Cardiovascular System: Vessels & Circulation

Vessel Tunics

• Tunica Intima (Innermost):
  • Endothelium (simple squamous epithelium).
  • Subendothelial layer (areolar connective tissue).
• Tunica Media (Middle):
  • Circularly arranged smooth muscle.
  • Elastic fibers.
• Tunica Externa (Outermost):
  • Areolar connective tissue.
  • Elastic and collagen fibers.

Vessel Distinctions

• Arteries: Thicker tunica media, narrower lumen, thicker walls.
• Capillaries: Only tunica intima (endothelium and basement membrane) for efficient exchange.
• Veins: Thicker tunica externa, wider lumen, thinner walls, valves.

Types of Arteries

• Elastic Arteries (Conducting Arteries):
  • Largest arteries near the heart (e.g., aorta).
  • High elastin content allows them to withstand and smooth out large blood pressure fluctuations.
• Muscular Arteries (Distributing Arteries):
  • Deliver blood to specific body regions and organs.
  • Thicker tunica media with more smooth muscle, enabling vasoconstriction and vasodilation.
• Arterioles:
  • Smallest arteries regulating blood flow into capillaries.
  • Smooth muscle in tunica media controls resistance.

Capillaries

• Structure: Endothelium and basement membrane.
• Function: Nutrient and waste exchange between blood and tissues.

Types of Capillaries

• Continuous Capillaries:
  • Endothelial cells form a continuous lining.
  • Intercellular clefts allow passage of small molecules.
  • Found in skin, muscles, and brain.
• Fenestrated Capillaries:
  • Endothelial cells have fenestrations (pores).
  • More permeable than continuous capillaries.
  • Found in kidneys, small intestine, and endocrine glands.
• Sinusoids:
  • Large gaps between endothelial cells allow passage of large molecules and cells.
  • Discontinuous basement membrane.
  • Found in liver, bone marrow, and spleen.

Capillary Beds

• Structure:
  • Metarteriole: vessel branch between arteriole and capillaries.
  • Precapillary sphincters: control blood flow into true capillaries.
• Blood Flow:
  • Through true capillaries when precapillary sphincters are relaxed.
  • Bypasses true capillaries when precapillary sphincters are contracted.

Veins

• Structure: Thinner walls, wider lumen than arteries.
• Function: Return blood to the heart; serve as blood reservoir.

Veins as Blood Reservoirs

• Veins hold about 60% of the body's blood volume at rest.
• Venoconstriction can shift blood from veins to arteries during times of increased demand.

Vessel Pathways

• Simple Pathway: One artery, one capillary bed, one vein.
• Alternative Pathways:
  • Anastomoses:
    • Arterial: Two or more arteries supply the same region.
    • Venous: Two or more veins drain the same region.
    • Arteriovenous: Artery flows directly into a vein, bypassing capillaries.
  • Portal System: Two capillary beds in series (e.g., hepatic portal system).

Tunica Media Tissue

• Smooth muscle.

Largest Artery

• Aorta (elastic artery).

Portal System

• Blood flows through two capillary beds before returning to the heart.

Leakiest Capillary

• Sinusoid.

Fenestration

• Pore in endothelial cells of fenestrated capillaries.

Sinusoid

• Large gap between endothelial cells in sinusoids.

Control of Blood Flow

• Precapillary sphincters regulate blood flow into capillary beds.

Cross-Sectional Area and Velocity

• Total cross-sectional area is inversely proportional to blood flow velocity.
• Capillaries have the largest total cross-sectional area resulting in slow blood flow.

Slow Blood Flow in Capillaries

• Allows sufficient time for nutrient and waste exchange.

Blood Flow Velocity

• Veins > Capillaries.

Capillary Exchange

• Diffusion: Movement of substances down their concentration gradients (e.g., oxygen, carbon dioxide).
• Vesicular Transport: Endocytosis and exocytosis.

Vasomotion

• Contraction and relaxation cycles of precapillary sphincters.

Vessel Valves

• Prevent backflow of blood in veins.
• Found mostly in veins of the limbs.

Blood Reservoir

• Veins.

Anastomosis

• Connection between blood vessels providing alternate routes for blood flow.

Bulk Flow

• Movement of fluid and solutes together due to pressure gradients.
• Filtration: Fluid moves out of capillaries into interstitial fluid.
• Reabsorption: Fluid moves back into capillaries from interstitial fluid.

Pressures in Capillaries

• Hydrostatic Pressure: Force of fluid against capillary wall.
  • Blood hydrostatic pressure (HPb): Promotes filtration.
  • Interstitial fluid hydrostatic pressure (HPif): Promotes reabsorption.
• Colloid Osmotic Pressure: Pressure due to proteins in the fluid.
  • Blood colloid osmotic pressure (COPb): Promotes reabsorption.
  • Interstitial fluid colloid osmotic pressure (COPif): Promotes filtration.

Net Filtration Pressure (NFP)

• NFP = (HPb + COP{if}) - (HP{if} + COPb)

Lymphatic System's Role

• Picks up excess fluid filtered out of capillaries and returns it to the bloodstream.

Oxygen Movement

• Diffusion.

Filtration

• Movement of fluid out of capillaries.

Colloid Osmotic Pressure Comparison

• COPb > COP{if}

Dominant Process at Arterial End

• Filtration.

Degree of Vascularization

• Extent of blood vessel distribution in a tissue.

Angiogenesis

• Formation of new blood vessels.
• Aids perfusion by increasing blood supply to tissues.

Myogenic Response

• Smooth muscle in blood vessel walls responds to changes in blood pressure to maintain constant blood flow.
• Increased pressure causes vasoconstriction; decreased pressure causes vasodilation.

Vasodilator vs. Vasoconstrictor

• Vasodilator: Substance that causes blood vessels to widen.
• Vasoconstrictor: Substance that causes blood vessels to narrow.

Tissue Autoregulation

• Tissues regulate their own blood flow based on metabolic needs.
• Increased metabolic activity leads to vasodilation; decreased activity leads to vasoconstriction.

Total Blood Flow vs. Local Blood Flow

• Total blood flow is relatively constant; local blood flow varies depending on tissue needs.

Carbon Dioxide and Lactic Acid Effects

• Vasodilation.

Histamine Trigger

• Vasodilation.

Myogenic Response to Increased Pressure

• Vasoconstriction.

Angiogenesis

• Formation of new blood vessels.

Blood Pressure and Gradient

• Blood Pressure: Force of blood against blood vessel walls.
• Blood Pressure Gradient: Difference in blood pressure between two points.

Blood Pressure in Vessels

• Arteries: Highest blood pressure; pulsatile.
• Capillaries: Lower blood pressure; relatively constant.
• Veins: Lowest blood pressure; relatively constant.

Pulse Pressure and MAP

• Pulse Pressure: Systolic pressure - Diastolic pressure.
• Mean Arterial Pressure (MAP): Diastolic pressure + 1/3(Pulse pressure).

Overcoming Pressure Gradient in Veins

• Valves, skeletal muscle pump, respiratory pump.

Resistance

• Opposition to blood flow.
• Influenced by blood viscosity, vessel length, and vessel radius.
• Resistance \propto \frac{Viscosity \cdot Vessel \ Length}{Radius^4}

Blood Flow Relationship

• Blood \ Flow = \frac{Pressure \ Gradient}{Resistance}

Blood Pressure and Resistance

• Blood pressure increases with increased resistance in the systemic circulation.

Blood Pressure Differences

• Arteries > Capillaries > Veins.

Pulse Pressure and MAP Example

• BP of 120/90 mm Hg:
  • Pulse Pressure = 120 - 90 = 30 mm Hg.
  • MAP = 90 + 1/3(30) = 100 mm Hg.

Mechanisms for Blood Flow in Veins

• Skeletal muscle pump, respiratory pump, valves.

Vasoconstriction Effects

• Increases resistance, decreases blood flow.

Short-Term Blood Pressure Regulation

• Baroreceptors, chemoreceptors, autonomic reflexes.

Baroreceptor Reflex

• Decreased blood pressure: Increased heart rate, increased vasoconstriction.
• Increased blood pressure: Decreased heart rate, decreased vasoconstriction.

Chemoreceptor Effects

• Respond to changes in blood oxygen, carbon dioxide, and pH.
• Increased carbon dioxide or decreased pH: Increased vasoconstriction, increased respiratory rate.

Autonomic Reflexes

• Cardioacceleratory center: Increases heart rate and contractility.
• Cardioinhibitory center: Decreases heart rate.
• Vasomotor center: Controls vasoconstriction and vasodilation.

Hormones Regulating Blood Pressure

• Epinephrine, norepinephrine, angiotensin II, antidiuretic hormone (ADH), atrial natriuretic peptide (ANP), aldosterone.

Renin-Angiotensin System

• Decreased blood pressure: Renin converts angiotensinogen to angiotensin I, ACE converts it to angiotensin II.
• Angiotensin II: Vasoconstriction, stimulates aldosterone and ADH release.

Hormone Effects on Blood Pressure

• Aldosterone, ADH, Angiotensin II: Increase blood pressure.
• Atrial Natriuretic Peptide (ANP): Decreases blood pressure.

Blood Pressure Drop Upon Standing

• Baroreceptor reflex: Increased heart rate, increased vasoconstriction.

Aldosterone and Blood Pressure

• Increases sodium and water reabsorption in kidneys, increasing blood volume and pressure.

ANP Effect on Blood Pressure

• Increases sodium and water excretion in kidneys, decreasing blood volume and pressure.

Vasomotor Center

• Controls blood vessel diameter through vasoconstriction and vasodilation.

Blood Flow During Rest vs. Exercise

• Rest: Most blood flow to digestive organs.
• Exercise: Increased blood flow to skeletal muscles, heart, and skin.

Tissues with Increased Blood Flow During Exercise

• Skeletal muscles, heart, skin.

Pulmonary Circulation

• Right ventricle → Pulmonary trunk → Pulmonary arteries → Lungs → Pulmonary veins → Left atrium.

Pulmonary vs. Systemic Circulation

• Pulmonary: Lower pressure, shorter pathway.

Pulmonary Veins Connection

• Left atrium.

Number of Pulmonary Veins

• Four.

Pressure Comparison

• Pressure in pulmonary trunk is lower than pressure in aorta.

Arteries from Left Ventricle

• Aorta, branching to systemic arteries.

Veins to Right Atrium

• Superior vena cava, inferior vena cava, coronary sinus.

Vessels off Aortic Arch

• Brachiocephalic trunk, left common carotid artery, left subclavian artery.

Vessels Forming Superior Vena Cava

• Brachiocephalic veins.

Head and Neck Vessels

• Arteries: Common carotid arteries, vertebral arteries.
• Veins: Internal jugular veins, external jugular veins.

Cerebral Arterial Circle (Circle of Willis)

• Provides alternate routes for blood to reach the brain.
• Formed by anterior and posterior communicating arteries, anterior and posterior cerebral arteries, and internal carotid arteries.

Dural Venous Sinuses

• Venous channels located between layers of the dura mater.
• Drain blood from the brain.

Arteries of Thoracic Wall

• Posterior intercostal arteries, internal thoracic artery.

Arteries of Abdominal Wall

• Inferior epigastric artery, superior epigastric artery, lumbar arteries.

Veins of Thoracic and Abdominal Walls

• Azygos vein, hemiazygos vein, intercostal veins, lumbar veins.

Vessels of Lungs, Esophagus, and Diaphragm

• Lungs: Pulmonary arteries and veins.
• Esophagus: Esophageal arteries and veins.
• Diaphragm: Phrenic arteries and veins.

Arteries to Gastrointestinal Tract

• Celiac trunk, superior mesenteric artery, inferior mesenteric artery.

Hepatic Portal System

• Carries blood from the gastrointestinal tract to the liver for processing.

Route of Blood

• Gastrointestinal tract → Hepatic portal vein → Liver → Hepatic veins → Inferior vena cava.

Vessels to Adrenal Glands, Kidneys, and Gonads

• Adrenal glands: Suprarenal arteries and veins.
• Kidneys: Renal arteries and veins.
• Gonads: Gonadal arteries and veins.

Vessels of Pelvis and Perineum

• Internal iliac artery and vein.

Path from Left Ventricle to Right Internal Carotid Artery

• Left ventricle → Aorta → Brachiocephalic trunk → Right common carotid artery → Right internal carotid artery.

Azygos Vein Location

• Right side of the body.

Hepatic Portal System

• Carries blood from the gastrointestinal tract to the liver.

Gastroduodenal Artery

• Emerges from the common hepatic artery (branch of the celiac trunk).

Arteries of Upper Limb

• Subclavian artery → Axillary artery → Brachial artery → Radial and ulnar arteries → Palmar arches → Digital arteries.

Veins of Upper Limb

• Superficial: Cephalic vein, basilic vein, median cubital vein.
• Deep: Radial veins, ulnar veins, brachial veins, axillary vein, subclavian vein.

Arteries of Lower Limb

• External iliac artery → Femoral artery → Popliteal artery → Anterior and posterior tibial arteries → Dorsalis pedis artery → Plantar arteries → Digital arteries.

Veins of Lower Limb

• Superficial: Great saphenous vein, small saphenous vein.
• Deep: Tibial veins, popliteal vein, femoral vein, external iliac vein, common iliac vein.

Path from Back of Knee to Right Atrium

• Popliteal vein → Femoral vein → External iliac vein → Common iliac vein → Inferior vena cava → Right atrium.

Vein Used for Blood Draw

• Median cubital vein (in the elbow).

Subclavian Artery

• Becomes the axillary artery after passing the first rib.

External Iliac Artery

• Becomes the femoral artery after passing the inguinal ligament.

Fetal Circulation

• Umbilical vein carries oxygenated blood from placenta to fetus.
• Ductus venosus bypasses the liver.
• Foramen ovale allows blood to flow from the right atrium to the left atrium, bypassing the lungs.
• Ductus arteriosus allows blood to flow from the pulmonary artery to the aorta, bypassing the lungs.
• Umbilical arteries carry deoxygenated blood from the fetus to the placenta.

Changes After Birth

• Umbilical vessels constrict and become ligaments.
• Ductus venosus closes and becomes the ligamentum venosum.
• Foramen ovale closes and becomes the fossa ovalis.
• Ductus arteriosus closes and becomes the ligamentum arteriosum.
• Pulmonary circulation begins.

Blood Flow After Umbilical Vein

• Liver or ductus venosus.

Remnant of Ductus Arteriosus

• Ligamentum arteriosum.

Closure of Foramen Ovale

• Increased pressure in the left atrium due to increased pulmonary blood flow.