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 = \text{Diastolic Pressure} + (\text{Systolic Pressure} - \text{Diastolic Pressure}) / 3
  • 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