Premed · Premed · Anatomy Physiology 2
Lecture 6: Blood Pressure Regulation
Anatomy and Physiology II
Learning Objectives
By the end of this lecture, students will be able to:
- Define blood pressure and explain the factors that determine it
- Describe the relationship between blood flow, blood pressure, and resistance
- Explain the neural mechanisms of blood pressure regulation
- Describe the hormonal mechanisms (RAAS, ADH, ANP) that regulate blood pressure
- Explain local (autoregulatory) control of blood flow
- Define hypertension and describe its consequences
Lecture Content
I. Blood Pressure — Fundamentals
Blood pressure (BP) is the force per unit area that blood exerts against the walls of the vessels through which it flows. When clinicians refer to "blood pressure," they typically mean arterial blood pressure, which is generated by ventricular contraction and maintained between beats by the elastic recoil of the arterial walls. Systolic pressure, the peak pressure during ventricular contraction, is normally about 120 mmHg. Diastolic pressure, the lowest pressure during ventricular relaxation, is normally about 80 mmHg. The difference between these two values is the pulse pressure, approximately 40 mmHg.
The mean arterial pressure (MAP) represents the average pressure driving blood through the systemic circulation and is calculated as diastolic pressure plus one-third of the pulse pressure, yielding approximately 93 mmHg at rest. A MAP of at least 60 mmHg is required to adequately perfuse the vital organs.
Pressure Changes Through the Vasculature
As blood moves from the aorta toward the periphery, pressure changes in a characteristic pattern. In the aorta and large arteries, pressure remains pulsatile at approximately 120/80 mmHg. The largest pressure drop occurs across the arterioles, where blood exits at roughly 35 mmHg and pulsation is dampened. In the capillaries, pressure falls from about 35 mmHg at the arteriolar end to 17 mmHg at the venular end, and flow becomes steady. In the venous system, pressure is approximately 15 mmHg in venules and approaches 0 mmHg at the right atrium. Importantly, flow velocity is slowest in the capillaries because their total cross-sectional area is the largest of any vascular segment, which maximizes the time available for exchange.
II. Hemodynamic Relationships
Flow, Pressure, and Resistance
Blood flow (F) is the volume of blood flowing through a vessel per unit time. It is described by the equation F = delta P / R, where delta P is the pressure gradient between two points and R is the resistance to flow. This is analogous to Ohm's law in electrical circuits. Since cardiac output represents total systemic blood flow, blood flow is directly proportional to the pressure gradient and inversely proportional to resistance.
Peripheral Resistance
Total peripheral resistance (TPR), also called systemic vascular resistance (SVR), is the combined resistance of all the systemic blood vessels. The fundamental equation of cardiovascular physiology states that MAP = CO x TPR, meaning that blood pressure is determined by the product of cardiac output and peripheral resistance.
Several factors affect resistance, as described by Poiseuille's law. Vessel radius is by far the most important: resistance is inversely proportional to the fourth power of the radius, so even small changes in radius cause dramatic changes in resistance. The arterioles, as the primary resistance vessels, are the main regulators of peripheral resistance. Blood viscosity also influences resistance; conditions such as polycythemia increase viscosity and therefore resistance. Vessel length contributes as well, though it remains relatively constant in the body.
III. Neural Regulation of Blood Pressure
Cardiovascular Center in the Medulla Oblongata
Three interconnected regions in the medulla coordinate cardiovascular function. The cardioacceleratory center increases heart rate and contractility through sympathetic nerves. The cardioinhibitory center decreases heart rate via the vagus nerve (parasympathetic). The vasomotor center controls the tone of vascular smooth muscle through sympathetic vasomotor fibers. A continuous low-level sympathetic discharge, called sympathetic vasomotor tone, keeps arterioles partially constricted at all times. Increasing sympathetic output causes vasoconstriction, raises TPR, and increases blood pressure, while decreasing sympathetic output allows vasodilation and lowers blood pressure.
Baroreceptor Reflex (Short-Term Regulation)
Baroreceptors are stretch-sensitive mechanoreceptors located in the walls of the carotid sinus (at the bifurcation of the common carotid artery) and the aortic arch. They continuously monitor arterial blood pressure and initiate rapid adjustments.
When blood pressure increases, baroreceptors are stretched and fire more frequently. Increased afferent signals travel via the glossopharyngeal nerve (CN IX, from the carotid sinus) and vagus nerve (CN X, from the aortic arch) to the medulla. The medulla responds by increasing parasympathetic output to decrease heart rate and decreasing sympathetic output to reduce heart rate, contractility, and vasomotor tone. The net effect is a decrease in cardiac output and TPR, bringing blood pressure back toward normal.
When blood pressure decreases, baroreceptors fire less frequently. The medulla responds by decreasing parasympathetic output (allowing heart rate to rise) and increasing sympathetic output (increasing heart rate, contractility, and vasoconstriction). The net effect is an increase in cardiac output and TPR, raising blood pressure toward normal.
The baroreceptor reflex operates on a moment-to-moment basis, responding within seconds to pressure changes. However, it adapts (resets) over the course of days if blood pressure is chronically altered, making it ineffective for long-term regulation.
Chemoreceptor Reflex
Peripheral chemoreceptors in the carotid bodies and aortic bodies detect decreased oxygen, increased carbon dioxide, or decreased pH in the blood. When stimulated, they activate the vasomotor center, causing vasoconstriction and increased blood pressure. They also increase respiratory rate, a response discussed in detail in the respiratory lectures.
<image>A diagram of the baroreceptor reflex. Panel A: An anatomical illustration showing the location of baroreceptors in the carotid sinus and aortic arch, with afferent nerve pathways (glossopharyngeal nerve from carotid sinus, vagus nerve from aortic arch) traveling to the cardiovascular center in the medulla oblongata. Panel B: A flowchart showing the response to increased BP — baroreceptors detect stretch, signal medulla, parasympathetic output increases (decreasing HR), sympathetic output decreases (decreasing HR, contractility, and vasomotor tone), resulting in decreased CO and TPR, lowering BP back to normal. Panel C: A complementary flowchart for the response to decreased BP — decreased baroreceptor firing, sympathetic output increases, parasympathetic output decreases, resulting in increased CO and TPR, raising BP back to normal.</image>
IV. Hormonal Regulation of Blood Pressure
Renin-Angiotensin-Aldosterone System (RAAS) — Long-Term Regulation
The RAAS is activated when juxtaglomerular (JG) cells in the kidney detect decreased blood pressure or decreased renal blood flow. Sympathetic stimulation of JG cells and decreased sodium delivery to the macula densa also trigger the cascade. JG cells release renin into the blood, which converts angiotensinogen (produced by the liver) to angiotensin I. Angiotensin-converting enzyme (ACE), located primarily in the pulmonary capillaries, then converts angiotensin I to angiotensin II, a potent vasoconstrictor that increases TPR and blood pressure. Angiotensin II also stimulates aldosterone release from the adrenal cortex, ADH release from the posterior pituitary, thirst, and sodium reabsorption in the proximal tubule. Aldosterone increases sodium and water reabsorption in the renal collecting ducts, expanding blood volume and further raising blood pressure.
Antidiuretic Hormone (ADH / Vasopressin)
ADH is released from the posterior pituitary in response to increased blood osmolarity or decreased blood volume and pressure. It promotes water reabsorption in the renal collecting ducts, which increases blood volume. At high concentrations, ADH also causes vasoconstriction, which explains its alternative name, vasopressin.
Atrial Natriuretic Peptide (ANP)
ANP is released by atrial cardiomyocytes when the atrial walls are stretched by increased blood volume. It opposes the RAAS by promoting sodium and water excretion (natriuresis and diuresis), causing vasodilation, and inhibiting renin and aldosterone release. The net effect is a decrease in blood volume and blood pressure.
Other Hormonal Influences
Epinephrine and norepinephrine from the adrenal medulla increase heart rate, contractility, and vasoconstriction through alpha-1 receptors, while epinephrine also causes vasodilation in skeletal muscle through beta-2 receptors. Endothelin, a powerful vasoconstrictor, is released by damaged endothelium. Nitric oxide (NO), a potent vasodilator, is released by healthy endothelial cells in response to shear stress and various chemical signals.
<image>A flowchart of the RAAS pathway. Starting with decreased blood pressure or renal blood flow, the diagram shows: JG cells releasing renin → renin converting angiotensinogen to angiotensin I → ACE in lung capillaries converting angiotensin I to angiotensin II. Angiotensin II branches to multiple effects: vasoconstriction (increasing TPR), aldosterone release from adrenal cortex (leading to Na+ and water reabsorption in kidneys, increasing blood volume), ADH release (water reabsorption), thirst stimulation, and direct renal Na+ reabsorption. All arrows ultimately converge on "increased blood pressure." Pharmacological intervention points are marked: ACE inhibitors blocking ACE, and ARBs blocking angiotensin II receptors.</image>
V. Local (Autoregulatory) Control of Blood Flow
Tissues have the ability to regulate their own blood flow based on their metabolic needs, independent of neural or hormonal input.
Metabolic Autoregulation
Active tissues produce metabolic vasodilators, including decreased oxygen, increased carbon dioxide, increased hydrogen ions, increased potassium, adenosine, lactic acid, and nitric oxide. These substances cause local arteriolar dilation, increasing blood flow to the tissue that needs it most. Active hyperemia refers to the increase in blood flow that matches increased metabolic activity, while reactive hyperemia is the surge of blood flow that follows a period of ischemia.
Myogenic Response
The myogenic response is an intrinsic property of vascular smooth muscle. When increased pressure stretches a vessel wall, the smooth muscle responds by contracting (vasoconstriction), which helps maintain constant flow despite the pressure increase. When pressure decreases, smooth muscle relaxes (vasodilation), preserving flow. This mechanism is particularly important in the brain and kidneys, where constant blood flow must be maintained despite fluctuations in systemic blood pressure.
Long-Term Autoregulation
Over longer time frames, tissues with chronically increased metabolic demands stimulate angiogenesis, the growth of new capillaries, and vascular remodeling to enhance their blood supply.
VI. Hypertension
Hypertension is defined as persistently elevated blood pressure with systolic readings of 130 mmHg or higher and/or diastolic readings of 80 mmHg or higher under current guidelines. Primary (essential) hypertension accounts for 90 to 95% of cases and has no single identifiable cause. Risk factors include genetics, obesity, high salt intake, smoking, stress, aging, and a sedentary lifestyle. Secondary hypertension, representing 5 to 10% of cases, results from an identifiable underlying condition such as renal artery stenosis, pheochromocytoma, Cushing syndrome, or coarctation of the aorta.
Chronic hypertension exacts a devastating toll on the body. It accelerates atherosclerosis and vascular damage, causes left ventricular hypertrophy that can progress to heart failure, increases the risk of stroke, damages the kidneys (nephrosclerosis), and harms the retina. Hypertension is often called the "silent killer" because it is frequently asymptomatic until significant organ damage has already occurred.
VII. Shock
Shock is a condition of inadequate blood flow to meet tissue metabolic needs. Hypovolemic shock results from loss of blood volume due to hemorrhage, dehydration, or burns. Cardiogenic shock occurs when the heart fails as a pump, as in myocardial infarction or heart failure. Vascular (distributive) shock involves excessive vasodilation and encompasses anaphylactic shock (severe allergic reaction), neurogenic shock (loss of vasomotor tone following spinal cord injury), and septic shock (systemic infection-induced vasodilation). The body initially compensates through the baroreceptor reflex, RAAS, ADH release, and sympathetic activation, but if these mechanisms are overwhelmed, decompensated shock ensues, with positive feedback loops driving irreversible organ failure.
