Venous

V. Venous System

a. Venules

  • Characteristics:

    • Small veins; collect blood from capillaries.

    • Very porous → allow fluid and white blood cells to pass into tissues.

    • Returns blood back to the heart

    • Postcapillary venules: smallest and made mostly of endothelium. Porous 

    • Large venules have 1 or 2 layers of smooth muscle and thin tunica externa

b. Veins

  • Characteristics:

    • Thinner walls, larger lumen, lower pressure.

    • Act as blood reservoirs (contain ~65% of blood).

  • Venous Valves:

    • Prevent backflow (especially in limbs).

    • Formed from folds of tunica intima.

    • 3 tunics, thinner walls, and larger lumens compared to arteries 

      • Tunica externa is most thick 

      • Lots of longitudinal bundles of collagen fiber and elastic networks 

      • Thin walls and large lumens = large blood volume (Blood Reservoirs) 

    • Venous valves: folds of the tunica intima • Limbs to oppose gravity

VI. Anastomoses

  • Definition: Interconnections between blood vessels.

  • Collateral Channels: Alternative routes for blood flow if one vessel is blocked.

  • Locations: Common in joints, heart, brain, abdominal organs.

VII. Physiology of Circulation Terminology

a. Blood Flow

  • Volume of blood flowing through vessel/organ/time (mL/min).

  • Equal to cardiac output.

  • At rest it is constant but changes when needed

b. Blood Pressure

  • Force exerted by blood on vessel walls (mmHg).

  • Pressure gradient: blood flows from high → low pressure.

c. Resistance

  • Opposition to blood flow.

  • Sources:

    1. Blood viscosity: Thickness of blood (↑ viscosity → ↑ resistance).

    2. Total vessel length: Longer = more resistance.

    3. Vessel diameter: Small diameter → ↑ resistance (main regulator).

VIII. Blood Pressure

  • Pumping fluids (blood) in a closed system. 

  • Pump = Heart (pressure highest closer to the pump) 

  • Heart Contractions result in blood flow. 

  • Blood Pressure results from the resistance to blood flow.

a. Arterial Blood Pressure

  • Depends on elasticity and volume of blood forced into arteries.

  • BP measurement closer to the heart and depends on: 

i. Compliance: Ability of vessels to expand (artery elasticity)

Volume of blood entering and moving through the arteries 
ii. Systolic BP: results from ventricular contraction (average 120 mm Hg) 
iii. Diastolic BP: results from elastic arteries recoiling and heart preventing back flow (average 80 mm Hg) 
iv. Pulse Pressure: Systolic – Diastolic pressure. This measures expansion of the elastic blood vessels as ventricles contract and force blood out (measures elasticity).
v. Mean Arterial Pressure (MAP):

  • Average pressure driving blood to tissues.

  • Formula: MAP = Diastolic + 1/3(Pulse Pressure)

b. Capillary Blood Pressure

  • Low (~35 mmHg → 15 mmHg).

  • Prevents rupture; allows nutrient exchange.

c. Venous Blood Pressure

  • Very low (~15 mmHg → 0 mmHg).

Venous return aided by:

  1. Respiratory Pump: Inhale → pressure changes → blood moves toward heart.

  • pressure changes to the ventral body cavity move blood towards the heart 

  1. Muscular Pump: Skeletal muscle contraction squeezes veins.

  2. Sympathetic Venoconstriction: Veins constrict → push blood to heart.

  • reduces the volume of blood in veins pushing blood to the heart

d. Blood Pressure Measurement

  • Pulse: Expansion/recoil of arteries; felt at pressure points.

  • Measuring BP:

    • Cuff (sphygmomanometer) on arm → inflate until artery collapses → slowly release and listen for Korotkoff sounds.