Unit 4 Vasculature and Blood Pressure

Tunica intima

Innermost layer of a blood vessel, consisting of a single layer of endothelium.

Tunica media

Middle layer of a blood vessel that changes the size of vessels and contains smooth muscle, playing a key role in blood pressure and blood flow.

Tunica externa

Outer layer of a blood vessel, made of loosely woven collagen that protects and reinforces blood vessels.

Arteries

Vessels that carry blood away from the heart, characterized by a high pressure system, three tunics, small lumen, thick walls, more muscle and elastin fibers, and no valves.

Capillaries

Vessels that exchange nutrients, gases, water, and solutes between blood and tissues or alveoli, featuring a very tiny lumen, thin walls (one tunic), large cross-sectional area, and no elastin or muscle fibers or valves.

Veins

Vessels that carry blood toward the heart, characterized by a low pressure system, three tunics, large lumen, thin walls, fewer muscle and elastin fibers, and valves to prevent backflow.

Compliance

The ability of a blood vessel to stretch.

Capacitance

The ability of a blood vessel to hold a large volume under low pressures.

Elastic artery

Large vessels such as the aorta and pulmonary artery that deliver blood to organs or lungs, featuring thick walls with elastin, smooth muscle, and connective tissue. Walls absorb high pressure blood flow.

Muscular artery

Medium-sized vessels that deliver oxygenated blood to organs, characterized by moderately thick walls with a moderate amount of elastin, smooth muscle, and connective tissue.

Arteriole

Smallest branches of arteries that function as the site of highest resistance to blood flow, with smooth muscle that constricts or dilates in response to the sympathetic nervous system.

alpha 1-stimulation

constricts arterioles (vasconstriction)

beta-2 stimulation

dilates arterioles (vasodilation)

Continuous capillary

Capillaries located in skin, muscle, and brain, allowing passage of lipid-soluble molecules across the lipid membrane and very small water-soluble molecules through intercellular clefts, with an intact basement membrane.

Fenestrated capillary

Capillaries located in the small intestine and kidney, characterized by fenestrations that allow passage of small molecules.

Sinusoidal capillary

Capillaries located in the liver, bone marrow, and spleen, with large fenestrations and an incomplete basement membrane allowing for exchange of large molecules, including whole cells.

High compliance

Characteristic of veins, indicating their ability to stretch and hold large volumes.

Low compliance

Characteristic of arteries, indicating their reduced ability to stretch.

High capacitance

Characteristic of veins, indicating their ability to hold large volumes under low pressures.

Low capacitance

Characteristic of arteries, indicating their reduced ability to hold large volumes under low pressures.

Velocity of blood flow

Cross-sectional area of a vessel is inversely related to the velocity of blood.

V

Velocity (cm/sec)

Q

Blood flow (mL/sec)

A

Cross sectional area

High cross sectional area

Capillary beds. Results in slow blood velocity allowing for complete exchange of nutrients and waste.

Ohm's Law

Describes the relationship between blood flow, pressure, and resistance.

Q=∆P/R

Q: blood flow, ∆P: change in pressure (pressure gradient), R: resistance to blood flow.

Pressure gradient

Directly related to blood flow.

Resistance

Inversely related to blood flow.

Poiseuille's Law

Describes the relationship between resistance & the radius and length of the vessel or tube.

R=8nl/πr4

R: resistance, N: viscosity of blood, L: length of vessel, r4: radius of vessel raised to the 4th power.

Resistance factors

Resistance is directly proportional to viscosity and length of the vessel; inversely proportional to the radius of the vessel.

Laminar flow

Layers of fluid moving in series, with each layer having a different velocity of flow, normal conditions.

Turbulent flow

During disease, blood flows in different directions, resulting in heart murmur.

Reynolds number

NR = pdv/n

NR

Reynolds number

p

Density of blood

d

Diameter of blood vessel

n

Viscosity of blood

Turbulence factors

Velocity: directly proportional to turbulence; Viscosity: inversely related to turbulence.

Ventricular septal defect

Small VSD=high velocity flow=increased turbulence=louder heart murmur; Large VSD=lower velocity flow=decreased turbulence=quieter murmur.

What is the equation that represents blood pressure?

Blood pressure (BP) is a product of cardiac output (CO) and vascular resistance (VR): BP = CO x VR.

What must be maintained at a constant level throughout the cardiovascular system?

Blood flow (Q) must be maintained at a constant level.

What happens to mean arterial blood pressure as arteries get smaller?

Mean arterial blood pressure is highest in large arteries and falls as arteries get smaller due to increased friction resistance.

What are the characteristics of elastic arteries?

Elastic arteries have low compliance and capacitance, and they hold a large volume of blood.

What is the role of arterioles in the cardiovascular system?

Arterioles have the highest resistance, which lowers blood flow (Q) and increases the pressure gradient. Keeps constant blood flow (Q).

What is the significance of capillaries in blood flow?

Capillaries have high frictional resistance and are sites of fluid loss.

What are the characteristics of venules and veins?

Venules and veins have high capacitance and low resistance.

What generates systolic pressure?

Systolic pressure is generated when ventricles pump blood into arteries, creating the highest arterial pressures during systole.

What occurs during diastolic pressure?

During diastolic pressure, ventricles relax, and pressure falls to the lowest level as blood is no longer ejected. Elastic properties of the aorta allow for continued movement of blood. Elastic recoil pushes blood forward.

What is the formula for calculating mean arterial pressure (MAP)?

MAP = (BPs + BPd + BPd)/3, where BPs is systolic blood pressure and BPd is diastolic blood pressure.

Mean pressure

Average pressure in a complete cardiac cycle and is the driving force for perfusion. Something to monitor under anesthesia.

What does pulse pressure represent?

Pulse pressure is the difference between systolic and diastolic blood pressures and represents stroke volume.

BPsystolic - BPdiastolic

stroke volume

volume of blood pumped out of the heart with each beat

What is subaortic stenosis?

a congenital heart disease characterized by narrowing of the outflow tract from the left ventricle to the aorta, resulting in a weak pulse.

What are the types of pulse characteristics?

Pulse can be normokinetic (adequate, strong), hyperkinetic (bounding), or hypokinetic (weak, thready).

What causes a hyperkinetic pulse?

A hyperkinetic pulse can be caused by conditions such as patent ductus arteriosus (PDA) or aortic regurgitation.

What causes a hypokinetic pulse?

A hypokinetic pulse can be caused by subaortic stenosis or hypovolemia.

How do pressures in the pulmonary circulation compare to systemic pressures?

Pressures in the pulmonary circulation are analogous to systemic pressures (same pattern) but are much lower, about 1/5th of systemic pressures.

Why is pulmonary resistance lower than systemic vascular resistance?

Pulmonary resistance is lower to maintain the same flow rates at lower pressures in the pulmonary circulation.

What factors influence blood pressure alterations?

Cardiac output, vascular resistance, stroke volume, heart rate, preload, afterload, contractility, and autonomic balance.

What are the two main divisions of the nervous system?

Central Nervous System (CNS) and Peripheral Nervous System (PNS).

What is the role of the autonomic nervous system?

It controls involuntary movements of smooth muscles and glands.

What are the two branches of the autonomic nervous system?

Sympathetic and Parasympathetic.

What is the sympathetic nervous system responsible for?

The 'fight or flight' response, which increases heart rate and blood pressure.

What is the parasympathetic nervous system responsible for?

The 'rest and digest' response, which decreases heart rate and blood pressure.

How does the baroreceptor reflex function?

It detects blood pressure changes and sends signals to the brain to adjust autonomic balance. Immediate -- sec/mins

Where are arterial baroreceptors located?

In the aortic arch and carotid sinus.

What cranial nerves are involved in the baroreceptor reflex?

Glossopharyngeal (CN IX) and Vagus (CN X) nerves to the nucleus tractus solitarius in the medulla oblongata.

What happens when blood pressure suddenly increases?

Baroreceptors detect vessel stretch, increasing their discharge frequency, which signals the brain to adjust autonomic balance. Mediates a reflex bradycardia, arteriolar vasodilation, and decreased contractility.

Cardiovascular center in the Pons

Stimulated or inhibited to adjust parasympathetic & sympathetic balance in the ANS.

Vasomotor control center, Cardiac control center (cardiac accelerator & cardiac decelerator)

Cardiac control center

Cardiac accelerator (increases HR & cardiac contracility -- sympathetic) and cardiac decelerator (decreases HR -- parasympathetic)

Vasomotor control center

Controls diameter of blood vessels

What is the effect of decreased baroreceptor signaling during low blood pressure?

It results in increased sympathetic tone and decreased parasympathetic tone, leading to increased heart rate and vasoconstriction and contractility.

What is the renin-angiotensin-aldosterone system (RAAS)?

A hormonal system that regulates blood pressure and fluid balance. Hormonally mediated. Long term -- mins/hrs

What does the juxtaglomerular apparatus consist of?

Juxtaglomerular cells, afferent arteriole of the glomerulus, macula densa cells

What triggers the release of renin from juxtaglomerular cells?

Low blood pressure detected by mechanoreceptors in the arteriolar wall.

Arteriolar baroreceptors detect low pressures and stimulate increased sympathetic flow which increases renin release.

Macula densa cells in the DCT monitor Na in the filtrate.

What is the role of angiotensin II in blood pressure regulation?

It constricts efferent arteriole, increases Na+ and water reabsorption in PT, stimulates increased thirst and ADH release, stimulates aldosterone release and norepinephrine release, systemic vasoconstrictor.

What does renin do?

Converts angiotensinogen to angiotensin I.

What does angiotensin converting enzyme (ACE) do?

It converts angiotensin I to angiotensin II.

How does ADH contribute to blood pressure regulation?

By inserting aquaporins in the distal convoluted tubule to promote water reabsorption.

What is the effect of aldosterone on blood pressure?

It increases sodium and water reabsorption in the distal convoluted tubule and collecting ducts. And potassium excretion.

What is the relationship between blood pressure and renal blood flow?

Low blood pressure leads to low renal blood flow, which stimulates renin release.

What are the immediate effects of the baroreceptor reflex?

It adjusts heart rate and vascular resistance within seconds to minutes.

What are the long-term effects of the renin-angiotensin-aldosterone system?

It regulates blood pressure over minutes to hours through hormonal changes.

What controls blood flow through the capillary bed?

Smooth muscle in arterioles.

What is the role of precapillary sphincters?

They can constrict or relax to control blood flow to tissues in capillary beds.

What is a metarteriole?

A vessel that serves as a vascular shunt when precapillary sphincters are closed.

What happens when precapillary sphincters are vasodilated?

They are relaxed, allowing increased blood flow through capillary beds.

What occurs when precapillary sphincters are vasoconstricted?

They are closed, reducing blood flow through capillary beds.

What mediates the individual blood flow needs of tissues?

Local vasodilators and vasoconstrictors.

Vasodilators

Low tissue O2, high CO2, histamine, adenosine, and bradykinin.

Vasconstrictors

Angiotensin II, epinephrine/norepinephrine, endothelin, and myogenic autoregulation.

What is microcirculation?

The exchange of substances across capillaries via simple diffusion and osmosis.

What types of substances diffuse through capillaries?

Lipid-soluble substances (O2, CO2) diffuse through cells; water-soluble substances (water, ions, glucose, amino acids) diffuse between cells.

Can proteins pass through capillaries?

Small proteins can pass through fenestrations. Large proteins (albumin & globulin) are generally to large to pass between cells in continuous and fenestrated capillaries. They remain in the vessel or pass through via vesicles.

What drives fluid exchange across capillaries?

Hydrostatic and oncotic pressures.

What happens at the arterial end of a capillary?

Fluid exits the capillary, resulting in net outflow (filtration). Hydrostatic pressure (out) is greater than oncotic pressure.

What occurs at the mid-capillary?

There is no net movement; hydrostatic pressure equals oncotic pressure.

What happens at the venous end of a capillary?

Fluid reenters the capillary, resulting in net inflow (absorption). Oncotic pressure is greater than hydrostatic pressure.