Blood Vessels

All blood vessels (arteries, veins, capillaries) have a….

Vessel walls are made of… what are the layers ?

lumen (the hollow space where blood flows).

Vessel walls are made of tunics (layers):

• Tunica intima (innermost): smooth lining of simple squamous epithelium + connective tissue → helps reduce friction.

• Tunica media (middle): smooth muscle + elastic fibers → controls diameter of vessel (vasoconstriction = narrow, vasodilation = wide). thicker in arteries

• Tunica externa (outermost): connective tissue with elastic + collagen fibers → protects and anchors vessel. thicker in veins

  • Big vessels may have their own blood supply here (vasa vasorum).

Difference between (what type of tunica, how is their lumen)

arteries 

veins 

capillaries 

which ones have valves ?

• Arteries

  • Thick tunica media, narrow lumen.

  • More elastic and collagen fibers → strong + resilient.

  • Resist changes in blood pressure.

• Veins

  • Thicker tunica externa, wide lumen.

  • Less elastic and collagen.

  • Many have valves to stop backflow (esp. in limbs).

• Capillaries

  • Only have tunica intima (super thin wall).

  • Perfect for gas/nutrient/waste exchange.

types of arteries

elastic

muscular

arterioles

• Elastic (conducting) arteries: largest, lots of elastic fibers (ex: aorta, pulmonary trunk). Stretch + recoil to keep blood moving.

• Muscular (distributing) arteries: medium size, more smooth muscle, control blood flow to regions (ex: brachial artery, coronary arteries).

• Arterioles: smallest, regulate blood pressure and flow via vasomotor tone (baseline constriction controlled by brainstem).

Capillaries connect what to what?

Types:

What are capillary beds? What are they controlled by?

• Connect arterioles to venules; RBCs move in single file.

• Types:

  • Continuous: most common, small gaps for water/nutrients (muscle, skin, CNS).

  • Fenestrated: pores for fluid exchange (kidneys, intestines).

  • Sinusoids: big gaps, allow proteins & cells through (bone marrow, spleen, liver, endocrine glands).

• Capillary beds: networks of capillaries. Controlled by precapillary sphincters (open/close → regulate blood flow). Only ~25% of capillary beds are open at once.

Veins

What are venules?

Medium & large veins have?

Veins act as what?

• Venules: smallest veins, merge into bigger veins.

• Medium & large veins: many have valves + rely on muscle contractions to move blood back to the heart.

• Veins act as blood reservoirs (hold ~70% of blood at rest). They can constrict to push blood back into circulation when needed.

What is the pathway of blood flow?

What are some alternative pathways?

Large arteries “elastic” → mediam arteries “muscular”→ arteriole → capillary bed → venule → medium vein → large vein (e.g., spleen circulation).

Alternative pathways:

• Arterial anastomosis: 2+ arteries supply the same region (backup blood supply).

• Venous anastomosis: 2+ veins drain same region (very common).

• Arteriovenous anastomosis (shunt): artery connects directly to vein (bypasses capillaries; useful in fingers, toes when cold).

• Portal system: 2 capillary beds in sequence (e.g., hepatic portal system between intestines → liver).

What is the cross-sectional area?

Which has a larger cross-sectional area? Capillaries or veins?

The greater the cross-sectional area, the slower….

• Cross-sectional area = lumen diameters added up for all vessels of a type.

• Capillaries have the largest total cross-sectional area (so many of them).

• Rule: The greater the cross-sectional area → the slower the blood moves.

  • Slowest in capillaries → allows time for nutrient & gas exchange.

capillaries are site of ….

What are the 3 processes?

Capillaries = exchange site (O₂, CO₂, nutrients, wastes, hormones).

3 Processes:

1. Diffusion → substances move high → low concentration.

   • O₂, nutrients, hormones go out of blood.

   • CO₂, wastes go into blood.

   • Small solutes = slip through cells/clefts.

   • Large solutes = use fenestrations/sinusoids.

2. Vesicular transport → vesicles move hormones or fatty acids across endothelial cells.

3. Bulk Flow = movement of fluid + solutes down pressure gradient.

   • Filtration (arteriole end): fluid pushed out.

   • Reabsorption (venule end): fluid pulled back in.

What is hydrostatic pressure (HP)?

Describe the following:

• Blood HP (HPb)

• Interstitial HP (HPif)

• Colloid osmotic pressure (COP)

• Blood COP (COPb)

• Interstitial COP (COPif)

• Hydrostatic pressure (HP) = pushing force.

  • Blood HP (HPb): pushes out of capillaries.

  • Interstitial HP (HPif): usually near zero.

• Colloid osmotic pressure (COP) = pulling force (due to proteins).

  • Blood COP (COPb): pulls fluid into blood.

  • Interstitial COP (COPif): near zero.

How do you calculate Net Filtration Pressure ?

NFP = (Hydrostatic P difference) − (Osmotic P difference).

• At arteriole end: HP > COP → filtration dominates. (+)

• At venule end: COP > HP → reabsorption dominates.(-)

Do all capillaries fill up at once?

Not all capillaries filled at once → the body prioritizes

How does the lymphatic system relate to fluid?

what happens if the lymphatic system fails ?

• Not all fluid is reabsorbed (about 15% left behind).

• Lymphatic system picks up excess fluid → filters → returns to venous circulation.

• If lymph system fails → swelling/edema.

Explain these regulation factors.

• Degree of vascularization

• Myogenic response

• Local short-term regulation

• Total flow = cardiac output.

• Degree of vascularization (more vessels = more flow, e.g., brain vs. tendons).

  • Angiogenesis = new vessel growth (exercise, fat gain, blocked coronaries).

  • Tumors hijack angiogenesis to grow.

• Myogenic response = vessels auto-adjust to maintain flow (stretch → contract; relax → dilate).

• Local short-term regulation

  • Vasodilators: ↑ flow (low O₂, high CO₂, lactic acid, H⁺, K⁺, nitric oxide, histamine).

  • Vasoconstrictors: ↓ flow (thromboxanes, leukotrienes).

  • Reactive hyperemia: big flush of blood after deprivation (cold hands → warm room).

• Total flow = cardiac output.

  • Rest ~5.25 L/min; increases with exercise.

Regulation of total flow depends on both the heart and the vessels

Gradient drives flow; is it highest in arteries or veins? Is it the lowest in arteries or veins?

How do you calculate pulse pressure?

How do you calculate mean arterial pressure?

Gradient drives flow; highest in arteries, lowest in veins.

• Systolic (contraction): top number (e.g., 120 mmHg).

• Diastolic (relaxation): bottom number (e.g., 80 mmHg).

• Pulse pressure = systolic − diastolic (reflects artery elasticity).

• Mean Arterial Pressure (MAP) = diastolic + ⅓ pulse pressure

  • MAP < 60 → poor perfusion.

• Capillaries: smooth, steady pressure; high enough for exchange but not damaging.

What is venous pressure?

How does it move?

What helps to get blood back to the heart?

• Venous pressure is the pressure of the blood in the veins as it returns to the heart. low and steady (not pulsing like arteries).

• Needs help to get blood back to the heart (against gravity).

• Helpers:

  • Pressure gradient – from heart contraction.

  • Skeletal muscle pump – muscles squeeze veins when contracting → valves keep blood from flowing backward.

  • Respiratory pump – breathing creates pressure changes:

    • Inspiration: ↑ abdominal pressure + ↓ thoracic pressure → blood pushed up toward chest.

    • Expiration: ↑ thoracic pressure + ↓ abdominal pressure → blood pushed into heart.

  • Venous valves – prevent backflow.

  • Exercise increases return; inactivity = blood pooling (esp. legs).

What is a systematic gradient?

What does higher cardiac output lead to?

• Systemic gradient = difference between arterial pressure near heart and pressure in vena cava.

• Driving force of blood flow.

• Higher cardiac output = higher gradient = more blood flow.

What is circulatory shock?

What are the causes?

• When tissues don’t get enough blood.

• Causes:

  • Heart problems (pump failure).

  • Low venous return (bleeding, dehydration, allergic reaction, blocked vein).

  • Venous pooling (immobility, too much dilation).

  • Toxins or brainstem injury (loss of vasomotor control).

Resistance depends on what?

Resistance depends on:

• Vessel length: Longer vessels = ↑ resistance; shorter vessels = ↓ resistance.

• Vessel lumen diameter: Narrow diameter = ↑ resistance; wide diameter = ↓ resistance.

• Blood viscosity: Thicker blood (more cells, proteins) = ↑ resistance.

• Turbulence: More swirling = ↑ resistance. Smooth flow = ↓ resistance.

• Vessel radius (most important): Small radius → more resistance. Flow is proportional to radius⁴ (F ∝ r⁴) → small changes = HUGE effect.

• Example: radius ↑ from 1 mm → 2 mm = 16× more flow.

What is resistance?

Friction blood faces in vessels.

Flow, Pressure, Resistance Relationship; calculation 

• Formula: F ∝ ΔP / R

  • Flow = Pressure gradient ÷ Resistance.

  • ↑ gradient → ↑ flow.

  • ↑ resistance → ↓ flow.

blood pressure regulation

Describe neural regulation.

Where is the cardiovascular center?

What does the vasomotor center do?

What do baro- and chemoreceptors do?

Where are the high brain centers located?

• Short-term: autonomic reflexes nervous system (neural reflexes).

• Long-term: Hormones, kidney control.

Neural Regulation

• Cardiovascular center (medulla):

  • Cardiac center: controls heart rate & output.

    • Sympathetic → ↑ HR, ↑ force (↑ BP).

    • Parasympathetic → ↓ HR (↓ BP).

  • Vasomotor center: controls vessel diameter.

    • Sympathetic → NE/EPI release:

      • α1 receptors = vasoconstriction (↑ BP).

      • β2 receptors = vasodilation (skeletal muscles, heart).

• Baroreceptors (stretch sensors):

  • Found in aortic arch & carotid sinuses.

  • If BP drops → sympathetic ↑, vessels constrict, HR ↑.

  • If BP rises → parasympathetic ↑, HR slows, vessels relax.

• Chemoreceptors: sense CO₂, O₂, pH.

  • High CO₂, low O₂, or low pH → ↑ BP & ↑ breathing.

• Higher brain centers:

  • Hypothalamus (temp, fight-or-flight).

  • Limbic system (emotions).

Describe hormonal regulation.

• Renin-Angiotensin-Aldosterone System (RAAS):

  • Kidneys release renin → activates angiotensin II.

  • Angiotensin II: ↑ BP (vasoconstriction, thirst, aldosterone).

• Aldosterone: retains Na⁺ + water in kidneys → ↑ volume, ↑ BP.

• ADH (vasopressin): water retention, thirst, vasoconstriction → ↑ BP.

• ANP: released by atria when stretched → vasodilation + more urine output → ↓ BP.

Blood Flow Distribution During Exercise

What does blood flow do during exercise?

How does that affect the heart?

Redistribution of blood flow…

• Total blood flow ↑ during exercise.

• Heart: Beats faster & stronger.

• Redistribution of blood flow:

  • Increased → Coronary vessels (heart wall), skeletal muscle (big increase), skin (to release heat).

  • Decreased → Abdomen, kidneys.

Pulmonary circulation 

• Right ventricle → pulmonary trunk → pulmonary arteries → arterioles → pulmonary capillaries (gas exchange).

• Oxygen into blood, CO₂ into alveoli.

• Capillaries → venules → pulmonary veins → left atrium.

General Arterial & Venous Flow

• Arteries:

  • LV → ascending aorta → aortic arch.

  • Branches of arch:

    1. Brachiocephalic trunk → right common carotid & right subclavian.

    2. Left common carotid.

    3. Left subclavian.

  • Arch → descending thoracic aorta → abdominal aorta → splits at L4 into common iliac arteries.

• Veins:

  • Blood returns to right atrium via superior vena cava, inferior vena cava, coronary sinus.

• Carotid arteries:

• Circle of Willis

• Veins

• Thoracic/Abdominal walls

• GI Tract arteries

• Celiac trunk

• Superior mesenteric artery

• Inferior mesenteric artery

• Hepatic portal system

• Carotid arteries:

  • External carotid → face & skull structures.

  • Internal carotid → brain.

• Circle of Willis = arterial anastomosis around brain.

• Veins: Internal jugular, external jugular, vertebral veins → brachiocephalic vein.

• Thoracic/Abdominal walls: Azygos system drains into superior vena cava.

• GI Tract arteries (from abdominal aorta):

  • Celiac trunk → left gastric, splenic, common hepatic.

  • Superior mesenteric artery → small intestine, ascending colon, part of transverse colon.

  • Inferior mesenteric artery → distal transverse, descending colon, rectum.

• Hepatic portal system: GI blood → hepatic portal vein (from gastric, splenic, superior & inferior mesenteric veins) → liver → filtered → hepatic veins → IVC.

Upper & Lower Limb Circulation

• Upper limb arteries: Subclavian → axillary → brachial → radial & ulnar → palmar arches → digital arteries.

• Upper limb veins:

  • Superficial: cephalic, basilic, median cubital.

  • Deep: radial, ulnar → brachial → axillary → subclavian → brachiocephalic.

• Lower limb arteries:

  • External iliac → femoral → popliteal → anterior tibial (→ dorsalis pedis), posterior tibial (→ plantar arteries).

• Lower limb veins:

  • Superficial: great & small saphenous.

  • Deep: anterior tibial, posterior tibial, fibular → popliteal → femoral → external iliac → IVC.

Artery / Drains

Vessel	Branches / Drains Into	Supplies or Carries Blood From

Brachiocephalic Trunk (only on right side)

Splits into: • Right Common Carotid • Right Subclavian

• Right Common Carotid → right side of head & neck • Right Subclavian → right upper limb & some thoracic structures

Left Common Carotid Artery

Direct branch of aortic arch

Supplies left side of head & neck

Left Subclavian Artery

Direct branch of aortic arch

Supplies left upper limb & some thoracic structures

Descending Thoracic Aorta

Several smaller thoracic branches

Supplies thoracic wall & thoracic organs

Descending Abdominal Aorta

Branches to abdominal wall & organs

Supplies abdominal organs & walls

Common Iliac Arteries (L & R, branch of abdominal aorta at L4)

Each splits into: • Internal Iliac Artery → pelvic structures • External Iliac Artery → lower limbs

Supplies pelvis & lower limbs

Vein / Drain

Vein	Formed From / Drains Into	Carries Blood From

Superior Vena Cava

Formed by left & right brachiocephalic veins

Head, neck, upper limbs, thoracic & abdominal wall

Inferior Vena Cava

Formed from veins below the diaphragm

Lower limbs, pelvis, perineum, abdominal structures

Coronary Sinus

Drains directly into right atrium

Myocardium (heart muscle)

Age-Related Cardiovascular Changes

what is arteriosclerosis?

what is atherosclerosis?

• Loss of elasticity (arteriosclerosis).

• Plaque build-up (atherosclerosis).

• Increased risk of aneurysm, hypertension, heart failure.

Vocabulary check

what is Aneurysm?

what is Capacitance?

what happens when there is a Cerebrovascular accident?

Aneurysm: bulge in artery wall.

Capacitance: veins’ ability to store blood.

Cerebrovascular accident (CVA): stroke, loss of blood flow to brain.