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Blood Vessels and Circulation

• Hemodynamics involves the forces related to blood circulation throughout the body.
• Blood vessels transport blood.
• Blood vessels are classified into five categories: arteries, arterioles, capillaries, venules, and veins

Blood Vessels

• Arteries carry blood away from the heart.
• Arterioles are small blood vessels formed from arteries splitting.
• Angiogenesis is the formation of new blood vessels.
• Capillaries are small vessels where gas and substance exchanges occur between tissues and the blood.
• Veins carry blood toward the heart.
• Venules are small blood vessels that merge to form veins.
• Anastomosis is a union of two or more blood vessels in the same body region.
• Anastomoses allows for alternative routes for blood flow to tissues or organs.
• Collateral circulation is the term given to this alternative blood flow.
• End artery is an artery that does not have anastomoses.
• Anastomoses may occur between two arteries, two veins, or an artery and a vein.

Blood Vessel Structure

• Blood vessel walls have three tunics, or layers.
• Tunica interna (intima) is the innermost layer.
  • It's the layer directly in contact with blood.
  • It is composed of three layers:
    • Endothelium: an epithelial layer for maintaining efficient blood flow, capillary permeability, and influencing smooth muscle tissue around the blood vessel.
    • Internal elastic lamina: Thin connective tissue that is elastic.
  • Basement membrane anchors the endothelium
• Tunica externa (adventitia) is the outermost layer.
  • It consists of collagen and elastic fibers
  • It contains nerves and blood vessels for the vessel wall called vasa vasorum.
• Tunica media is between the tunica interna and the tunica externa.
  • It contains smooth muscle fibers and elastic fibers.
  • It controls blood flow by altering the width of the lumen.
  • The external elastic lamina is the tunica media's outermost part.
• Blood flows through the lumen (interior opening).

Arteries

• They are three types
• Elastic (conducting) arteries propel blood toward the rest of the body.
  • They have thin vessel walls
  • Their tunica media is composed of elastic fibers called elastic lamellae.
• Muscular (distributing) arteries maintain proper blood pressure and flow.
  • They have thick vessel walls
  • Their tunica media is made primarily from smooth muscle.
• Resistance arteries are the smallest.

Arterioles

• Arterioles are branches of arteries.
• They regulate blood flow into the capillaries.
• The metarteriole links arterioles and venules, and capillaries with both blood vessels.
• Precapillary sphincters are muscle fibers in arterioles that can close off blood flow to capillaries.
• The tunica externa of arterioles has many nerve cells that control blood flow.

Capillaries

• Microcirculation is the flow of blood from the arterioles into the capillaries and venules.
• Capillary bed is composed of 50-100 capillaries originating from a single metarteriole.
• There are three types of capillaries: continuous, fenestrated, and sinusoids.
• Continuous capillaries have a mostly continuous endothelium
  • Intercellular clefts (gaps between endothelial cells) form occasional interruptions.
  • Location: CNS, lungs, muscles, and skin
• Fenestrated capillaries have many pores in the endothelium.
  • Location: kidneys, villi of the small intestine, and most endocrine glands
• Sinusoids are wider and more winding than continuous or fenestrated capillaries.
  • Sinusoids have an incomplete basement membrane
  • The intercellular clefts in sinusoids are much largest.
  • They allow proteins and even blood cells to pass from a tissue into the bloodstream.
  • They may also contain specialized cells, depending on the function of the tissue it's supplying.

Blood Circulation

• Most blood follows conventional blood flow properties.
• Blood flows from arteries into arterioles, capillaries, venules, and veins.
• Portal veins serve portions of the body with one capillary system providing blood to another
• Transport of products from on body region directly to another occurs via the portal system.

Venules

• Venules usually possess much thinner walls than their arteriole counterparts.
• Postcapillary venules drain blood from the capillaries.
  • They begin the process of returning blood to the heart.
  • They are also very porous and form part of the microcirculation.
• Muscular venules have thicker walls, and often serve as blood reservoirs.

Veins

• Veins have much thinner walls than arteries.
• They consists of very thin tunica interna and media layers.
• Veins lack internal and external elastic laminae.
• Most veins have unidirectional valves to prevent backflow because blood pressure is far less than that of arteries.
• Vascular sinus is a vein without smooth muscle fibers to alter its diameter.

Blood Circulation

• Systemic veins and venules hold ≈ two-thirds of the blood volume.
• Function as blood reservoirs and provide blood when needed.
• Brain sends signals to veins to constrict during increased muscular activity.
• Vasoconstriction delivers greater blood flow to the muscles.
• Vasoconstriction compensates for blood loss during hemorrhage.

Capillary Exchange

• Capillary exchange is the movement of substances between blood and interstitial fluid.
• The primary purpose of blood flow throughout the body is capillary exchange
• Substance exchange functions through three mechanisms: diffusion, transcytosis, and bulk flow.
  • Diffusion: Movement down the concentration gradient.
  • Transcytosis: Transport through endothelial cells.
  • Bulk flow: Movement due to pressure differences.

Blood Flow

• Blood flow is the volume of blood that flows through any tissue over a period of time.
• Blood pressure determines flow direction from high to low pressure regions.
• Vascular resistance is the opposition to blood flow due to friction between blood and the vessel walls.
• Venous return refers to blood flowing back to the heart through the systemic veins.
  • Skeletal muscle and respiratory pumps assist venous return.

Control of Blood Flow

• The Cardiovascular (CV) center regulates blood flow and blood pressure
• Baroreceptors are for pressure-sensitive sensory receptors in major arteries.
  • There are two negative feedback loops involved with regulating blood pressure that occur as reflexes.
  • CV center alters heart rate and blood vessel diameters to return blood pressure to normal when it fluctuates, called the Baroreceptor Reflex.
• Chemoreceptors are sensory receptors located close to baroreceptors within major arteries.
  • These receptors detect changes the concentrations of oxygen, carbon dioxide, and protons within the blood, stimulating blood pressure changes via the CV center.
  • Changes in the rate of breathing balance the return the blood composition to normal.

Hormone Regulation

• Cardiac output increases by epinephrine and norepinephrine.
• Vasoconstriction increases blood pressure, and may be caused by angiotensin II, vasopressin, or aldosterone.
• Hormones play a role in regulating BP
• Vasodilation decreases blood pressure, and may be caused by atrial natriuretic peptide (ANP), epinephrine, or nitric oxide.
• An increase in blood volume increases blood pressure by aldosterone and vasopressin
• A decrease in blood volume decreases blood pressure by ANP.

Autoregulation

• Autoregulation refers to the adjustment of blood flows by tissues to fit their requirements.
• Region-specific blood flow fluctuate according to the activity being performed.
• Physical and chemical changes induce autoregulatory changes.
  • Physical: Temperature or muscle stretching
  • Chemical: Chemicals released by tissues that affect blood vessel diameter

Shock and Homeostasis

• Shock is the failure of the cardiovascular system to deliver oxygen and nutrients to the body.
• There are four types of shock
  • Hypovolemic shock: Due to decreased blood volume.
  • Cardiogenic shock: Due to low heart function.
  • Vascular shock: Due to inappropriate vasodilation (anaphylactic, neurogenic, and septic shock).
  • Obstructive shock: Due to blood flow.
• The body employs hormones and stimulates baro/chemoreceptors to initiate responses from the autonomic nervous system.
• Local autoregulatory processes can also reduce the effects of shock.
• The body may not resist the effects of shock if blood volume decreases by more than 10-20%.

Circulation and Vessels

• The systemic circulation delivers oxygen and nutrients to the body.
  • Components:
    • Arteries and arterioles that carry blood from the left ventricle.
    • Systemic capillaries
    • Venules and veins that carry blood from the body to the right atrium.
• The pulmonary circulation oxygenates the blood.

Systemic Circulation

• Its subdivisions are:
  • Coronary circulation which supplies the heart
  • Cerebral circulation supplies the brain
  • Bronchial circulation supplies the lungs
    • Bronchial and pulmonary circulation roles are distinctly different
  • Hepatic portal circulation for conveying blood from the GI tract to the liver.

Notable Arteries

• Aorta descending/thoracic/abdominal)
• Brachiocephalic trunk
• Left/Right (L/R) carotid (common, internal, and external)
• L/R vertebral, and renal
• Cerebral arterial circle (circle of Willis)
• Includes inferior suprarenal
• Includes L/R suprarenal (superior and middle)
• Celiac trunk including common hepatic, left gastric, splenic branches
• Superior and inferior mesenteric
• L/R subclavian, axillary, brachial, radial, and ulnar
• L/R iliac and femoral (common, internal, and external)
• Includes L/R popliteal, tibial (anterior and posterior), and peroneal

Notable Veins

• Superior and inferior vena cava
• Left/Right brachiocephalic and subclavian
• Hepatic portal vein (includes mesenteric, splenic, L/R gastric)
• Dural venous sinuses
• L/R jugular, hepatic, renal, and suprarenal
• L/R radial, ulnar, brachial, cephalic, basilic, and axillary
• L/R saphenous has great and small branches
• Includes L/R iliac, femoral, popliteal, tibial, and peroneal

Hypertension

• Persistently high blood pressure is hypertension.
• It affects >50 million indiv in US
• Systolic blood pressure is measured during the force of blood on arterial walls after ventricular contraction.
• Diastolic blood pressure is measured and corresponding to force of blood on arterial walls after ventricular relaxation.
• Normal blood pressure is less than 120 (systolic) over less than 80 (diastolic)
• Prehypertension is 120-139 over 80-89
• Stage 1 hypertension is 140-159 over 90-99
• Stage 2 hypertension is 160-179 over 100-109
• Hypertensive crisis is over 180 over 110
• It falls into primary and secondary categories.
  • 90-95% of hypertension cases are primary hypertension with idiopathic cause.
  • Secondary hypertension can be attributed to a specific cause (5–10% of all hypertension cases).
• Hypertension can damage blood vessels and the heart and is a major risk factor for heart disease and stroke.

Nomenclature

• Hypoxia: Decrease of [O2] in the blood
• Acidosis: Decrease of pH in the blood
• Alkalosis: Increase of pH in the blood
• Hypercapnia: Increase of [CO2] in the blood
• Tachycardia: Rapid resting heart rate (>100 beats/minute
• Bradycardia: Slow resting heart rate (<50 beats/minute)
• Aneurysm: Expansion of weakened blood vessel, forming bulging sac.
• Aortagraphy: X-ray examination of the aorta and its main branches.
• Deep vein thrombosis: Formation of blood clot in a deep vein of the lower limbs. Femoral angiography: X-ray examination of the femoral artery and its branches.
• Hypotension: Low blood pressure.
• Occlusion: Closure or blockage of the lumen of a structure, such as a blood vessel.
• Postural hypotension: Low blood pressure caused by assuming a standing position.
• Phlebitis: Inflammation of a vein.
• Thrombectomy: Surgical removal of a blood clot.
• Thrombophlebitis: Inflammation of a vein caused by formation of blood clots.