Blood Vessels and Circulation

Blood Vessels and Circulation

Overview of Blood Flow

• Pulmonary Circuit:

  • Blood flows from the right ventricle to the pulmonary arteries.

  • Gas exchange occurs in the capillaries of the lungs.

  • Blood returns to the left atrium via the pulmonary veins.

• Systemic Circuit:

  • Blood flows from the left ventricle to the systemic arteries.

  • Blood reaches the head, neck, and upper limbs via capillaries.

  • Systemic veins carry blood from the trunk and lower limbs back to the right atrium.

Arteries and Veins: Structure

• Three Layers (Tunics):

  • Tunica Intima (Interna):

    • Innermost layer.

    • Composed of endothelial cells, connective tissue, and elastic fibers.

  • Tunica Media:

    • Middle layer.

    • Contains concentric sheets of smooth muscle.

  • Tunica Externa (Adventitia):

    • Outermost layer.

    • Connective tissue sheath.

    • Anchors the vessel to surrounding tissues.

Arteries and Veins: Types of Vessels

• Five General Classes:

  • Arteries

    • Elastic Arteries

    • Muscular Arteries

  • Arterioles

  • Capillaries

  • Venules

  • Veins

    • Medium-sized Veins

    • Large Veins

Capillaries

• Consist of a tube of endothelial cells with a delicate basement membrane.

• Two Major Types:

  • Continuous Capillaries:

    • Have a complete endothelial lining.

    • Endothelial cells are tightly connected but allow for the passage of small molecules.

    • Contain vesicles for transporting materials across the endothelial cells.

  • Fenestrated Capillaries:

    • Contain fenestrations, or pores, that span the endothelial lining.

    • Permit rapid exchange of water and solutes between blood and interstitial fluid.

    • Found in locations where rapid absorption or filtration occurs (e.g., kidneys, small intestine).

  • Sinusoids:

    • Resemble fenestrated capillaries but have larger gaps between adjacent cells.

Capillary Beds

• Interconnected networks of capillaries.

• Supplied by more than one artery via collaterals (arterial anastomosis).

• Arteriovenous anastomosis directly connects arteriole to venule, bypassing the capillary bed.

• Thoroughfare Channel:

  • Metarterioles.

• Precapillary Sphincters:

  • Control blood flow into capillaries.

• Vasomotion:

  • Cyclic contraction and relaxation of precapillary sphincters.

Venous Functional Anatomy

• Blood pressure in peripheral venules is less than 10% of that in the ascending aorta.

• Mechanisms to Maintain Blood Flow Against Gravity:

  • Valves:

    • Prevent backflow of blood.

  • Contraction of Skeletal Muscles:

    • Compresses veins, aiding in blood flow.

• Blood Distribution:

  • Systemic venous system: 64%

  • Systemic capillaries: 7%

  • Systemic arterial system: 13%

  • Heart: 7%

  • Pulmonary circuit: 9%

Venoconstriction

• Contraction of smooth muscle fibers in veins.

• Maintains blood volume in the arterial system during blood loss.

• Controlled by the vasomotor center in the medulla oblongata.

• Sympathetic nerves stimulate smooth muscles in medium-sized veins.

Cardiovascular Regulation

• Adjustments to both:

  • Cardiac Output:

    • Must generate enough pressure to force blood through peripheral capillaries.

  • Blood Distribution within systemic and pulmonary circuits.

• Regulation Mechanisms:

  • Neural

  • Hormonal

Neural and Hormonal Regulation

• Coordinated adjustments to:

  • Heart rate and stroke volume.

  • Peripheral resistance.

  • Venous pressure.

Pressure and Resistance

• Blood pressure is higher in arteries than in veins.

• Flow through blood vessels is influenced by resistance.

• Peripheral Resistance:

  • Resistance of the arterial system.

  • Increases as vessels get smaller.

Blood Flow in Capillaries

• Very slow flow and low pressure.

• Allows time for capillary exchange (diffusion).

Blood Pressure in Veins

• Maintained by:

  • Valves.

  • Muscular compression of peripheral veins.

• Vessels get larger, and resistance decreases as blood moves toward the heart.

• Venous Return:

  • Amount of blood arriving at the right atrium each minute.

  • Equal to the cardiac output on average.

Factors Affecting Peripheral Resistance

• Total Peripheral Resistance:

  • Resistance of the entire cardiovascular system.

  • Overcome by sufficient pressure from the heart.

  • Depends on:

    • Vascular resistance.

    • Viscosity.

    • Turbulence.

Vascular Resistance

• Opposition to blood flow in vessels.

• Largest component of total peripheral resistance.

• Results from friction between blood and vessel walls.

• Depends on:

  • Vessel Length:

  • Vessel Diameter:

    • Diameter = 2 cm, Resistance to flow = 1

    • Diameter = 1 cm, Resistance to flow = 16

Viscosity

• Resistance to flow caused by interactions of solutes and suspended materials in a liquid.

• Blood viscosity is ~5 times that of water due to cells and plasma proteins.

• Normally stable.

Turbulence

• Type of fluid flow with eddies and swirls.

• Caused by high flow rates, irregular surfaces, and sudden changes in vessel diameter.

• Increased turbulence = increased resistance = slow blood flow.

Factors Affecting Blood Flow

• Blood flow is directly proportional to blood pressure and inversely proportional to peripheral resistance.

• Pressure Gradient:

  • Difference in pressure from one end of vessel to other.

• Changes in diameter affect resistance and flow.

  • Aorta to Capillaries: Decreasing diameter increases resistance = decreased flow.

  • Capillaries to Venae Cavae: Increasing diameter decreases resistance = increased flow.

Pressure Changes

• Highest pressure at the aorta.

• Pressure drops at each branching in arterial system.

• Smaller, more numerous vessels produce more resistance, reducing pressure.

• Capillary pressure:

  • Start of peripheral capillaries: 35 mm Hg

  • Venules: 18 mm Hg

Blood Flow Changes

• Highest flow in the aorta.

• Slowest in the capillaries.

• Flow accelerates in venous system.

Arterial Pressure

• Rises during ventricular systole (systolic pressure).

• Declines during ventricular diastole (diastolic pressure).

• Pulse Pressure:

  • Difference between systolic and diastolic pressure.

  • Example: 120 – 90 = 30 mm Hg

• Mean Arterial Pressure (MAP):

  • Adding 1/3 of pulse pressure to diastolic pressure.

  • MAP = Diastolic Pressure + (Systolic Pressure - Diastolic Pressure)

  • Example: 90 + (120 – 90)/3 = 100 mm Hg

Capillary Exchange

• Involves a combination of diffusion, osmosis, and filtration.

• Capillary Hydrostatic Pressure (CHP)

Diffusion

• Net movement of substances from an area of higher concentration to lower concentration.

• Occurs most rapidly when distances are short, concentration gradient is large, and ions/molecules are small.

Filtration

• At the arterial end of the capillary:

  • CHP is highest near arteriole.

  • As filtration occurs, blood colloid osmotic pressure (BCOP) increases.

  • CHP > BCOP = fluid forced out of capillary.

• Net Filtration Pressure (NFP):

  • Difference between capillary hydrostatic and blood colloid osmotic pressure.

  • $NFP = CHP – BCOP$

  • Is positive at beginning of capillary: Filtration.

  • Becomes negative by end of capillary: Reabsorption.

Reabsorption

• At roughly 2/3 of the way along the capillary:

  • No net movement: CHP = BCOP.

  • NFP = CHP – BCOP = 0

• At the venule end of the capillary:

  • Reabsorption predominates: CHP < BCOP.

  • Water moves into capillary.

• Overall: more water leaves bloodstream than is reabsorbed.

  • Difference (about 3.6 L/day) enters the lymphatic vessels.

Variations in Capillary Exchange

• Any condition affecting blood pressure or osmotic pressure shifts balance of hydrostatic and osmotic forces.

• Hemorrhaging:

  • Blood volume and pressure decline (CHP reduced) = increased capillary reabsorption.

• Dehydration:

  • Plasma volume and blood pressure decline (CHP reduced and BCOP increases).

• If CHP rises or BCOP declines:

  • Filtration increases = Fluid builds up in peripheral tissues (edema).

Cardiovascular Regulatory Mechanisms

• Ensure adequate tissue perfusion (blood flow through tissues).

• Two Regulatory Pathways:

  • Autoregulation.

  • Central regulation (Neural and endocrine control).

Autoregulation

• Involves changes in blood flow within capillary beds.

• Regulated by precapillary sphincters.

• Vasodilators: Local chemicals that increase blood flow.

• Vasomotion.

Central Regulation

• Involves both neural and endocrine mechanisms.

• Neural:

  • Activation of cardioacceleratory center.

  • Activation of vasomotor center.

  • Peripheral vasoconstriction.

  • Arteriole vasodilation in skeletal muscle and the brain.

  • Increases cardiac output and reduces blood flow to nonessential tissues.

• Endocrine:

  • Release of vasoconstrictor (primarily NE).

Baroreceptor Reflexes

• Respond to changes in blood pressure.

• Receptors located in walls of carotid sinuses, aortic sinuses, and right atrium.

Chemoreceptor Reflexes

• Respond to changes in blood and cerebrospinal fluid.

• Increased CO2 levels, decreased pH and O2 levels in blood and CSF increases respiratory rate, cardiac output and blood pressure, and vasoconstriction occurs.

Endocrine Responses

• Endocrine system regulates cardiovascular function through:

  • The heart

  • The kidneys

  • The hypothalamus/posterior pituitary gland

Hormonal Response to Low Blood Pressure

• Immediate Response:

  • Release of epinephrine and norepinephrine from the adrenal medullae.

• Long-Term Response:

  • Antidiuretic hormone (ADH).

  • Angiotensin II.

  • Erythropoietin (EPO).

  • Aldosterone.

Hormonal Response to High Blood Pressure

• Natriuretic peptides (ANP and BNP) released by the heart:

  • Increased Na+ and water loss in urine.

  • Reduced thirst.

  • Inhibition of ADH, aldosterone, epinephrine, and norepinephrine release.

  • Peripheral vasodilation.

Cardiovascular Adjustments During Exercise

• Light Exercise:

  • Vasodilation occurs.

  • Peripheral resistance drops.

  • Capillary blood flow increases.

  • Venous return increases.

  • Cardiac output increases to 9500 mL/min.

• Heavy Exercise:

  • Cardiac output approaches maximal levels (~17,500 mL/min).

  • Increased flow to skeletal muscles and skin.

  • Reduced flow to digestive viscera and kidneys.

  • Brain blood flow remains unchanged.

Cardiovascular Performance and Training

• Trained athletes have bigger hearts and greater stroke volumes.

• Can maintain normal blood flow with lower heart rate.

• Maximal cardiac output can be 50% higher than in non-athletes.

Fetal Circulation

• Umbilical Arteries:

  • Carry blood from the fetus to the placenta.

• Umbilical Vein:

  • Carries blood from the placenta to the fetus.

  • Drains into the ductus venosus.

• All umbilical vessels degenerate after birth.

• Ductus Venosus:

  • Vascular connection to veins within the liver.

  • Empties into inferior vena cava.

• Foramen Ovale (Interatrial Opening):

  • Allows blood to pass from right atrium to left atrium.

• Ductus Arteriosus:

  • Bypass between pulmonary trunk and aorta.

  • Sends blood from right ventricle to systemic circuit.

Changes in Circulation at Birth

• Occur due to expansion of the pulmonary blood vessels and pressure changes.

• Increasing pressure in the left atrium closes foramen ovale.

  • Remnant is a shallow depression called the fossa ovalis.

• Rising oxygen levels cause ductus arteriosus to constrict and close.

  • Remnant is a fibrous cord called the ligamentum arteriosum.

Fetal Circulation - Congenital Defects

• Ventricular septal defects

• Patent foramen ovale

• Patent ductus arteriosus

• Tetralogy of Fallot

• Atrioventricular septal defect

• Transposition of the great vessels