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