Physiology Quiz on Cardiovascular System

Blood Pressure and Control Mechanisms

• Pressure in the cardiovascular (CV) system is a vital factor
• Factors influencing blood pressure include peripheral resistance and cardiac output
• Volume control is crucial for blood pressure homeostasis
• Homeostatic control of cardiovascular functions is managed by the autonomic nervous system and the renin-angiotensin-aldosterone system (RAAS)
• Endothelial factors, nitric oxide (NO), and prostacyclin (PGI2) are also involved
• Natriuretic peptides play a role in blood pressure regulation

Blood Pressure

• Blood pressure is generated by ventricular contraction, causing pulsatile surges in arteries
• Systolic and diastolic pressures are measured, for example, 120mm Hg/80 mm Hg (120/80)
• Elastic rebound in arteries maintains pressure

Pulse and Mean Arterial Pressures

• Systolic pressure is the peak pressure during ventricular contraction
• Diastolic pressure is the minimum pressure during ventricular relaxation
• Pulse pressure is the difference between systolic and diastolic pressures
• Mean arterial pressure (MAP) is the average pressure throughout the cardiac cycle

Role of Arteries

• During ventricular contraction, the semilunar valve opens, allowing blood to enter the aorta and arteries
• Arteries expand and store pressure in their elastic walls
• During ventricular relaxation, the semilunar valve closes, and elastic recoil of the arteries propels blood forward through the circulatory system.

Factors Affecting Blood Pressure

• Cardiac output is a key factor
• Peripheral vascular resistance is another key factor
• Blood volume is also a factor influencing blood pressure

Mean Arterial Pressure (MAP)

• MAP is the average arterial pressure during a cardiac cycle
• The formula for MAP is: Diastolic pressure + (1/3)Pulse Pressure

Mean Arterial Pressure (Important Equations)

• Mean arterial pressure (MAP) is calculated as cardiac output (CO) multiplied by peripheral vascular resistance (PVR)
• Cardiac output (CO) is determined by stroke volume (SV) and heart rate (HR)

Factors Affecting Cardiac Output

• Heart rate (HR) is influenced by autonomic innervation, hormones, fitness levels, and age
• Stroke volume (SV) depends on heart size, fitness levels, gender, contractility, preload (end-diastolic volume), and afterload (resistance)

Factors Affecting Stroke Volume

• Contractility, preload (stretch during diastole), and afterload (pressure against ejection) are key factors determining stroke volume
• An increase in any of these factors generally results in a higher stroke volume

Factors Affecting Contractility

• Starling's Law of the Heart describes the relationship between preload and contractility.
• Increased contractility is often seen with adrenergic agonists
• Decreased contractility can be a sign of heart failure

Pressure, Flow, and Resistance

• Fluid flows from high to low pressure
• Resistance depends on vessel length, vessel radius, and blood viscosity.
• A smaller radius leads to a much larger increase in resistance.

Resistance to Blood Flow (Poiseuille equation)

• The Poiseuille equation describes the relationship between resistance (R), viscosity (η), length (L), and radius (r) of a blood vessel
• R=(8nL)/(πr^4)

Effect of Blood Volume on Blood Pressure

• Blood volume is approximately 5 liters
• Arteries contain a smaller percentage of the total blood volume compared to veins.
• Increased blood volume (hypervolemia) results in increased blood pressure.
• Decreased blood volume (hypovolemia) results in decreased blood pressure

Factors Affecting Blood Volume

• Oedema is the accumulation of fluid in interstitial space due to disruptions in fluid exchange
• Increased hydrostatic pressure, decreased plasma protein concentration, and increased interstitial proteins can disrupt fluid balance.

Questions

• Edema in the case of left ventricular failure is most likely to occur in the lower extremities or abdomen due to fluid buildup from inadequate pumping.
• Malnutrition can lead to ascites—or fluid buildup in the abdomen—because malnutrition can lead to decreased plasma protein concentration, causing lower osmotic pressure.
• Lower osmotic pressure causes fluid leakage from blood vessels to tissue compartments.

Vascular Tone

• Vascular tone refers to a blood vessel's constriction relative to its maximum dilation—the widest the vessel can be.
• All arterial and venous vessels under basal conditions exhibit some smooth muscle contraction
• Vascular tone is influenced by the composition of the vessels and control by intrinsic and extrinsic factors.

Smooth Muscle Contraction

• Extrinsic and intrinsic mechanisms influencing vasoconstriction and vasodilation involve signal transduction pathways
• Calcium changes in smooth muscle cells are linked to contraction or relaxation

Cardiovascular Physiology (Control Mechanisms)

• Nervous, cytokine, and autoregulatory mechanisms influence the cardiovascular system
• Autonomic nervous system (ANS) and renin-angiotensin-aldosterone system (RAAS) are key components
• Nitric oxide (NO) and prostaglandins (PGs) are also significant factors

Baroreceptors

• Baroreceptors are stretch-sensitive mechanoreceptors located in the aortic arch and carotid sinus.
• Increased or decreased blood pressure leads to baroreceptor firing rate changes
• Firing rate changes alter sympathetic and parasympathetic activity to regulate blood pressure.

Autonomic Nervous System

• The sympathetic nervous system (SNS) is associated with "fight or flight" responses, often increasing HR
• The parasympathetic nervous system (PNS) promotes "rest and digest," typically decreasing HR.

What effect will the sympathetic nervous system have on?

• Heart rate: increase
• Force of contraction of the heart: increase
• Blood pressure: increase

Systems work to oppose each other

• Sympathetic (SNS) nerves increase heart rate through chronotropy and inotropy (force of contraction) and also increase dromotropy (impulse conduction), while excitability increases
• Parasympathetic (PNS) nerves decrease heart rate, whereas dromotropy, inotropy, and lusitropy decrease.

Innervation of the Heart

• The sympathetic nervous system innervates the atria, sinoatrial (SA) node, and ventricles.
• The vagus nerve (X) innervates the heart, but sometimes overlaps in function with sympathetic nerves

Adrenergic receptors

• Alpha1 receptors are primarily involved in vasoconstriction
• Beta1 receptors are primarily involved in increasing heart rate and contractility
• Beta2 receptors are primarily involved in vasodilation

Muscarinic receptors

• M2 receptors primarily affect heart rate, including decreasing heart rate and contractility.

Blood Vessels

• Alpha1 receptors are primarily involved in vasoconstriction; Beta2 receptors are primarily involved in vasodilation.

Renin-Angiotensin-Aldosterone System (RAAS)

• RAAS is a hormonal system that regulates blood pressure.
• Renin is released in response to decreased blood pressure and initiates a cascade of events, ultimately contributing to blood pressure regulation
• ACE converts angiotensin I to angiotensin II, which is a potent vasoconstrictor and stimulates the release of aldosterone

Angiotensin-Converting Enzyme (ACE)

• ACE converts angiotensin I to angiotensin II
• ACE is inhibited by medications like captopril
• ACE2 is involved in COVID-19 pathogenesis

Angiotensin II

• Angiotensin II acts via AT1 and AT2 receptors
• Angiotensin II causes vasoconstriction, ADH release, increased sympathetic activity, and increases sodium/water reabsorption.
• AT1 receptors are primary for vasoconstriction and for stimulating ADH release

Aldosterone

• Aldosterone is a mineralocorticoid hormone
• Primary actions include increasing sodium reabsorption and secretion of potassium.
• Aldosterone release is stimulated by angiotensin II

Natriuretic Peptides

• Natriuretic peptides (ANP and BNP) are released by the heart in response to stretch
• ANP and BNP act to oppose the effects of Angiotensin II; they increase natriuresis and diuresis (increasing sodium and water excretion)
• BNP is a biomarker for heart failure.

Nitric Oxide (NO)

• NO is a vasodilator
• NO is produced in response to stimulation by acetylcholine, histamine, bradykinin, serotonin, shear stress and other stimuli.
• NO acts through increasing cyclic GMP, leading to smooth muscle relaxation

Endothelin

• Endothelin is a potent vasoconstrictor
• Endothelin-1 is primarily responsible for cardiovascular effects.
• Endothelin effects are largely opposite to those of NO.

Prostaglandin 12 (PGI2)

• PGI2 is produced from arachidonic acid
• PGI2 is a vasodilator and inhibits platelet aggregation