the principle of arterial hemodynamics in various physiological conditions. ## Functional Organization The arterial system carries blood from the heart to the tissues. It consists of the aorta, large and small arteries, arterioles, and metarterioles. These vessels have all three layers in their walls, namely tunica intima, tunica media and tunica adventitia. The tunica media containing smooth muscle is thicker in the arterial compartment than the other compartments of the circulatory system. However, the amount of smooth muscle and elastic tissues present varies in different parts of the arterial system (for details, refer to Fig. 41.7, Chapter 41). ## Innervation The arterial system is richly supplied by sympathetic fibers. Sympathetic fibers originate from the thoracic and lumbar segment of the spinal cord. - Normally at rest, there is a tonic discharge of sympathetic fibers to arteries in systemic circulation. This is called sympathetic tone. Stimulation of sympathetic fibers results in vasoconstriction. Therefore, sympathetic tone is also known as vasoconstrictor tone. - Vasoconstrictor tone is essential for maintaining normal blood pressure. Increases in vasoconstrictor tone increase and decreases in vasoconstrictor tone decrease blood pressure. - There is no parasympathetic innervation of blood vessels in general circulation. ## Types of Vessels There are two types of vessels in the arterial system: ### Elastic Vessels Elastic vessels are the aorta and large arteries. - In these vessels, the quantity of the elastic component is more than the muscle component. - Therefore, these arteries are more compliant (stretchable). - These are called Windkessel vessels (elastic reservoir). ### Muscular Vessels These include small arteries, arterioles, and metarterioles. - In these vessels, the quantity of smooth muscle is more than elastic tissues. - Therefore, these vessels provide maximum resistance to blood flow. - These are called resistance vessels. - Arterioles and metarterioles are the primary seat of peripheral resistance. - In fact, due to vascular resistance, a significant fall in blood pressure occurs when blood passes through the arterioles. ## Functions of Arteries 1. Arteries transport blood from the heart to the tissues. Thus, they supply oxygen and nutrients to the different parts of the body.  2. The aorta and the large arteries due to their elastic recoil property, maintain forward movement of blood during diastole (details, described later).  3. Small arteries, arterioles, and metarterioles are richly innervated by sympathetic fibers and offer maximum resistance to blood flow. They are the seat of peripheral resistance. Therefore, these vessels mainly control blood pressure. ## Functional aspects ### Arterial Elasticity The aorta and large arteries have more compliance due to the presence of more elastic elements in their wall. When blood is ejected forcefully into the aorta and its major branches during ventricular systole, these vessels are distended. During diastole, the aortic wall immediately recoils back to its previous position. This property of recoiling is known as the Windkessel effect. Due to the Windkessel effect, the vessel wall that recoils back on the blood column pushes blood to move in a forward direction during diastole. (Note: During systole, forward movement of blood is due to the energy created by the forceful ejection of blood that occurs due to ventricular contraction.) Had the aorta and large arteries been stiff and rigid (no recoiling effect), flow of blood during diastole would have stopped and that would have resulted in intermittent blood flow only during systole, not the continuous flow of blood. Thus, blood moves continuously during systole and diastole due to the Windkessel effect of elastic arteries. Continuous arterial flow is essential for adequate tissue perfusion. ### Arterial Pressure Pulse The arterial pressure pulse is the pressure wave that travels along the wall of the arteries created by the forceful ejection of blood into the arterial system during ventricular systole. These pressure waves are felt as arterial pulses when clinically examined by the physician. - The velocity of transmission of pulse wave in the wall of the artery is 15 times the velocity of flow of blood in the lumen of the artery. For example, in the aorta, the velocity of flow is 0.33 m/s, whereas the velocity of transmission of the pressure wave is about 4 m/s. - The velocity of transmission of a pressure wave increases towards the periphery. In large arteries, it is about 8 m/s and in small arteries about 20 m/s. Thus, the radial pulse in the wrist is felt in 0.1 s from the peak of ventricular ejection. - The amplitude and the pattern of the arterial pulse also change from central arteries to peripheral arteries. Arterial pulses are recorded (pulse tracing) by a sphygmograph or a physiograph. The central arterial pulse has a higher amplitude, a steep ascending limb, less sharp peak and incisura in the upper part of the descending limb, which is less steep. The peripheral arterial pulse has a steep ascending limb, a sharp peak, steep descending limbs, and the dicrotic notch (instead of incisura) present towards the lower part in the descending limb. ### Arterial Pulse Waves In arterial pulse, there are two waves: a percussion wave and a dicrotic wave - The percussion wave or tidal wave occurs due to the ejection of blood during ventricular systole. This corresponds to the maximum ejection phase. - The dicrotic wave occurs due to the rebound of blood against the closed aortic valve during diastole. The dicrotic notch represents closure of aortic valve (for details, refer to Fig. 49.1; Chapter 49). ## Arterial Pressure Arterial pressure is defined as the lateral pressure exerted by the column of blood on the walls of the arteries. Blood pressure usually means the arterial pressure. The pressure in the arteries fluctuates during systole and diastole of the cardiac cycle. - The maximum pressure is recorded during systole (systolic blood pressure) and the minimum pressure is recorded during diastole (diastolic blood pressure). - In adults, the normal systolic pressure ranges between 100 mm Hg and 119 mm Hg and diastolic pressure ranges between 60 mm Hg and 79 mm Hg. ## Pulse Pressure Pulse pressure (PP) is the difference between the systolic and diastolic pressures. Normally, it ranges between 20 mm Hg and 50 mm Hg. - The change in pressure during systole and diastole of a cardiac cycle produces pulse pressure. - The pulse pressure is high in the aorta and large artery. It grossly decreases across the arterioles, almost negligible in capillaries and nil in veins. ## Mean Arterial Pressure Mean arterial pressure (MAP) is the average pressure recorded during the cardiac cycle. It is calculated as: MAP = Diastolic pressure + 1/3 PP As the duration of systole is less than the duration of diastole, MAP is slightly less than the value halfway between systolic and diastolic pressure. Normal MAP ranges between 80 mm Hg and 105 mm Hg. ## Measurement of Blood Pressure Methods of measurement of blood pressure are broadly divided into two categories: 1. Direct methods, and 2. Indirect methods. ### Direct Methods The blood pressure is measured directly by placing a cannula in the artery and connecting the cannula to a mercury manometer or a pressure transducer. This is the method used for recording blood pressure in experimental animals. In human beings, blood pressure is usually measured by indirect methods. ### Indirect Methods Blood pressure is usually measured with the help of a sphygmomanometer. The procedure is called sphygmomanometry. In this method, the Riva Rocci cuff of the sphygmomanometer is wrapped around the arm of the subject. The cuff is then inflated until the air pressure in the cuff overcomes the arterial pressure and obliterates the arterial lumen. This is confirmed by palpating the radial pulse that disappears when the cuff pressure is raised above the arterial pressure. Pressure is then raised further by about 20 mm Hg and then slowly reduced. When pressure in the cuff reaches just below the arterial pressure, blood escapes beyond the occlusion into the peripheral part of the artery and pulse starts reappearing. This is detected by the appearance of sounds on the stethoscope, which is taken as the systolic pressure. Then the quality of the sound changes and finally disappears. The level where sound disappears is noted as the diastolic pressure. Sound disappears because the flow in the blood becomes laminar. The blood pressure can be measured by three methods: 1. Palpatory method 2. Auscultatory method 3. Oscillatory method ## Palpatory Method In the palpatory method, pressure in the cuff is progressively raised and radial artery pulse is palpated simultaneously. The point where the pulsation disappears is the systolic pressure. Diastolic pressure cannot be measured by the palpatory method. ## Auscultatory Method In the auscultatory method, pressure in the cuff is raised by about 20 mm Hg above the palpatory level and then progressively lowered during which the brachial artery is auscultated for sounds by placing the diaphragm of a stethoscope on it. The sounds undergo a series of changes in their quality and intensity. These sounds are known as Korotkov sounds (described by the Russian scientist Korotkov in 1905). The sounds are heard in five different phases: 1. **Phase I:** Sudden appearance of faint tapping sound which becomes gradually louder and clearer during the succeeding 10 mm Hg fall in pressure. 2. **Phase II:** The sound becomes murmurish in the next 10 mm Hg fall in pressure. 3. **Phase III:** The sound changes little in quality but becomes clearer and louder in the next 15 mm Hg fall in pressure. 4. **Phase IV:** Sounds become muffling in character during the next 5 mm Hg fall. 5. **Phase V:** Sounds completely disappear. The appearance of the sound is recorded as systolic blood pressure and disappearance of sound is recorded as diastolic blood pressure. In persons having severe hypertension, muffling rather than the disappearance of sound is taken as diastolic pressure. In children, muffling is also noted as diastolic pressure. ## Oscillatory Method In the oscillatory method, the procedure is the same as that of the palpatory method. But, instead of palpating the artery, oscillations of the mercury column in the sphygmomanometer is noted to record BP. The pressure in the cuff is raised and the appearance and the disappearance of the oscillations of the mercury column are noted. The point of appearance of the oscillation gives systolic pressure and the point of disappearance of the oscillations gives diastolic pressure. ## Key Concepts - The Windkessel effect is due to arterial elasticity, which facilitates the movement of blood in arteries in a forward direction during diastole. - The more elastic elements in the wall of the aorta and large arteries make these vessels resistant to distension. - Therefore, these vessels are called Windkessel vessels. Dense sympathetic innervation also contributes to this. - Arterioles and metarterioles have more smooth muscle vessels. - Vasoconstrictor tone mainly functions to control arterial blood pressure. ## Important to Know (Must Read) - Windkessel effect, Korotkov sound, Sympathetic vasoconstrictor tone, functions of arteries. - What are the types of vessels in the arterial system, what are the different segments of the circulatory system, what are systolic and diastole. - Define arterial blood pressure. - Define pulse pressures. - What is the palpatory method of measuring BP? What is the auscultatory method of measuring BP? What is the oscillatory method of measuring BP? What are the phases of Korotkov sound?  - How does blood move continuously during systole and diastole?  - What is the sympathetic vasoconstriction tone? - Why is arterial elasticity important? - What the functions of arteries?