Unit III Lecture 6 - Arteries and Arterial Blood Pressure Notes

Structure of Blood Vessels

• Arterial and venous walls are composed of three tunics: tunica interna, tunica media, and tunica externa.
• The three tunics surround the central blood‐containing space, the lumen.

Tunica Interna (Tunica Intima)

• The endothelial layer that lines the lumen of all vessels.

Tunica Media

• Consists of smooth muscle and an elastic fiber layer
• Regulated by the sympathetic nervous system.
• Controls the vasoconstriction and vasodilation of vessels.

Tunica Externa (Tunica Adventitia)

• Contains collagen fibers that protect and reinforce the vessel wall.
• Larger vessels contain vasa vasorum.
• Capillaries are composed of endothelium with sparse basal lamina.

Arteries

• Arteries carry blood away from the heart.

Types of Arteries

Elastic (Conducting) Arteries

• Thick-walled arteries near the heart (the aorta and its major branches).
• Contain elastin that helps to withstand and smooth out large blood pressure fluctuations and allow blood to flow fairly continuously through the body.

Muscular (Distributing) Arteries

• Found distal to elastic arteries.
• Deliver blood to body organs.
• Have a thick tunica media with more smooth muscle and less elastic tissue
• Active in vasoconstriction.

Arterioles

• Smallest arteries; they lead to capillary beds.
• Control flow into capillary beds via vasodilation and constriction.

Blood Pressure (BP)

• The force per unit area exerted by blood on the vessel wall.
• Expressed in millimeters of mercury (mm Hg).
• Measured in reference to systemic arterial BP in the large arteries near the heart.
• Differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas.

Resistance (Peripheral Resistance)

• The opposition to flow.
• Measures the amount of friction that the blood encounters as it passes through the vessels.

Sources of Resistance

Blood Viscosity

• Represents the thickness or “stickiness” of the blood.
• Directly proportional to resistance.

Blood Vessel Length

• The longer the vessel, the greater the resistance encountered.

Blood Vessel Diameter

• Changes in vessel diameter are frequent and most significant to peripheral resistance.
• Small‐diameter arterioles are the major determinants of peripheral resistance.

Blood Flow

• Defined as the volume of blood flowing through a vessel, an organ, or the entire circulation in a given period of time.
• Measured in ml/min.
• Blood flow in the entire vascular system is equal to cardiac output (CO).
• Blood flow through each organ varies widely through individual organs, dependant upon its immediate needs.
• Blood flow is directly proportional to the difference in blood pressure ($\Delta P$) between two points in the circulatory pathway.
  • If $\Delta P$ increases, blood flow speeds up; if $\Delta P$ decreases, blood flow declines.
• Blood flow is inversely proportional to resistance ($R$).
  • If $R$ increases, blood flow decreases.
• $Blood Flow = \frac{Pressure gradient}{Resistance}$

Systemic Blood Pressure

• The pumping action of the heart generates blood flow through the vessels.
• Pressure results when flow is opposed by resistance.
• Systemic pressure is highest in the aorta and declines throughout the length of the pathway.
• It is 0 mmHg in the right atrium.
• The steepest change in blood pressure occurs in the arterioles.
• Blood pressure in elastic arteries near the heart is pulsatile (BP rises and falls).

Systolic Pressure

• The pressure exerted on the arterial walls during ventricular contraction.
• The normal value is less than 120mmHg.

Diastolic Pressure

• The lowest level of arterial pressure during a ventricular cycle.
• The normal value is less than 80mmHg.

Pulse Pressure

• The difference between systolic and diastolic pressure:
• $Pulse Pressure = P<em>{systolic} – P</em>{diastolic}$

Mean Arterial Pressure (MAP)

• The pressure that propels the blood to the tissues.
• $MAP = P_{diastolic} + \frac{1}{3}Pulse Pressure$

Example Calculation

• Calculate mean arterial pressure if the blood pressure at the left arm is 120/90mmHg

Maintaining Blood Pressure

• Maintaining blood pressure requires the cooperation of the heart, blood vessels, kidneys, and the brain.

• The main factors influencing blood pressure are cardiac output (CO), peripheral resistance (PR), and blood volume.

• Blood pressure varies directly with CO, PR, and blood volume:

  • $BP = CO \cdot PR$

Regulation of Blood Pressure

• Includes short and long-term mechanisms.

Short-Term Mechanisms

• Mediated by the nervous system and blood-borne chemicals that counteract moment-to-moment fluctuations in blood pressure by altering peripheral resistance.

Long-Term Mechanisms

• Regulate blood volume.

Short-Term Mechanisms: Baroreceptor-Initiated Reflexes

• High blood pressure increases the stimulation of baroreceptors in the aortic arch and the carotid sinuses.
• The stimulus is processed by the cardiovascular center.
• Results in an increase of parasympathetic activity (decreasing heart rate and contractility), leading to a decrease of CO, and therefore blood pressure.
• The cardiovascular center also decreases the sympathetic activity (increasing vessel diameter), leading to a decrease in peripheral resistance and therefore, blood pressure.
• Declining blood pressure decreases the rate of firing of baroreceptors.
• The stimulus is processed by the cardiovascular center and leads to an increase in the sympathetic stimulation.
• Resulting vasoconstriction leads to an increase in peripheral vascular resistance; an increase in heart rate and contractility lead to an increase in cardiac output.
• Blood pressure rises back to normal.

Long‐Term Mechanisms: Renal Regulation

• Long-term mechanisms control BP by altering blood volume.
• Baroreceptors adapt to chronic high or low blood pressure.

Increased BP

• Stimulates the kidneys to eliminate water, thus reducing BP.

Decreased BP

• Stimulates the kidneys to increase blood volume and BP.
• The kidneys act directly and indirectly to maintain long-term blood pressure.

Direct Renal Mechanism

• Leads to increase urine production, lower blood volume and, hence, decrease in BP.

Indirect Renal Mechanism: Renin-Angiotensin Mechanism

• Declining BP causes the release of renin by kidneys.
• Renin converts blood-born angiotensinogen into angiotensin I.
• In the lungs, angiotensin-converting enzyme (ACE) further converts angiotensin I into angiotensin II.

Effects of Angiotensin II

• It is a potent vasoconstrictor which increases resistance and increases BP.
• It stimulates aldosterone and ADH secretion.
• These hormones enhances renal reabsorption and lead to water retention.
• In turn, increased blood volume leads to preload increase and SV and CO and BP increase.