II | cardiac output and hemodynamics

blood vessel structure

endothelium: layer of cells that lines blood vessels and regulates exchanges

smooth muscle: regulates diameter of lumen

elastic fibers: in all 3 layers (tunica externa, media, and intima/interna), recoil moderates pressure changes

arteries vs veins

arteries:

• function: carry oxygenates blood

• wall: thick, muscular, elastic to withstand high pressure

veins:

• function: carry deoxygenated blood

• wall: thinner, less muscular, less elastic

different types of blood vessels

large arteries: carry oxygenated blood away from the heart to various parts of the body

• thick, elastic walls

• muscular for pressure regulation

• ex: aorta, pulmonary arteries

mid-sized arteries: distribute blood to specific regions and organs

• muscular walls, less elastic

• connect arteries to arterioles

arterioles: regulate blood flow into capillaries

• smaller, muscular vessels

• control blood flow via smooth muscle contraction

• connect arteries to capillaries

capillaries: facilitate exchange of gases and nutrients with tissues

• single-cell layer walls for easy diffusion

• small diameter, extensive network

• connect arterioles to venules

• smallest blood vessel

• do NOT contain smooth muscle

venules: connect blood from capillaries

• smaller than veins

• merge to form larger veins

mid-sized veins: collect blood from venules, transport toward heart

• thinner walls compared to arteries

large veins: carry deoxygenated blood back to the heart

• thinner walls than arteries

• ex: superior and inferior vena cava

hemodynamics

hemodynamics: mechanical and physical forces that determine blood flow

blood pressure

blood pressure (BP): force exerted on a given area of vessel wall by blood (mmHg)

• force mainly generated by ventricular contraction

• area = diameters of vessel

  • ↑ area (vasodilation) → ↓ resistance → ↓ bp

  • ↓ area (vasoconstriction) → ↑ resistance → ↑ bp

pressure (P) = force / area

pulse pressure - systolic pressure - diastolic pressure

mean arterial pressure (MAP) = diastolic pressure + 1/3 pulse pressure

compliance vs elasticity

compliance: a measure of how easily a structure changes its volume in response to a change in pressure

• volume and pressure are inverse

• arterial compliance maintains blood flow during systole

• accommodates volume changes

elasticity: ability of a blood vessel to expand and recoil (and return back to original state)

• arterial elasticity maintains blood flow during diastole

• maintains blood flow and pressure

compliance and elasticity are more prominent in aorta and large arteries and are mainly due to vascular elastic fibers

*the aorta and arteries sustain the driving pressure for blood flow during ventricular relaxation to keep blood moving continuously

resistance

resistance: the friction encountered by blood that opposes flow

• affected by: radius, viscosity (blood composition), length

• resistance opposes flow (as resistance increases flow decreasing)

poiseuille’s law: vascular resistance R = Lη / r ⁴

• L = length of vessel

• η = viscosity of blood

• r = radius of vessel (vasoconstriction vs vasodilation)

capillary exchange

hydrostatic pressure: physical pressure that fluid exerts on the structures around it (ex: capillary wall)

colloid osmotic pressure: dissolved particles that cannot diffuse across the capillary wall (ex: plasma, proteins) which “pull” water toward them

net filtration pressure = pressures to filtrate - pressures to reabsorb

→ [ capillary hydrostatic pressure (CHP) + interstitial fluid osmotic pressure (IFCOP) ] - [ interstitial fluid hydrostatic pressure (IFHP) + blood colloid osmotic pressure (BCOP) ]

short-term regulation of blood pressure by nervous system

baroreceptors initiate negative feedback mechanisms in response to changes in blood pressure

↑ blood pressure: stimulate various baroreceptors, excite the cardioinhibitory center, inhibit the cardioacceleratory and vasomotor centers, decrease heart rate, and cause vasodilation to ultimately lower blood pressure

↓ blood pressure: baroreceptors are inhibited, exciting the cardioacceleratory and vasomotor centers, inhibiting the cardioinhibitory center, increasing heart rate, and causing vasoconstriction to increase blood pressyre

short-term regulation of blood pressure by endocrine system

blood osmolarity: osmoreceptors in hypothalamus detect increase in blood osmolarity → posterior pituitary is stimulated to secrete ADH (aka vasopressin) → ADH binds to receptors in the kidney, stimulates insertion of aquaporins in collecting duct, which increases H2O reabsorption → more water adjusts osmolarity of blood (left image)

juxtaglomerular apparatus (JGA) for low blood volume/pressure: kidney synthesizes, stores, and secretes the enzyme renin → renin converted angiotensinogen to angiotensin II

→ binds to receptors in adrenal gland → stimulates the secretion of aldosterone, which increases Na+ and H2O reabsorption → ↑ blood volume

→ contracts vascular smooth muscle of arterioles → ↑ blood pressure

effects of cardiovascular disease and aging on circulation

arterial stiffening (hardening): reduced elasticity of arteries → increased afterload on the heart, contributing to hypertension and impaired blood flow

decreased elasticity of heart tissues: reduced compliance of heart muscle → impaired filling of the heart chambers, potentially leading to heart failure

reduced cardiac output: decline in the heart’s ability to pump blood → decreased blood flow to tissues and organs

valve changes: thickening and stiffening of heart valves → impaired valve function, potentially leading to regurgitation or stenosis

decline in baroreceptor sensitive: reduced responsiveness of baroreceptors in blood vessels → impaired ability to regulate blood pressure in response to changes

increased risk of atrial fibrilation: higher likelihood of irregular heart rhythms → elevated risk of stroke due to blood clot formation