Systemic Circulation & Blood Flow Regulation

Plasma

• Plasma proteins

• Solutes

• Water

Formed Elements

• Platelets

• WBC

• RBC

Blood

• Fluid connective tissue (unique fluid property)

• Consists of a matrix (plasma) and formed elements (cells & cell fragments

• Assessed quantitatively

  • Hematocrit (% Whole Blood Volume)

    • Increase with Diabetes, more blood coming around, increase in hypertension

  • Packed Cell Volume (Size & Shape)

    • Ex: sickle cell disease

Red Blood Cells (Erthrocyets)

• Made in the bone marrow

• Carry Hb

• Expressed as HEMATOCRIT = % of blood

• 7.5 micro m in diameter, 2 micro m thick

• Biconcave shape

  • Large SA:V ratio

  • Form stacks: Rouleaux

  • Bend & Flex

White Blood Cells (Leukocytes)

• Mobile units of the body’s protective system

• Partially formed in bone marrow & lymph tissue, then transported in blood to areas of infection & inflammation

  • Rapid & potent defense against infectious agents

Types of WBCs

1. Neutrophils (62%)

2. Eosinophils (2.3%)

3. Basophils (0.4%)

4. Monocytes (5.3%)

5. Lymphocytes (30%)

6. Plasma cells (~0%)

• 1-4: protect the body against invading organisms through:

  • Ingestion (phagocytosis)

  • Antimicrobial/inflammatory substance release

• 5-6: recognize & destroy foreign cells, tumor cells, infected cells

Platelets

• Small granulated bodies that aggregate at sites of vascular injury

• Anucleated cells

  • Lack nuclei

  • 2-4 micro m in diameter

Megakaryocytes

Form platelets by pinching off bits of cytoplasm & extruding them into the circulation

Plasma

• Fluid portion of the blood

• Transport:

  • Ions

  • Inorganic molecules (H2O, O2, CO2)

  • Organic molecules

• Contain blood clotting factors (Fibrinogen)

  • Proteins that prevent excessive bleeding

Serum

• Remaining fluid after the removal of clotting

• INCREASE serotonin content

  • Breakdown of platelets during clotting

Arteries

• Conduct blood from heart → capillaries

Tunica Intima

Innermost coat with endothelial cells

Tunica Media

Middle coat with smooth muscle cells & collagen fibers

Tunica Externa

Outer coat consisting of connective tissue

Veins

• Thin walls compared to arteries of similar size

• High capacitance (concentration) is attributed to the distensibility of their wall

• Blood content is LARGE

• Has tunics like arteries, with slight differences

  • Tunica media is THINNER

Functional Characteristics of Veins

• Holds 64% of blood volume

• Have inherent vasomotor activity

• Movement of blood back to the heart relies on external forces

  • Contraction of skeletal muscles

  • Contraction of valves

Structural Hierarchy

• Starts at end of capillary bed with Postcapillary Venule

• Converge into Collecting Venules

• Enlarge into Muscular Venules

• Arteries → postcapillary venules → collecting venules → muscular venules → veins

Continuous Capillaries

• Have continuous basal lamina to block diffusion of molecules

  • Tight & occluding junction

• Found in iris of eye

• Prevent leakage

Fenestrated Capillaries

• More selective

• Contains pores which allow for exchange of specialized molecules

• Found in ciliary body & choroid of eye

• Found in renal filtration system

Sinusoid Capillaries

• MAXIMIZE molecular exchange between blood & tissue

• Found in bone marrow, spleen, & lymph nodes

• Production aspect

Organization of the Circulatory System

• Consists of arteries & veins that transport blood between heart & lungs

• Arteries & veins branch into smaller arterioles & venules where capillaries interconnect

• Starts at RV and ends at LA

• LV pumps oxygen-rich blood to the peripheral tissues

Systemic Circulation

1. Oxygenated blood flows from LA to LV, then pumped into aorta

2. Blood passes from aorta → elastic arteries → muscular arteries → arterioles

3. Blood in arterioles enters systemic capillaries for exchange of gases & nutrients

4. Deoxygenated blood exits capillary beds into venules → veins

5. Deoxygenated blood is conducted to either the superior or inferior vena cava, then enters RA of heart

Pulmonary Circulation

1. Blood flows from RA → RV → pumped to pulmonary trunk

2. Pulmonary trunk conducts deoxygenated blood → pulmonary arteries → lungs

3. Blood passes through smaller & smaller arteries before entering pulmonary capillaries for gas exchange

4. Oxygenated blood exits lung via a series of progressively larger veins that merge → pulmonary veins

5. Pulmonary veins drain → LA

6. Cycle repeats

Macrovasculature

• Arteries & veins

• > 130 micro m

Microvasculature

• Arterioles, Venules, Capillaries

• < 130 micro m

Functional Parts of the Circulation

• Arteries transport blood under high P to tissues

  • Strong vascular walls to manage high flow & velocity rates

• Arterioles are the last small branches of the arterial system

  • Strong muscular walls for constricting & dilating

    • Vastly alter blood flow

  • Capillaries exchange fluid, nutrients, waste produces between blood & interstitial fluid

    • Walls are thin for water & other small molecular transport (capillary pores)

• Venules collect blood from the capillaries and coalesce into larger veins

  • Veins transport from the venules back to heart

    • Reservoir of extra blood

      • Contain larger volumes of blood to maintain circulation & blood P

Volume of Blood

• CSA of veins are MUCH larger than arteries

  • 4x, have large BLOOD STORAGE capacities

• Velocity of blood (v) is INVERSELY proportional to CSA

• V = F/A

Basic Principles of Circulatory Function

1. Blood flow to most tissues is controlled according to tissue needs

2. Cardiac output = sum of all tissue flows

3. Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control

Ohm’s Law

• F = change in P/R

• Blood P means the force exerted by the blood against any unit area of the vessel wall

Laminar Flow

• AKA Newtonian Flow

• When blood flows at a steady rate through a long, smooth blood vessel

Parabolic

Velocity of flow in center of vessel is FAR GREATER than that toward outer edges

Turbulent

When flow becomes too great or disorderly

Reynold’s Number

• Quantifies tendency for turbulence to occur

• Increase Re, Increase turbulence

• Re > 200-400, turbulent flow will occur in branched vessels

• Re > 2000, turbulence will occur in straight smooth vessels

Conductance

• Measure of blood flow through a vessel for a given pressure difference

• C = 1/R

• Conductance DIRECT proportional to d^4

Poiseuille’s Law

• Flow is directly proportional to the 4th power of the radius of the vessel

• F = pi deltaPr^4 / 8*n*l

Blood Flow Resistance

• Impediment to blood flow in a vessel

• Rate of blood flow through the entire circulatory system = rate of blood pumping by the heart (cardiac output)

• When blood vessels are arranged in series, flow through each blood vessel is the same, and the total is the sum of the resistance of each vessel

• R total = R1 + R2 + R3

• Blood vessels branch extensively to form parallel circuits that supply blood to many organs & tissues of body

• 1/R total = 1/R1 + 1/R2 +1/R3

Autoregulation

Local control of constricting & dilating efficiently to meet metabolic demands

Vascular Wall Tension

• Develops in response to increase in pressure gradients

• Causes vascular smooth muscle & endothelial cells to stretch in all directions

Shear Stress

Frictional force on endothelial cells as blood flows through a vessel

Local Blood Flow Regulation

1. Delivery of oxygen to tissues

2. Delivery of other nutrients (glucose, AA, fatty acids)

3. Removal of CO2 from tissues

4. Removal of H+ from tissues

5. Maintenance of proper concentrations of ions in the tissues

6. Transport of various hormones & other substances to the different tissues

Oxygen Demand for Local Blood Flow

• Smooth muscles & precapillary sphincters modulate flow based on tissue demand for O & nutrients

• Strength of contraction increases with [O2]

Reactive Hyperemia

• Occlusion, lack of flow → vasodilation → O2 to be diffused

• Tissue after temporary occlusion of the artery supplying blood flow

Active Hyperemia

• Constant increase of metabolic activity

Vascular Remodeling

• From chronic changes is critical for tissue growth

  • Ex: chronic exercise & training increase vascularity

• Adaptive response to chronic pressure changes

  • Long-term high blood P causes arteries/arterioles to accommodate mechanical wall stress

• Vasoconstriction decreases lumen diameter → normalizing vascular wall tension

  • Laplace’s equation: T = r * P

Inward Eutrophic Remodeling

• Smooth muscle cells/endothelial cells rearrange around lumen with no change to CSA

Hypertrophic Remodeling

• Wall increase

• Increase in CSA with increase size of smooth muscle cells & extracellular protein

  • Results in stiffer vessels as seen in chronic hypertension

Outward Remodeling

• Increase in lumen diameter with little changes in wall thickness → increase in CSA

• Lumen

Outward Hypertrophic Remodeling

• Increase in lumen diameter & increase in wall thickness → increase in CSA

  • Found in long-term chronic conditions

Vasoconstrictors

• Norepinephrine & Epinephrine

  • Released from sympathetic nervous system

• Angiotensin II

• Vasopressin

Vasodilators

• Bradykinin

  • Arteriolar dilation & increase in capillary permeability

• Histamine

  • From damaged or inflamed tissues

    • Mast cells & basophils (blood)

  • Dilation of arterioles & increase in capillary porosity

• Poor regulation of bradykinin & histamine → edema

  • No control (leakage or swelling)