Lecture 11 - Circulatory System

Circulatory System: Vascular Component

• Arteries: Carry blood away from the heart.
• Veins: Carry blood towards the heart.
• Capillaries: Smallest blood vessels for exchange between blood and tissue fluid.

Circulatory Plan

• Blood is carried on a closed loop that starts and ends with the heart.

Structure of Blood Vessels

• Lumen: The space within a blood vessel through which blood travels.
• Tunics: Main components of blood vessels.
  • Tunica Interna (Intima): Innermost layer, endothelial lining of all vessels.
  • Tunica Media: Middle layer, contains smooth muscle and elastic fibers, regulated by the sympathetic nervous system for vasoconstriction and vasodilation.
    • Vasoconstriction: Narrowing of blood vessels.
    • Vasodilation: Widening of blood vessels.
  • Tunica Externa (Adventitia): Outermost layer, contains collagen fibers for protection and reinforcement.

Arterial System

• Three main types of blood vessels:
  • Elastic Arteries:
    • Largest diameter: 1 to 2.5 cm (approximately 1 inch).
    • Also called conducting arteries.
    • Allow for a large blood flow with little fluctuation.
    • Thick-walled, located closer to the heart.
    • Low resistance to blood flow.
  • Muscular Arteries:
    • Diameter: 3 mm to 1 cm.
    • Also called distributing arteries.
    • Deliver blood to the body's organs.
    • Thick tunica media with more smooth muscle than elastic fibers.
    • Involved in vasoconstriction.
  • Arterioles:
    • Smallest diameter: 10 micrometers to 0.3 mm.
    • Important for regulating blood flow into capillary beds through vasodilation and vasoconstriction.

Capillaries

• Walls of thin tunica interna, one cell thick.
• Narrow diameter: Allows only a single red blood cell to pass.
  • Red blood cell diameter: ~7.5 micrometers.
• Movement of substances:
  • Fat-soluble substances can pass across the cell membrane.
  • Water-soluble substances can undergo transcytosis.
  • Paracellular transport occurs between adjacent epithelial cells.

Types of Capillaries (Least to Most Leaky)

• Continuous Capillaries:
  • Least leaky.
  • Found in the skin, lungs, and muscles.
  • Complete and continuous lining connected by tight junctions.
• Fenestrated Capillaries:
  • Have pores or fenestrations that allow small molecules to pass.
  • Found in the intestines, endocrine system, and kidneys.
• Sinusoidal Capillaries:
  • Most leaky.
  • Found in the spleen, bone marrow, and liver.
  • Few tight junctions, allowing larger molecules to pass through, such as blood cells.

Blood Flow Through Capillary Beds

• Regulated by vasomotor nerves and local chemical conditions.
• Blood can flow through capillary beds or bypass them via thoroughfare channels.
• Precapillary sphincters regulate blood flow into true capillaries.
• All cells in the body are close to a capillary.
• Only ~5% of capillaries are open at any one time.

Venous System

• United capillary beds form venules.
• Venules unite to form veins.
• Veins act as blood reservoirs, containing approximately 60% of the blood supply.
• Veins have thinner walls and lower blood pressure than arteries.
• Large diameter lumens limit resistance to blood flow.
• Valves prevent backflow of blood.
• Deep veins pass through skeletal muscle groups; muscle movement aids in pumping blood towards the heart.

Blood Pressure

• Systole: Contraction of a heart chamber.
• Diastole: Relaxation of a heart chamber.
• Normal blood pressure: 120/80 (systolic/diastolic).
• Blood pressure gradient: Highest in the aorta, lowest in the veins (almost zero in the right atrium).
• Measured in millimeters of mercury (mm Hg).
• Short-term controls: Nervous system and blood-borne chemicals.
• Long-term controls: Organ system changes, maintaining blood volume.
• Steepest change in blood pressure occurs in the arterioles and capillaries.

Pulmonary Circulation

• Pulmonary trunk carries oxygen-poor blood from the right ventricle to the pulmonary arteries for oxygenation in the lungs.
• Pulmonary veins return oxygen-rich blood from the lungs to the left atrium.

Systemic Circulation

• Blood vessels extend to and from the heart.
• Delivers oxygen to tissues.
• Returns waste products like carbon dioxide.
• Great Vessels: Aorta and vena cavae.
  • Aorta: Ascending aorta, aortic arch, descending aorta (thoracic aorta, abdominal aorta).
  • Vena Cavae: Superior and inferior vena cavae.
• Aortic Arch Branches:
  • Brachiocephalic trunk (branches into right common carotid and right subclavian arteries).
  • Left common carotid artery.
  • Left subclavian artery.

Systemic Circulation: Arteries

Head and Neck

• Circle of Willis:
  • Located in the brain.
  • Provides alternative channels for blood to supply the brain if blockages occur.
  • Equalizes blood pressure in the brain.
• Four pairs of arteries supply the head and neck region:
  • External common carotid arteries: Supply tongue, face, skin, muscles, occipital, temporal, and maxillary regions.
  • Internal common carotid arteries: Supply ophthalmic region and the anastomosis of the brain.
• Subclavian Artery Branches:
  • Vertebral artery: Supplies blood to the posterior brain.
  • Thyrocervical trunk: Supplies blood to scapular muscles and thyroid gland.
  • Costocervical trunk: Supplies blood to deep muscles of the neck.

Upper Limb

• Subclavian artery becomes the axillary artery in the armpit region.
• Axillary artery becomes the brachial artery along the humerus.
• Brachial artery branches into the radial and ulnar arteries in the forearm.
• Radial and ulnar arteries supply blood to the forearm (lateral and medial sides, respectively) and then lead into the palmar arches.
• Palmar arches supply blood to the hand.

Thorax

• Thoracic Arteries:
  • Internal Thoracic (branch of subclavian artery):
    *Supplies the anterior thoracic wall and mammary gland.
    *Anterior intercostal: Supplies the ribs and the space between the ribs.
    *Posterior intercostal: Supplies the posterior wall more so for the posterior one and also the posterior portion of the diaphragm.
    *Bronchial: Supplies the lungs.
    *Thoracic Aorta: Supplies mediastinum, pericardium, and esophagus.

Abdominal and Pelvic Arteries

• Celiac Trunk
• Inferior phrenic artery
• Superior mesenteric artery
• Suprarenal arteries
• Renal arteries
• Gonadal arteries
• Inferior mesenteric artery
• Common iliac arteries

Celiac Trunk

• Located in the superior part of the abdominal cavity.
  • Left gastric artery: Supplies the stomach.
  • Splenic artery: Supplies the spleen.
  • Common hepatic artery: Supplies the liver, gallbladder, duodenum, and head of the pancreas.

Pelvic Arteries

• Internal iliac artery.
• External iliac artery.
• Femoral artery: Supplies the thigh.
• Popliteal artery.

Systemic Circulation: Veins

Generalizations: Systemic Arteries vs. Systemic Veins

• One major artery (aorta) exiting vs. three major veins (superior/inferior vena cava and coronary sinus) entering the heart.
• Arteries are usually deep, veins are more superficial.
• Veins may form plexuses (venous plexuses).
• Unusual venous drainage: Dural sinuses in the brain and hepatic portal system.
• Unusual arterial vessels: Circle of Willis.

Vena Cavae

• Superior vena cava: Drains the superior portion of the systemic system.
• Inferior vena cava: Drains the inferior portion of the systemic system.
• Both join at the right atrium.

Dural Sinuses in the Cranium

• Most brain veins drain into intracranial dural sinuses.
• Dural Sinuses: (Sigmoid sinus, etc.)

Head and Neck Veins

• Superficial temporal vein.
• Facial vein.
• Posterior auricular vein.
• Jugular veins (external, internal).

Upper Limb Veins

• Deep veins generally follow the same pattern as arteries.
• Superficial veins are more distinct:
  • Cephalic vein (lateral side).
  • Basilic vein (medial side).
  • Median cubital vein: Usually used to draw blood.

Thorax Wall Veins

• Major branches: right subclavian, left subclavian vein.
  *When you get to the armpit, it's called axillary.
  When you get into the where the humerus is going along there, it's brachial.
  But you can see the basilic and the cephalic vein
• Basilic and cephalic veins.

Azygos Venous System

• Azygos vein: Drains blood from intercostal areas and part of the vertebral column; empties into the superior vena cava
• Hemiazygos: The left side of the vertebral column.
  Accessory hemiazygos

Abdominal Veins

• Lumbar
• Gonadal
• Renal
• Suprarenal
• Hepatic
• Superior and inferior mesenteric
• Splenic
• Veins of the same name as arteries perform the same function (draining instead of supplying blood).

Lower Limb Veins

• Deep veins share the same name as the corresponding artery.
• Superficial Veins:
  • Great saphenous vein: Longest vein in the human body.
  • Small saphenous vein