Chapter 14 - Blood Vessels, Blood Pressure and Blood Flow

CHAPTER 14 - BLOOD VESSELS, BLOOD PRESSURE AND BLOOD FLOW

I. PHYSICAL LAWS GOVERNING BLOOD FLOW AND BLOOD PRESSURE

• The movement of blood through the cardiovascular system is influenced by two primary factors: pressure gradient and resistance.

  • Flow Equation: $\text{Flow} = \frac{\text{Pressure gradient}}{\text{Resistance}} \ (P/R)$

  • Pressure:

  • Defined as the driving force that pushes blood through the vessels from a region of higher pressure to one of lower pressure.

  • Resistance:

  • Refers to all factors that inhibit blood flow. The general rules regarding resistance:

    • Increased resistance leads to decreased flow rate.

    • Decreased resistance leads to increased flow rate.

A. Pressure Gradient Differences

1. A pressure difference between two locations acts as the driving force for blood flow.

2. Role of Pressure Gradients in Driving Blood Flow:

   • The heart acts as the primary pump in the cardiovascular system, generating the pressure necessary to drive blood through the vessels.

   • Pumping blood into the arteries creates a pressure gradient between arteries and veins, driving blood flow.

3. Pressure Gradients Across Circulatory Circuits:

   • Mean Arterial Pressure (MAP):

     • Defined as the average pressure in the aorta throughout the cardiac cycle.

     • Approximately $85 ext{ mm Hg}$.

   • Central Venous Pressure (CVP):

     • Pressure in large veins returning to the heart (Superior and Inferior Vena Cava).

     • Approximately $2-8 ext{ mm Hg}$.

   • The difference between MAP and CVP drives blood flow through systemic circulation. Since CVP is low, the pressure gradient for systemic blood flow approximates MAP.

   • A low pressure gradient necessitates low resistance in the circulatory system.

II. OVERVIEW OF THE BLOOD VESSELS OF THE HUMAN BODY

• Blood vessels are categorized based on the direction they carry blood (away from or towards the heart) and by size.

• Arteries and Arterioles: Carry oxygenated blood away from the heart to the body, delivering it to capillaries. Capillaries are subsequently drained by venules and larger veins that carry deoxygenated blood.

III. ARTERIES

• Structure of Arterial Walls:

  • Composed of three layers (tunics):

  1. Tunica Interna (Tunica Intima):

     • Innermost layer surrounding the lumen.

     • Lined with endothelium (simple squamous epithelium), which reduces friction during blood flow.

  2. Tunica Media:

     • Contains smooth muscle tissue and elastic connective tissue.

     • Regulated by the autonomic nervous system and various chemicals affecting vasoconstriction and vasodilation.

     • Typically the thickest layer and regulates blood pressure and flow.

  3. Tunica Externa (Tunica Adventitia):

     • Outermost layer predominantly made of connective tissue.

     • Contains the vasa vasorum (small blood vessels supplying the arterial walls).

• Functionality of Arteries:

  • Arteries withstand high pressure due to their elastic and connective tissue composition, allowing them to expand during systole and recoil during diastole, aiding in forward blood pumping and creating a pulse.

  • Confirming arterial health involves measuring blood pressure using a sphygmomanometer, with normal readings being $120/80 ext{ mm Hg}$, although variations exist.

• Aorta:

  • Largest human artery with an internal diameter of $12.5 ext{ mm}$ and a wall thickness of $2 ext{ mm}$. Other arteries range from $2-6 ext{ mm}$ in diameter with a wall thickness of $1 ext{ mm}$.

B. Arterioles

1. Lead into capillaries and significantly regulate blood flow by controlling resistance through their muscular walls.

2. Arterioles possess the greatest resistance compared to other vessels due to their reduced lumen diameter.

3. Functions:

   • Control blood flow into capillary beds.

   • Regulate Mean Arterial Pressure (MAP).

4. Arteriolar Tone:

   • Represents partial contraction of arteriolar smooth muscle to maintain minimal pressure without stimulation.

C. Distribution of Blood Flow to Organs

1. Blood flow is unequally distributed among organs based on metabolic need.

2. Smooth muscle in arterioles regulates this distribution; local nerve cells provide intrinsic control.

3. Perfusion Pressure:

   • Pressure gradient driving blood flow through an organ; equated to MAP.

4. Hyperemia:

   • Refers to an elevated blood flow rate.

D. Intrinsic Control of Blood Flow and Pressure to Organs

1. Based on Metabolic Activity:

   • Active Hyperemia:

     • Increases blood flow corresponding to heightened metabolic activity. A decline in oxygen and rise in carbon dioxide levels stimulate vasodilation; vasoconstriction occurs when normal oxygen and carbon dioxide levels are restored.

2. Based on Blood Flow:

   • Reactive Hyperemia:

     • Increase in blood flow due to prior blood flow reduction, often caused by vascular blockage.

3. Response to Stretch in Arteriolar Smooth Muscle:

   • Myogenic Response:

     • Stretch-sensitive fibers in arterioles contract upon stretch, enhancing vessel resistance. This response helps maintain flow autoregulation.

4. Chemical Regulation:

   • Various locally secreted chemical messengers affect vascular smooth muscle activity:

     • Nitric Oxide: Released by endothelial cells, prompting vasodilation; histamine from inflamed tissue stimulates nitric oxide synthesis.

     • Adenosine: Released when oxygen is low, induces vasodilation in coronary arteries.

     • Prostacyclin: A vasodilator preventing blood clotting.

E. Extrinsic Control of Blood Flow and Pressure to Organs

1. Governed by factors external to vascular smooth muscle.

2. Sympathetic Division Neurons: Innervates arteriolar smooth muscle, which features Alpha and Beta receptors.

3. Epinephrine: Binds to alpha receptors inducing vasoconstriction; binds to beta receptors in cardiac muscle for vasodilation. Associated with Fight or Flight responses.

4. Vasopressin (Antidiuretic Hormone): Promotes renal vasoconstriction, enhancing blood water reabsorption.

5. Angiotensin II: A hormone causing vasoconstriction.

IV. CAPILLARIES

• Primary exchange site for nutrients and waste products between blood and tissues.

• Features of Capillaries:

  • Smallest blood vessels, approximately $1 ext{ mm}$ long and $5-10 ext{ micrometers}$ in diameter.

  • One cell layer thick, facilitating material diffusion.

  • Highly branched, forming capillary beds ensuring cellular proximity.

  • Increased cross-sectional area leads to decreased blood flow velocity, allowing for efficient material exchange (analogous to rivers flowing into a lake).

• Capillary Density: Related to the metabolic activity of associated tissues.

A. Types of Capillaries (Based on Leakiness)

1. Continuous Capillaries:

   • Most common; permeable to small, water-soluble substances but restricts large materials.

2. Fenestrated Capillaries:

   • Contain large pores for passing water-soluble materials, common in kidneys.

3. Sinusoidal Capillaries:

   • Feature large blood-filled spaces, facilitating substance exchange; found in liver and spleen.

4. Discontinuous Capillaries:

   • Connect fenestrated capillaries to sinusoids.

B. Local Control of Blood Flow Through Capillary Beds

1. Smooth muscle regulates blood flow; Precapillary Sphincters control capillary flow by contraction.

   • Relaxation allows blood through capillaries; contraction redirects flow through metarterioles, bypassing capillaries.

C. Exchange of Materials Across Capillary Walls

1. Mechanisms of Exchange:

   • Simple Diffusion: Movement of small solutes based on concentration gradients.

   • Bulk Flow: Movement of fluids and solutes together; includes filtration (from blood to interstitial fluid) and absorption (from interstitial fluid back to blood).

   • Mediated Transport: Movement via protein carriers in membranes.

   • Transcytosis: Molecule transport via vesicles.

2. Starling Forces:

   • Influence fluid movement into/out of capillaries:

     • Capillary Hydrostatic Pressure (PCAP): Pressure within capillaries (essentially capillary blood pressure).

     • Interstitial Fluid Hydrostatic Pressure (PIF): Pressure outside the capillaries.

     • Colloid Osmotic Pressure: Created by plasma proteins.

   • Net Filtration Pressure (NFP): The difference between filtration pressures and absorption pressures.

     • Positive NFP indicates filtration; negative NFP indicates absorption. Generally, filtration occurs at the arteriole end, while absorption occurs at the venule end.

3. Factors Affecting Filtration and Absorption:

   • Changes in hydrostatic pressure or pathological conditions (e.g., kidney, liver, cardiovascular diseases) can significantly impact tissue fluid levels.

V. VENULES

• Characteristics:

  1. Slightly smaller than arterioles.

  2. Very thin walls lacking smooth muscle.

  3. Smallest venules can function similarly to capillaries.

VI. VEINS

• Formed from the confluence of venules.

• Characteristics:

  1. Larger lumen diameter (approximately $5 ext{ mm}$) compared to arteries.

  2. Thin walls (approximately $0.5 ext{ mm}$) akin to arteries but structured in a similar way.

  3. Lower blood pressure compared to arteries.

  4. Unique presence of valves facilitating unidirectional flow.

  5. High compliance allows for easy stretch due to thin walls, making veins "Volume Reservoirs" compared to arteries which are "Pressure Reservoirs."

  6. Blood volume status can be shifted rapidly to arterial sides during increased demand via heart rate signals.

A. Venous Return to the Heart

• Driven by a pressure gradient of approximately $15 ext{ mm Hg}$ between peripheral veins and the right atrium.

B. Factors Influencing Venous Pressure and Return

1. Skeletal Muscle Pump:

   • One-way valves promote blood flow; muscle contractions compress veins aiding flow towards the heart.

2. Respiratory Pump:

   • Diaphragm actions create thoracic vacuum assisting blood return.

3. Blood Volume:

   • Higher blood volumes increase venous pressure supporting heart return.

4. Venomotor Tone:

   • Sympathetic signals can induce venous smooth muscle contraction, aiding blood movement towards the heart.

VII. REGULATORY PROCESSES RELATED TO BLOOD VESSELS

• A. Cardiac Output:

  • Significantly influences blood pressure affecting artery and vein flow.

• B. Cardiovascular Center of the Medulla: Adjusts heart activity according to oxygen demand.

• C. Baroreceptor Reflex:

  • Baroreceptors located in major blood vessels create action potentials stimulating the sympathetic nervous system, enhancing heart activity during metabolic demand.

• D. Hormonal Control:

  • Various hormones like epinephrine, vasopressin, and angiotensin II affect heart rate and vessel action.

VIII. DISORDERS RELATED TO BLOOD VESSELS AND BLOOD FLOW

1. Hypertension: Persistently high blood pressure (above $120/80$) caused by conditions like atherosclerosis, genetics, poor diet.

   • Treatment Options: Various lifestyle and pharmacological interventions.

2. Venous Pooling:

   • Accumulation of blood in veins leading to symptoms such as light-headedness.

3. Hypotension: Reduced blood pressure, causing dizziness due to insufficient brain blood flow.

   • Compensation mechanisms: Normally reflexes can mitigate this effect under non-critical conditions.