BIO 304 · Human Anatomy & Physiology
Blood Vessels & Hemodynamics
Cardiovascular System · Module 11
A reference for the Blood Vessels video. Blood pressure is generated by the heart and distributed by the vessels. Resistance is mostly set at the arterioles. Capillary exchange is where the work actually happens.
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- Compare the structure and function of arteries, capillaries, and veins.
- Explain how cardiac output, peripheral resistance, and blood volume determine arterial pressure.
- Describe the Starling forces that govern capillary fluid exchange.
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Vessel Types
Three tunics (in all vessels)
- Tunica intimaendothelium + thin connective tissue; smooth inner surface
- Tunica mediasmooth muscle + elastic fibers; thickest in arteries
- Tunica externaconnective tissue; anchors vessel; thickest in veins
Arteries & arterioles
- Elastic (conducting) arteriesaorta and major branches; recoil dampens pulsatile pressure
- Muscular (distributing) arteriesdeliver blood to organs; thick muscular media
- Arteriolessmall; smooth muscle controls diameter; principal site of resistance
- Vasoconstriction / vasodilationchanges resistance and redirects blood
Capillaries
- Single endothelial layerthin enough for diffusion
- Continuoustight; most tissues (muscle, skin)
- Fenestratedpores; kidney, intestine, endocrine glands
- Sinusoidalgaps; liver, spleen, bone marrow
- Precapillary sphincterscontrol flow into capillary bed
Venules & veins
- Thin wallslow pressure; thinner media than arteries
- Larger lumenholds more blood (60-70% of total blood volume)
- Valvesprevent backflow; especially in limbs
- Skeletal muscle pumpmuscle contraction squeezes veins toward heart
- Respiratory pumpthoracic pressure changes aid venous return
Hemodynamics
Blood pressure basics
- Systolicpeak pressure during ventricular systole (~120 mmHg)
- Diastolicminimum pressure during diastole (~80 mmHg)
- Pulse pressuresystolic minus diastolic
- Mean arterial pressure (MAP)diastolic + 1/3 pulse pressure
- Required MAP> 60 mmHg to perfuse vital organs
What sets pressure
- BP ≈ CO × SVRcardiac output times systemic vascular resistance
- Cardiac outputHR × stroke volume
- Resistancemostly arteriolar tone; also blood viscosity and vessel length
- Blood volumelow volume = low pressure (hemorrhage, dehydration)
Control of pressure
- Baroreceptorscarotid sinus & aortic arch; respond second-by-second; alter ANS output
- Chemoreceptorsdetect O₂, CO₂, pH; modulate respiration and BP
- Renin-angiotensin-aldosterone (RAAS)long-term; kidney pulls strings on Na+/water and vascular tone
- ADHlow BP triggers ADH; water retention
- ANPatria stretched by high volume; promotes Na+/water loss
Starling forces at capillary
- Hydrostatic pressure (HP)pushes fluid out of capillary; higher at arteriolar end
- Oncotic pressure (OP)plasma proteins (mainly albumin) pull fluid back in
- Net filtration at arteriolar endHP > OP → fluid leaves capillary
- Net reabsorption at venular endHP < OP → fluid returns
- Lymphaticspick up the small excess that does not return
When Starling fails → edema
- High capillary HPheart failure, venous obstruction
- Low plasma OPliver disease (low albumin), nephrotic syndrome
- Increased capillary permeabilityinflammation, sepsis
- Lymphatic obstructionlymphedema (filariasis, post-mastectomy)
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