Medical School · Year 1 · Cardiovascular · includes a quiz and discussion video

Lecture 6: Blood Pressure Regulation

Unit 1.7: Cardiovascular System

Learning Objectives

By the end of this lecture, students will be able to:

1. Define arterial blood pressure and describe its determinants
2. Explain the short-term neural mechanisms of blood pressure regulation
3. Describe the intermediate-term humoral mechanisms of blood pressure control
4. Explain the long-term renal mechanisms of blood pressure regulation
5. Describe the integration of multiple regulatory systems
6. Apply blood pressure physiology to hypertension and hypotension

Arterial Blood Pressure Fundamentals

Arterial blood pressure is the force exerted by blood against the walls of the arteries and is essential for driving blood flow to the tissues. Blood pressure fluctuates throughout the cardiac cycle, reaching its peak during ventricular ejection and its nadir during diastole.

Systolic blood pressure represents the maximum pressure in the arteries during ventricular ejection, normally approximately 120 mmHg in healthy adults. Diastolic blood pressure represents the minimum pressure during ventricular relaxation when the aortic valve is closed, normally approximately 80 mmHg. Pulse pressure is the difference between systolic and diastolic pressures (normally about 40 mmHg) and reflects stroke volume and arterial compliance. Wide pulse pressure occurs with increased stroke volume (as in aortic regurgitation) or decreased arterial compliance (as in aging).

Mean arterial pressure (MAP) represents the average pressure throughout the cardiac cycle and is the driving force for tissue perfusion. Because the heart spends more time in diastole than systole, MAP is closer to diastolic than to systolic pressure. It can be calculated as diastolic pressure plus one-third of pulse pressure, or equivalently as the sum of systolic and twice diastolic pressures divided by three. Normal MAP is approximately 93 mmHg. At rapid heart rates, where systole and diastole approach equal duration, MAP approximates the simple average of systolic and diastolic pressures.

The fundamental relationship governing blood pressure is that mean arterial pressure equals cardiac output multiplied by total peripheral resistance. This relationship highlights that blood pressure can be increased either by increasing cardiac output (heart rate, stroke volume) or by increasing peripheral resistance (vasoconstriction). All blood pressure regulatory mechanisms ultimately act through one or both of these factors.

<image>Panel A: Arterial pressure waveform over several cardiac cycles showing systolic pressure (120 mmHg peak), diastolic pressure (80 mmHg trough), and pulse pressure (40 mmHg amplitude). Panel B: Mean arterial pressure (93 mmHg) shown as horizontal line positioned closer to diastolic with calculation MAP = DBP + 1/3(PP) = 93 mmHg. Panel C: Fundamental equation MAP = CO x TPR with CO broken down as HR x SV, showing arrows indicating increased CO or TPR increases blood pressure. Panel D: Cardiovascular system diagram illustrating heart as pump (determining CO) and arterioles as major resistance vessels (determining TPR).</image>

Short-Term Regulation: The Baroreceptor Reflex

The baroreceptor reflex is the most important mechanism for rapid, moment-to-moment blood pressure regulation. Operating within seconds, it can restore blood pressure toward normal following acute changes, such as those occurring with postural changes.

The sensors of this reflex are baroreceptors, specialized stretch-sensitive mechanoreceptors located in the walls of the carotid sinus (at the bifurcation of the common carotid artery) and the aortic arch. When blood pressure rises, these arterial walls stretch, increasing baroreceptor firing rate. When blood pressure falls, baroreceptor firing decreases. The carotid sinus baroreceptors are particularly important and are most sensitive to pressures around 80 to 150 mmHg.

Afferent signals travel via the glossopharyngeal nerve (cranial nerve IX) from the carotid sinus and via the vagus nerve (cranial nerve X) from the aortic arch to the nucleus tractus solitarius (NTS) in the medulla oblongata, which serves as the central integration center.

The response to increased blood pressure illustrates the reflex arc. Elevated pressure stretches the baroreceptors, increasing afferent firing to the NTS. This activates the parasympathetic nuclei (nucleus ambiguus, dorsal motor nucleus of the vagus) while inhibiting the sympathetic centers in the rostral ventrolateral medulla (RVLM). Increased vagal output to the heart decreases heart rate. Decreased sympathetic output decreases heart rate, decreases contractility, causes venodilation (reducing preload), and causes arteriolar dilation (reducing total peripheral resistance). The net effect is decreased cardiac output and decreased resistance, lowering blood pressure toward normal.

The response to decreased blood pressure is the opposite. Reduced baroreceptor stretch decreases afferent firing. Parasympathetic output decreases while sympathetic output increases. Heart rate and contractility increase, venoconstriction augments preload, and arteriolar constriction increases resistance. The net effect is increased cardiac output and increased resistance, raising blood pressure.

The baroreceptor reflex has important characteristics and limitations. It responds within seconds, making it ideal for rapid compensation. However, the baroreceptors adapt to sustained pressure changes over one to two days, resetting to the new baseline. This adaptation explains why the baroreceptor reflex does not provide effective long-term control of blood pressure and why chronic hypertension is not corrected by this mechanism.

<image>Panel A: Anatomical components showing baroreceptors in carotid sinus and aortic arch with afferent fibers via CN IX and CN X to nucleus tractus solitarius in medulla connecting to parasympathetic and sympathetic centers. Panel B: Response to increased BP flowchart showing increased baroreceptor stretch, increased parasympathetic and decreased sympathetic output, leading to decreased HR, contractility, vasodilation, and BP returning toward normal. Panel C: Response to decreased BP with opposite cascade showing decreased baroreceptor firing leading to increased sympathetic output and BP restoration. Panel D: Inset graph of baroreceptor firing rate versus arterial pressure showing sigmoid relationship with maximum sensitivity around normal MAP and curve reset in chronic hypertension.</image>

Other Short-Term Mechanisms

Several additional rapid-acting mechanisms contribute to blood pressure regulation.

Chemoreceptors in the carotid and aortic bodies primarily regulate respiration but also influence cardiovascular function. Peripheral chemoreceptors respond to decreased arterial oxygen, increased carbon dioxide, and decreased pH. When activated, they increase sympathetic outflow, causing vasoconstriction and increased blood pressure. This response helps maintain perfusion when oxygen delivery is compromised. Central chemoreceptors in the medulla respond primarily to carbon dioxide and pH in the cerebrospinal fluid, with similar cardiovascular effects.

The CNS ischemic response is a powerful emergency mechanism activated when brain blood flow decreases severely, causing cerebral ischemia. Accumulation of carbon dioxide and lactate in the vasomotor center triggers massive sympathetic discharge, producing intense vasoconstriction and raising blood pressure dramatically. This "last-ditch" mechanism can raise mean arterial pressure to over 200 mmHg temporarily, attempting to restore cerebral perfusion. The Cushing reflex (or response) is a specific form of this mechanism occurring with elevated intracranial pressure. Increased intracranial pressure compresses cerebral blood vessels, causing brain ischemia. The resulting hypertension, combined with reflex bradycardia (from baroreceptor activation by the elevated blood pressure), constitutes the classic Cushing triad of hypertension, bradycardia, and irregular respirations.

The Bainbridge reflex responds to increased blood volume reaching the right atrium. Stretch receptors in the right atrium detect increased filling and send signals via vagal afferents that increase heart rate. This helps prevent venous congestion by accelerating the heart to pump out the increased returning blood volume.

Low-pressure baroreceptors in the atria and pulmonary vessels sense blood volume rather than arterial pressure. Volume expansion activates these receptors, suppressing ADH release (promoting water excretion) and stimulating ANP release (promoting natriuresis). Volume depletion has the opposite effect. These receptors provide an interface between blood volume and renal handling of sodium and water.

<image>Panel A: Chemoreceptors showing carotid body and aortic body with afferent connections via CN IX and X, flowchart showing decreased O2, increased CO2, or decreased pH activating sympathetic output and vasoconstriction. Panel B: CNS ischemic response showing brainstem vasomotor center with CO2 and lactate accumulation triggering massive sympathetic outflow, and Cushing reflex with elevated ICP causing brain ischemia, hypertension, and bradycardia. Panel C: Bainbridge reflex showing right atrium stretch receptors with pathway of increased atrial stretch leading to vagal afferents and increased HR to accommodate volume. Panel D: Low-pressure baroreceptors in atria and pulmonary vessels showing effects on ADH and ANP release in response to volume changes.</image>

Intermediate-Term Regulation: The Renin-Angiotensin-Aldosterone System

The renin-angiotensin-aldosterone system (RAAS) operates over minutes to hours and provides powerful regulation of blood pressure and blood volume.

Renin is an enzyme released from juxtaglomerular cells in the kidney in response to three stimuli. First, decreased renal perfusion pressure is sensed directly by baroreceptor mechanisms in the afferent arteriole. Second, decreased sodium delivery to the macula densa cells of the distal tubule triggers renin release through tubuloglomerular feedback. Third, increased sympathetic nervous system activity directly stimulates renin release via beta-1 receptors on juxtaglomerular cells.

Once released, renin initiates a cascade. Renin cleaves angiotensinogen (produced constitutively by the liver) to form angiotensin I, a relatively inactive decapeptide. Angiotensin-converting enzyme (ACE), located primarily on pulmonary endothelial cells but also present elsewhere, removes two amino acids from angiotensin I to produce angiotensin II, a potent octapeptide with multiple cardiovascular effects.

Angiotensin II acts on AT1 receptors to produce both immediate and delayed effects. Directly, it causes potent arteriolar vasoconstriction, rapidly increasing total peripheral resistance and blood pressure. It also stimulates the adrenal cortex to release aldosterone. Aldosterone acts on the distal tubule and collecting duct of the kidney to increase sodium reabsorption and potassium excretion, expanding extracellular fluid volume and thus increasing blood volume and preload. Angiotensin II acts on the hypothalamus to stimulate thirst and ADH release, both of which promote water retention. It enhances sympathetic nervous system activity by facilitating norepinephrine release from sympathetic nerve terminals and by acting centrally. Chronically, angiotensin II promotes cardiac and vascular remodeling, contributing to hypertrophy and fibrosis.

The RAAS is the target of several important antihypertensive drug classes. ACE inhibitors (such as lisinopril) block the conversion of angiotensin I to angiotensin II. Angiotensin receptor blockers (such as losartan) block AT1 receptors. Aldosterone receptor antagonists (such as spironolactone) block the effects of aldosterone on the kidney. All of these reduce blood pressure and blood volume.

<image>Panel A: Kidney with juxtaglomerular apparatus showing afferent arteriole, macula densa, and juxtaglomerular cells with three renin release stimuli: decreased perfusion pressure, decreased NaCl delivery, and sympathetic stimulation. Panel B: RAAS cascade showing angiotensinogen cleaved by renin to angiotensin I then converted by ACE to angiotensin II. Panel C: Angiotensin II effects on arterioles (vasoconstriction increasing TPR), adrenal cortex (aldosterone causing Na+ retention), hypothalamus (thirst and ADH), sympathetic terminals (NE release), and heart/vessels (remodeling). Panel D: Drug targets showing ACE inhibitors blocking ACE, ARBs blocking AT1 receptors, and aldosterone antagonists blocking mineralocorticoid receptor.</image>

Additional Intermediate-Term Mechanisms

Several other mechanisms operate over minutes to hours to help regulate blood pressure.

Antidiuretic hormone (ADH, also called vasopressin) is released from the posterior pituitary in response to increased plasma osmolality (the primary stimulus) and decreased blood volume or blood pressure (secondary stimuli). Through V2 receptors in the renal collecting duct, ADH increases water reabsorption by inserting aquaporin channels into the apical membrane. This conserves water, expands blood volume, and increases preload. At higher concentrations, ADH also acts on V1 receptors on vascular smooth muscle to cause vasoconstriction, directly increasing total peripheral resistance. In normal physiology, ADH's primary role is osmoregulation, but in states of severe hypotension or hypovolemia, its vasopressor effect becomes significant.

Atrial natriuretic peptide (ANP) is released from atrial myocytes in response to atrial stretch, signaling volume expansion. ANP promotes natriuresis (sodium excretion) and diuresis (water excretion) by the kidney, reduces aldosterone and renin secretion, and causes vasodilation. The net effect is reduced blood volume and blood pressure, counterbalancing the effects of RAAS. Brain natriuretic peptide (BNP) is released primarily from ventricular myocytes in response to ventricular stretch and has similar effects. BNP levels are clinically useful as biomarkers of heart failure.

Capillary fluid shift is a passive mechanism that helps buffer blood pressure changes. When blood pressure increases, capillary hydrostatic pressure rises, promoting filtration of fluid from the capillaries into the interstitial space. This reduces intravascular volume, which tends to lower blood pressure. Conversely, when blood pressure falls, capillary hydrostatic pressure decreases, favoring reabsorption of interstitial fluid into the capillaries, expanding intravascular volume. This mechanism can shift several liters of fluid over hours and is particularly important following hemorrhage.

Stress relaxation refers to the gradual adjustment of vascular smooth muscle to changes in volume. When blood volume acutely increases, the vessels initially stretch passively, but over minutes to hours, the smooth muscle relaxes, accommodating the increased volume with less rise in pressure. Reverse stress relaxation occurs with acute volume depletion, where the vessels constrict to maintain filling pressure.

Endothelial factors provide local regulation that contributes to overall vascular resistance. Nitric oxide (NO), synthesized by endothelial nitric oxide synthase (eNOS) in response to shear stress and various chemical signals, diffuses to vascular smooth muscle, activates guanylyl cyclase, and causes vasodilation. Endothelin-1 (ET-1), produced by endothelial cells in response to various stimuli, is a potent vasoconstrictor acting through ETA receptors on smooth muscle.

<image>Panel A: ADH showing hypothalamus and posterior pituitary with release stimuli (increased osmolality, decreased blood volume/BP), V2 receptors in collecting duct (aquaporin insertion, water reabsorption), and V1 receptors on vessels (vasoconstriction). Panel B: Natriuretic peptides showing stretched atria releasing ANP and stretched ventricles releasing BNP with effects of natriuresis, diuresis, vasodilation, and RAAS suppression. Panel C: Capillary fluid shift showing high BP (increased Pc favoring filtration) and low BP (decreased Pc favoring reabsorption), and stress relaxation with blood vessel gradually accommodating volume. Panel D: Endothelial factors showing NO production from eNOS causing smooth muscle relaxation via cGMP and ET-1 production causing smooth muscle contraction via ETA receptors.</image>

Long-Term Regulation: Renal Mechanisms

The kidney is the ultimate arbiter of long-term blood pressure control through its ability to regulate sodium and water balance. Guyton and colleagues demonstrated that the renal mechanism has "infinite gain," meaning it will continue adjusting sodium and water excretion until blood pressure returns to the setpoint.

Pressure natriuresis is the key phenomenon. When arterial pressure rises, the kidney increases sodium and water excretion. Several mechanisms contribute. Increased renal perfusion pressure directly increases glomerular filtration rate and decreases tubular reabsorption. Reduced angiotensin II (resulting from pressure-mediated suppression of renin release) decreases sodium reabsorption. Increased renal interstitial pressure from higher perfusion may also reduce tubular reabsorption. The net effect is that higher blood pressure leads to greater sodium and water excretion, which reduces blood volume and lowers blood pressure back toward normal.

The renal function curve plots steady-state sodium and water excretion against arterial pressure. At higher pressures, excretion is greater; at lower pressures, excretion is reduced. The equilibrium point is where the renal function curve intersects with sodium and water intake—this is the setpoint for long-term blood pressure.

Chronic hypertension results from a rightward shift of the renal function curve. For any level of sodium intake, a higher blood pressure is required to achieve sodium balance. Factors that shift this curve rightward include high sodium intake, RAAS activation (angiotensin II causes sodium retention), renal disease (reduced ability to excrete sodium), and sympathetic hyperactivity. ACE inhibitors shift the curve leftward, reducing the pressure required for sodium balance.

Understanding the infinite gain of the renal mechanism explains why all other blood pressure regulatory systems cannot fully compensate for chronic changes. Baroreceptors adapt to new pressure levels and cannot provide long-term control. RAAS and sympathetic systems can only shift the renal function curve, determining where the equilibrium occurs. Only the kidney, by adjusting fluid volume to match vascular capacity, determines the long-term level of blood pressure.

<image>Panel A: Renal function curve (pressure-natriuresis curve) with arterial pressure on x-axis and sodium/water excretion on y-axis, horizontal intake line intersection defining equilibrium blood pressure. Panel B: Curve shifts showing rightward shift (red, high salt, RAAS activation, renal disease) resulting in higher equilibrium BP and leftward shift (green, ACE inhibitors, low salt) resulting in lower equilibrium BP. Panel C: Pressure natriuresis mechanism showing increased BP leading to increased renal perfusion, GFR, decreased tubular reabsorption, increased excretion, decreased blood volume, and BP returning to setpoint. Panel D: Infinite gain concept showing feedback loop with continued renal adjustment until pressure returns to normal unlike baroreceptor reflex which adapts.</image>

Integration of Regulatory Systems

Blood pressure regulation involves multiple mechanisms operating over different timescales, providing redundant, overlapping control.

In terms of timing, the baroreceptor reflex operates within seconds, providing the fastest response to pressure changes such as those occurring with postural changes. Chemoreceptors and the CNS ischemic response also operate within seconds to minutes in specific emergency situations. The RAAS, ADH, and capillary fluid shift operate over minutes to hours, providing intermediate-term regulation. Renal pressure natriuresis operates over hours to days, providing long-term control.

The response to hemorrhage illustrates the integrated function of these systems. In the immediate phase (seconds to minutes), decreased blood volume reduces venous return, cardiac output, and blood pressure. Baroreceptors detect the fall in pressure, and the reflex response increases heart rate and causes vasoconstriction, partially maintaining blood pressure. Venoconstriction shifts blood from capacitance vessels to maintain cardiac filling.

In the short-term phase (minutes to hours), RAAS activation causes vasoconstriction and initiates sodium retention. ADH release promotes water retention and contributes to vasoconstriction. Sympathetic activation enhances all of these responses. Reduced capillary hydrostatic pressure promotes fluid reabsorption from the interstitium, partially restoring intravascular volume.

In the long-term phase (hours to days), renal sodium and water retention continues until blood volume is restored. Thirst promotes fluid intake. If blood loss was significant, erythropoietin release stimulates red blood cell production to restore oxygen-carrying capacity.

The response to standing (orthostatic challenge) similarly involves multiple systems. Blood pools in the lower extremities due to gravity, reducing venous return and cardiac output. The immediate baroreceptor response increases heart rate and causes vasoconstriction within seconds, preventing syncope in most individuals. RAAS activation over hours contributes to sodium retention that helps maintain volume. Failure of these compensatory mechanisms results in orthostatic hypotension.

<image>Panel A: Timeline with x-axis from seconds to days showing mechanism active periods: baroreceptor reflex (seconds), chemoreceptors (seconds-minutes), CNS ischemic response (minutes), RAAS (minutes-hours), ADH (minutes-hours). Panel B: Capillary fluid shift (minutes-hours), stress relaxation (hours), and renal pressure natriuresis (hours-days extending indefinitely) with gain graph showing baroreceptors (high acute, no long-term), RAAS (sustained intermediate), and renal (infinite long-term). Panel C: Integrated hemorrhage response immediate phase (baroreceptor reflex with increased HR, vasoconstriction) and short-term phase (RAAS, ADH, capillary fluid shift). Panel D: Long-term phase (renal sodium retention, fluid intake, erythropoiesis) all working to restore blood volume and pressure.</image>

Vascular Resistance and Blood Flow Distribution

Total peripheral resistance, one of the two factors determining blood pressure, is governed primarily by arteriolar diameter.

Poiseuille's law describes the factors determining resistance to flow through a tube: resistance equals 8 times viscosity times length divided by pi times radius to the fourth power. Because resistance is inversely proportional to the fourth power of the radius, small changes in vessel diameter produce large changes in resistance. Halving the radius increases resistance sixteenfold.

Arterioles are the primary resistance vessels, containing smooth muscle that can dramatically alter vessel diameter. Arteriolar constriction increases resistance and reduces blood flow to the tissue, while arteriolar dilation decreases resistance and increases flow. Total peripheral resistance is the sum of resistances in all vascular beds, but because vascular beds are largely in parallel, the total resistance is less than the resistance of any individual bed.

Regional distribution of blood flow is accomplished by differential arteriolar resistance in different organs. At rest, the kidneys receive about 20 percent of cardiac output despite representing only about 0.5 percent of body mass—reflecting their high metabolic demands for filtration and their specialized vascular anatomy. Skeletal muscle receives about 20 percent at rest but can receive up to 80 percent during maximal exercise when vasodilation reduces its vascular resistance. The gastrointestinal tract receives about 25 percent at rest, increasing after meals. The brain receives about 15 percent, carefully maintained by autoregulation. The heart receives about 5 percent, essentially all of its oxygen is extracted, so increased demand is met by increased flow.

Control of regional resistance involves neural, hormonal, and local mechanisms. Sympathetic tone provides a baseline level of vasoconstriction that can be modulated. Local metabolic factors (oxygen, carbon dioxide, pH, adenosine) provide autoregulation that matches flow to tissue needs. Endothelial factors (nitric oxide, endothelin) modulate resistance locally.

<image>Panel A: Poiseuille's law with formula R = 8nL/pi r to the fourth emphasizing radius importance with two vessels showing normal and half diameter (16x resistance). Panel B: Systemic circulation schematic with arterioles as major resistance vessels and diagram showing parallel resistance vessels produce total resistance less than any individual. Panel C: Regional blood flow distribution at rest pie chart (kidneys 20%, muscle 20%, GI 25%, brain 15%, heart 5%, other 15%) with arrows showing exercise changes (muscle increasing to 80%). Panel D: Table of regulatory mechanisms for each vascular bed: renal (autoregulation, RAAS), muscle (metabolic, sympathetic), GI (metabolic, sympathetic), brain (autoregulation, metabolic), heart (metabolic), skin (thermoregulation, sympathetic).</image>

Hypertension

Hypertension is defined as sustained elevation of blood pressure and is a major risk factor for cardiovascular disease, stroke, and kidney disease.

Current guidelines classify blood pressure as follows. Normal blood pressure is systolic less than 120 and diastolic less than 80 mmHg. Elevated blood pressure is systolic 120 to 129 with diastolic less than 80. Stage 1 hypertension is systolic 130 to 139 or diastolic 80 to 89. Stage 2 hypertension is systolic 140 or greater or diastolic 90 or greater. Hypertensive crisis is systolic greater than 180 or diastolic greater than 120, requiring immediate attention.

Primary (essential) hypertension accounts for 90 to 95 percent of cases and has no single identifiable cause. It is multifactorial, involving genetic predisposition, environmental factors, and pathophysiological mechanisms. Risk factors include family history, age, obesity, high sodium intake, low potassium intake, physical inactivity, and excessive alcohol consumption. Pathophysiologically, the renal function curve is shifted rightward, requiring higher pressure to maintain sodium balance. This shift may result from subtle renal abnormalities, RAAS overactivity, sympathetic hyperactivity, or endothelial dysfunction.

Secondary hypertension (5 to 10 percent of cases) has an identifiable, potentially reversible cause. Renovascular hypertension results from renal artery stenosis causing increased renin release from the ischemic kidney. Primary aldosteronism (Conn syndrome) results from aldosterone-producing adrenal adenomas or hyperplasia, causing sodium retention. Pheochromocytoma, a catecholamine-secreting tumor of the adrenal medulla, causes episodic or sustained hypertension from excessive catecholamine effects. Cushing syndrome, with cortisol excess, causes hypertension through mineralocorticoid effects and other mechanisms. Coarctation of the aorta causes hypertension proximal to the narrowing. Sleep apnea causes hypertension through intermittent hypoxia and sympathetic activation.

Untreated hypertension leads to end-organ damage. The heart develops left ventricular hypertrophy (an adaptive response to increased afterload), which eventually leads to diastolic dysfunction, heart failure, and increased risk of coronary artery disease and arrhythmias. The brain is at increased risk of ischemic and hemorrhagic stroke and vascular dementia. The kidneys develop hypertensive nephrosclerosis and chronic kidney disease. The eyes develop hypertensive retinopathy with arteriolar narrowing, hemorrhages, and in severe cases, papilledema. The vessels develop accelerated atherosclerosis and may develop aneurysms.

<image>Panel A: Blood pressure classification table color-coded from green (normal) to red (hypertensive crisis) with primary hypertension showing rightward-shifted renal function curve and contributing factors (genetics, obesity, salt, RAAS, sympathetic). Panel B: Secondary causes with diagrams showing renal artery stenosis (increased renin), primary aldosteronism (adrenal adenoma), pheochromocytoma (increased catecholamines), Cushing syndrome, coarctation (upper/lower BP difference), and sleep apnea. Panel C: End-organ damage showing heart (LVH leading to HF), brain (ischemic and hemorrhagic stroke), and kidney (nephrosclerosis). Panel D: Additional end-organ damage showing eye (retinal hemorrhages, papilledema) and vessels (atherosclerotic plaque, aneurysm).</image>

Hypotension and Shock

Hypotension is defined as systolic blood pressure below 90 mmHg or mean arterial pressure below 65 mmHg, or a significant drop from the patient's baseline. While hypertension causes long-term damage, hypotension threatens immediate organ perfusion.

Orthostatic hypotension is defined as a fall of at least 20 mmHg in systolic pressure or 10 mmHg in diastolic pressure within three minutes of standing. Causes include volume depletion (the most common cause), autonomic dysfunction (diabetes, Parkinson disease, multiple system atrophy), and medications (antihypertensives, particularly alpha-blockers and diuretics). Symptoms include lightheadedness, dizziness, and syncope upon standing.

Shock is a state of inadequate tissue perfusion and oxygenation, leading to cellular dysfunction and, if prolonged, organ failure and death. Different types of shock have different hemodynamic profiles.

Cardiogenic shock results from pump failure. The heart cannot generate adequate cardiac output despite adequate filling. Causes include myocardial infarction, severe heart failure, arrhythmias, and valvular catastrophes. Hemodynamically, cardiac output is reduced, while preload (filling pressures) is elevated and systemic vascular resistance is elevated (compensatory vasoconstriction). Treatment targets improving cardiac function with inotropes and potentially mechanical support.

Hypovolemic shock results from loss of intravascular volume—blood loss (hemorrhage) or fluid loss (dehydration, burns). Cardiac output is reduced due to decreased preload. Filling pressures are low. Systemic vascular resistance is elevated (compensatory vasoconstriction). Treatment involves volume resuscitation.

Distributive shock results from widespread vasodilation, reducing systemic vascular resistance and creating relative hypovolemia despite normal or even high blood volume. The most common forms are septic shock (due to inflammatory mediators), anaphylactic shock (due to histamine and other mediators), and neurogenic shock (due to loss of sympathetic tone, as in spinal cord injury). In the early hyperdynamic phase of septic shock, cardiac output may actually be elevated despite low blood pressure. Treatment involves vasopressors to restore vascular tone and addressing the underlying cause.

Obstructive shock results from mechanical obstruction to blood flow. Cardiac tamponade (pericardial fluid compressing the heart), tension pneumothorax (increased intrathoracic pressure compressing great vessels), and massive pulmonary embolism (obstruction of pulmonary outflow) are examples. Treatment requires relieving the obstruction.

All shock states activate compensatory mechanisms—baroreceptor reflexes, RAAS, ADH, and sympathetic activation—which help maintain blood pressure and vital organ perfusion initially but may prove insufficient or maladaptive if shock persists.

<image>Panel A: Orthostatic hypotension showing figure transitioning supine to standing with BP measurements before (120/80) and after (90/60) and gravitational blood pooling in legs. Panel B: Shock types showing cardiogenic (CO down, Preload up, SVR up) with infarcted heart and hypovolemic (CO down, Preload down, SVR up) with blood loss diagram. Panel C: Distributive shock (CO variable, SVR down) with dilated vessels and obstructive shock (CO down, SVR up) with tamponade, tension pneumothorax, and PE diagrams. Panel D: Compensatory mechanisms showing baroreceptor activation (increased HR, vasoconstriction), RAAS activation (Na retention), ADH release (water retention), and sympathetic activation with treatment principles for each shock type.</image>

Summary

Blood pressure is determined by cardiac output times total peripheral resistance. Mean arterial pressure equals diastolic pressure plus one-third of pulse pressure, normally approximately 93 mmHg.

Short-term regulation (seconds) is primarily through the baroreceptor reflex: increased pressure stretches baroreceptors, increases parasympathetic tone and decreases sympathetic tone, reducing heart rate and causing vasodilation to lower pressure. Chemoreceptors and the CNS ischemic response provide emergency backup.

Intermediate-term regulation (minutes to hours) involves the RAAS (angiotensin II causes vasoconstriction and aldosterone causes sodium retention), ADH (water retention, vasoconstriction), ANP (natriuresis, vasodilation), and capillary fluid shift.

Long-term regulation (hours to days) is through renal pressure natriuresis, which has infinite gain. Higher pressure causes greater sodium and water excretion until blood volume adjusts to return pressure to the setpoint.

Hypertension is classified as primary (95%) or secondary (5%), with secondary causes including renovascular disease, primary aldosteronism, pheochromocytoma, and coarctation. Untreated hypertension causes end-organ damage to the heart, brain, kidneys, eyes, and vessels.

Shock states (cardiogenic, hypovolemic, distributive, obstructive) have different hemodynamic profiles and require targeted treatment.

Key Terms

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