Lecture #17: Arterial Blood Pressure Regulation

What equation relates mean arterial pressure, cardiac output, and total peripheral resistance?

MAP = CO × TPR. Therefore, an increase in CO or TPR increases arterial blood pressure, while a decrease in CO or TPR decreases arterial blood pressure.

What determines cardiac output (CO)?

CO = SV × HR. Cardiac output is the volume of blood pumped by the heart per minute and is approximately 5 L/min; stroke volume is affected by preload, afterload, and contractility.

What is the minimum MAP needed for adequate tissue perfusion according to the lecture?

MAP should be >60 mmHg to maintain adequate tissue perfusion.

What are the major short-term regulators of arterial blood pressure?

The baroreceptor reflex, chemoreceptor reflex, CNS-ischemic response, and atrial reflex regulate BP within seconds to minutes primarily through neural pathways that alter vessel diameter, HR, and contractility.

Where are the major arterial baroreceptors located?

Baroreceptors are stretch-sensitive receptors located in the carotid sinuses and aortic arch that sense changes in arterial blood pressure.

Which cranial nerves carry baroreceptor afferent signals?

CN IX (glossopharyngeal nerve) carries signals from the carotid sinus, while CN X (vagus nerve) carries signals from the aortic arch.

Where are baroreceptor afferent signals integrated?

Baroreceptor afferent signals travel to the nucleus tractus solitarius (NTS), the cardiovascular control center located in the medulla.

What happens to the baroreceptor reflex when arterial BP increases?

↑BP → ↑vascular wall stretch → ↑baroreceptor firing → afferent signals through CN IX and X to the NTS → ↑parasympathetic and ↓sympathetic stimulation → ↑ACh and ↓NE → ↓HR, ↓contractility, and vasodilation → ↓CO and ↓TPR → ↓BP.

What happens to the baroreceptor reflex when arterial BP decreases?

↓BP → ↓vascular wall stretch → ↓baroreceptor firing → signals through CN IX and X to the NTS → ↓parasympathetic and ↑sympathetic stimulation → ↓ACh and ↑NE → ↑HR, ↑contractility, and vasoconstriction → ↑CO and ↑TPR → ↑BP.

How does sympathetic activation restore BP during hypotension?

Sympathetic activation increases HR and myocardial contraction force, increasing CO, while also producing vasoconstriction that increases peripheral resistance; both effects raise BP.

How does parasympathetic activation lower BP during hypertension?

Increased parasympathetic activity increases ACh release and decreases HR, while reduced sympathetic activity decreases NE, contractility, and vascular tone; CO and peripheral resistance fall, decreasing BP.

Why can standing up quickly activate the baroreceptor reflex?

A rapid positional change can transiently decrease arterial pressure and baroreceptor stretch, activating sympathetic activity to increase HR, SV, and BP.

What is the Valsalva maneuver?

The Valsalva maneuver is forceful exhalation against a closed airway. Increased intrathoracic pressure increases baroreceptor stretch and firing, increasing parasympathetic and decreasing sympathetic activity, which can decrease HR.

What happens to venous return and cardiac output during continued Valsalva strain?

Increased intrathoracic pressure decreases venous return (preload), which decreases cardiac output.

How does carotid massage decrease heart rate?

Carotid massage increases pressure on carotid sinus baroreceptors, simulating increased BP → ↑baroreceptor firing → CN IX carries the signal to the NTS → NTS increases parasympathetic activity through CN X → ↑AV node refractory period → ↓HR.

When is carotid massage contraindicated?

Avoid carotid massage in patients with prior TIA/CVA or carotid bruits because carotid stenosis may contain embolic material that can be dislodged and cause TIA/CVA.

What is carotid sinus hypersensitivity syndrome?

It is increased sensitivity of carotid sinus baroreceptors, often associated with arteriosclerotic changes in older patients; stimulation of the carotid sinus can produce excessive reflex responses and syncope or presyncope.

What is a classic clinical presentation of carotid sinus hypersensitivity?

An older male experiencing syncope or presyncope while buttoning a shirt or shaving is a classic vignette because pressure over the carotid sinus triggers an exaggerated baroreceptor response.

What do peripheral chemoreceptors detect?

Peripheral chemoreceptors in the carotid and aortic bodies detect ↑CO2, ↓O2, and ↓pH.

How do peripheral chemoreceptors affect BP?

↑CO2, ↓O2, or ↓pH activates peripheral chemoreceptors → afferent signals through CN IX and X → stimulation of the medullary respiratory center and vasomotor center → ↑sympathetic activity → ↑BP.

How do baroreceptors and chemoreceptors differ?

Baroreceptors are primary stretch-sensitive sensors that detect changes in vascular wall stretch/BP, whereas chemoreceptors are secondary sensors that detect changes in O2, CO2, and pH.

What is the CNS-ischemic reflex?

It is an emergency BP response triggered by a severe decrease in cerebral blood flow: ↓cerebral blood flow → ischemia → ↑CO2 accumulation → excitation of vasomotor center neurons → powerful vasoconstriction → ↑BP.

What is the Cushing reflex?

The Cushing reflex is a life-threatening type of CNS-ischemic response caused by increased intracranial pressure that reduces cerebral blood flow and triggers compensatory sympathetic activation.

What is the Cushing reflex triad?

Hypertension, bradycardia, and respiratory depression.

How does increased intracranial pressure produce hypertension in the Cushing reflex?

↑ICP → constriction of arterioles and ↓cerebral blood flow → cerebral ischemia → ↑PCO2 and ↓pH → compensatory sympathetic activation → vasoconstriction and ↑BP in an attempt to restore cerebral blood flow.

Why does bradycardia occur in the Cushing reflex despite sympathetic activation?

Sympathetic activation initially increases BP; carotid sinus and aortic arch baroreceptors then detect the increased BP → ↑baroreceptor firing → reflex decrease in HR.

What conditions can cause the Cushing reflex?

Potential causes include head injury, intracranial hemorrhage, tumor, CNS inflammation/infection, excess CSF such as hydrocephalus, and metabolic changes such as hyponatremia.

What is the atrial/Bainbridge reflex?

The Bainbridge reflex is a physiologic reflex mediated by atrial stretch receptors (B-fibers) that responds to increased venous return and atrial distension by increasing HR.

What is the mechanism of the Bainbridge reflex?

↑Blood volume/venous return → ↑atrial stretch → activation of atrial B-fibers → signals to the vasomotor center → ↑sympathetic activity and inhibition of parasympathetic activity → ↑HR.

What stimulates atrial natriuretic peptide (ANP) secretion?

Increased blood volume stretches atrial cardiomyocytes, causing a non-neural response that increases ANP secretion.

Through what second messenger does ANP act?

ANP acts through cGMP.

What are the major cardiovascular effects of ANP?

ANP relaxes vascular smooth muscle → vasodilation → ↓TPR; it also promotes renal NaCl and water excretion, ultimately decreasing BP.

What are the major renal effects of ANP?

ANP decreases Na+ reabsorption in the renal collecting tubule, inhibits renin, dilates afferent arterioles, and constricts efferent arterioles, promoting diuresis and NaCl/water excretion.

How do the Bainbridge reflex and ANP work together during increased blood volume?

Atrial stretch triggers a neural Bainbridge reflex that increases HR and a non-neural ANP response that promotes vasodilation, natriuresis, and diuresis to help eliminate excess circulating fluid.

How does long-term BP regulation differ from short-term regulation?

Short-term regulation occurs within seconds to minutes primarily through neural reflexes, whereas long-term regulation occurs over hours to days through hormonally mediated pathways, especially the renin-angiotensin-aldosterone system (RAAS).

Where are the renal baroreceptors involved in long-term BP regulation?

Renal baroreceptors are located in the afferent arterioles and detect changes in renal perfusion pressure.

Which renal cells secrete renin?

Juxtaglomerular (JG) cells, also called granular cells, secrete renin.

What are the three major triggers for renin secretion?

↓Renal perfusion pressure detected by afferent arteriole baroreceptors, ↑renal sympathetic discharge, and ↓NaCl delivery to macula densa cells.

What does the macula densa detect to regulate renin release?

The macula densa detects decreased NaCl delivery; ↓NaCl stimulates increased renin release from JG cells.

What is the first enzymatic action of renin in RAAS?

Renin converts liver-derived angiotensinogen into angiotensin I.

How is angiotensin I converted to angiotensin II?

Angiotensin-converting enzyme (ACE) converts angiotensin I into angiotensin II.

What is the complete RAAS response to hypotension?

↓BP → ↓renal perfusion pressure and ↓NaCl delivery to macula densa → ↑renin secretion by JG cells → angiotensinogen converted to angiotensin I → ACE converts angiotensin I to angiotensin II → ↑sympathetic activity, arteriolar vasoconstriction, renal NaCl reabsorption, aldosterone secretion, and ADH secretion → ↑salt and water retention → ↑BP.

How does angiotensin II increase TPR?

Angiotensin II causes arteriolar vasoconstriction, increasing total peripheral resistance and therefore increasing MAP.

How does RAAS increase cardiac output?

RAAS increases extracellular fluid and blood volume through salt and water retention → ↑venous return → ↑cardiac output → ↑MAP.

How does RAAS restore MAP through both components of MAP = CO × TPR?

RAAS increases extracellular fluid and blood volume → ↑venous return → ↑CO, while simultaneously causing arteriolar vasoconstriction → ↑TPR. Increasing both CO and TPR raises MAP.

What is shock?

Shock is a life-threatening circulatory disorder in which disruption of circulation causes inadequate organ perfusion, tissue hypoxia, metabolic disturbances, irreversible organ damage, and potentially death.

How does acute hemorrhage initially affect blood pressure?

Hemorrhage causes loss of circulating blood volume → ↓venous return and cardiovascular filling → ↓BP, triggering compensatory short-term baroreceptor responses and long-term renal/RAAS responses.

What is the baroreceptor response to acute hemorrhage?

Hemorrhage → ↓blood volume → ↓BP → ↓baroreceptor stretch and firing → NTS → ↓parasympathetic and ↑sympathetic stimulation → ↑HR, ↑contractility, and vasoconstriction → ↑CO and peripheral resistance to restore BP.

What is the renal/RAAS response to acute hemorrhage?

↓Blood volume and BP → ↓renal perfusion → renin release → angiotensin I → ACE → angiotensin II → ↑sympathetic activity, arteriolar vasoconstriction, renal NaCl reabsorption, aldosterone secretion, and ADH secretion → ↑salt and water retention → restoration of BP.

How are short-term and long-term BP mechanisms integrated during hypovolemic shock from bleeding?

Acute bleeding lowers blood volume and BP. The baroreceptor reflex rapidly increases sympathetic activity, HR, contractility, and vasoconstriction, while decreased renal perfusion activates RAAS, producing additional sympathetic activity and vasoconstriction plus aldosterone/ADH-mediated salt and water retention. Together these mechanisms increase CO, TPR, blood volume, and BP.