CVS NOTE
THE CIRCULATION
Function of the Circulation:
The primary functions to service the needs of body tissues include:
Transport of nutrients to body tissues.
Removal of waste products from tissues.
Transport of hormones from one part of the body to another.
Rate of Blood Flow:
Controlled based on tissue needs for nutrients, ensuring efficient supply.
Characteristics of the Circulation
Divisions of Circulation:
Systemic Circulation:
Supplies blood to all tissues of the body except the lungs; also known as greater or peripheral circulation.
Pulmonary Circulation:
Responsible for supplying blood to the lungs for oxygenation.
Functional Parts of the Circulation
Arteries:
Transport blood under high pressure to tissues.
Arterioles:
Smallest branches of the arterial system; act as control conduits for blood flow into capillaries.
Capillaries:
Sites for exchange of fluids, nutrients, electrolytes, hormones, and other substances between blood and interstitial fluid.
Venules:
Collect blood from capillaries, merging into larger veins.
Veins:
Conduct blood from venules back to the heart; also serve as major reservoirs for extra blood.
Blood Volume Distribution
Cross-sectional Areas of Systemic Vessels:
Veins possess a cross-sectional area (CA) four times larger than arteries, explaining their capacity for blood storage.
The same volume of blood must flow through each segment of the circulation within a minute, leading to:
Velocity of Blood Flow: Inversely proportional to vascular cross-sectional area; for example, average velocity is:
Aorta: approximately 33 cm/sec.
Capillaries: approximately 0.3 mm/sec.
Blood stays in capillaries for only 1 to 3 seconds due to their short length of 0.3 to 1 millimeter.
Pressures in the Circulation
Systemic Circulation Pressures:
Mean Aortic Pressure: about 100 mm Hg.
Arterial pressure alternates:
Systolic Pressure: 120 mm Hg.
Diastolic Pressure: 80 mm Hg.
Define Blood Pressure: The force exerted by blood against vessel walls.
Relationship between Blood Pressure (BP), Cardiac Output (CO), and Total Peripheral Resistance (TPR).
Define Pulse Pressure and Mean Arterial Pressure (MAP).
As blood flows through the systemic circulation, mean pressure falls to about 0 mm Hg by the right atrium.
Pulmonary Circulation Pressures:
Pressures in the pulmonary arteries are lower:
Pulmonary Artery Systolic Pressure: averages 25 mm Hg.
Diastolic Pressure: 8 mm Hg.
Mean Pulmonary Arterial Pressure: 16 mm Hg.
Basic Principles of Circulatory System
Rate of blood flow is controlled precisely in relation to tissue needs.
Cardiac output is influenced mainly by sum of local tissue flows.
Arterial pressure is controlled independently of local blood flow control or cardiac output control.
Interrelationships Among Pressure, Flow, and Resistance
Blood Flow Definition:
Quantity of blood passing a point in circulation over time.
Units of Measurement: milliliters per minute, liters per minute, or milliliters per second.
Average total circulation flow at rest: about 5000 ml/min (CO).
Blood Flow Determinants
Determined by:
Pressure Difference between the two ends of the vessel (pressure gradient).
Vascular Resistance: Impedance to blood flow in vessels.
Types of Blood Flow
Two Types of Flow:
Laminar Flow:
Blood flows smoothly in streamlined layers; velocity is highest at the center.
Turbulent Flow:
Blood flows in all directions and mixes within the vessel, occurring under conditions like:
High blood flow rates.
Passing of blood by an obstruction.
Sharp turns in vessels.
Flowing over rough surfaces.
Characteristics of Turbulent Flow
Presence of eddy currents increases resistance due to added friction.
Tendency for turbulent flow depends on:
Directly proportional to blood flow velocity, vessel diameter, and blood density.
Inversely proportional to blood viscosity.
Blood Pressure
Definition:
Force exerted by blood against unit area of vessel wall.
Standard Units of Measurement:
Millimeters of mercury (mm Hg).
Occasionally, centimeters of water (cm H2O).
Resistance to Blood Flow
Resistance Defined:
Impedance to blood flow in vessels.
Respective terminologies:
Total Peripheral Resistance: Resistance in systemic circulation.
Total Pulmonary Vascular Resistance: Resistance in pulmonary circulation.
Arrangement of Blood Vessels
Blood vessels can be arranged in series or parallel:
Series Arrangement:
Same flow through vessels (R_total = R1 + R2 + …).
Parallel Arrangement:
Total resistance is less than the resistance of any single vessel, given by the formula:
\frac{1}{R{total}} = \sum{i=1}^{n} \frac{1}{R_{i}} .
Poiseuille’s Law
Flow Equations:
Flow rate ( extit{F}) is influenced by:
\Delta P (pressure difference), r (radius of vessel), l (length of vessel), and η (viscosity of blood).
Effect of Blood Viscosity on Blood Flow
Increased viscosity reduces flow through vessels.
Viscosity correlates with the number of red blood cells (hematocrit).
Greater hematocrit results in greater viscosity.
Conductance of Blood in a Vessel
Conductance Definition:
Measure of blood flow through a vessel for a given pressure difference; it is the reciprocal of resistance.
Small changes in vessel diameter cause large changes in conductance, increasing with the fourth power of diameter.
Effects of Pressure on Blood Flow
Increase in arterial pressure not only pushes blood through vessels but also distends them, decreasing resistance.
Decreased sympathetic activity influences vessel dilation, significantly increasing blood flow.
Strong sympathetic stimulation may constrict vessels, occasionally reducing blood flow to zero.
Vascular Distensibility
Definition:
Pressure increases within blood vessels cause them to dilate, reducing resistance, and promoting increased blood flow.
Distensibility Ranking:
Veins are the most compliant vessels, acting as reservoirs for blood.
Vascular Compliance
Definition of Vascular Compliance:
Total blood volume that can be stored per millimeter of mercury pressure rise.
Compliance of systemic veins is about 24 times that of corresponding arteries due to their higher distensibility and greater volume.
Arterial Pressure Pulsations
Systolic Pressure: about 120 mm Hg; Diastolic Pressure: about 80 mm Hg.
Pulse Pressure: difference between systolic and diastolic pressures; averages around 40 mm Hg.
Factors Affecting Pulse Pressure
Stroke volume output of the heart.
Compliance of the arterial tree.
Ejection characteristics from the heart during systole.
Abnormal Conditions
Pulse pressure may rise to twice normal levels in old age.
Conditions like aortic stenosis decrease pressure pulse; patent ductus arteriosus increases pressure, while aortic regurgitation alters contour due to valve absence.
Damping of Pressure Pulses
Definition: Progressive decrease of pulsations seen in peripheral circulation.
Causes include:
Resistance to blood movement in vessels.
Compliance of the vessels.
Mean Arterial Pressure (MAP): Average pressure in arterial circulation, weighted significantly by diastolic pressure.
Central Venous Pressure
Pressure measured in the right atrium [Central Venous Pressure (CVP)].
Regulated by the balance:
Heart's ability to pump blood.
Blood return from peripheral veins (venous return).
Factors Impacting CVP
Increased blood volume.
Increased large vessel tone.
Arteriole dilation.
Normal right atrial pressure is 0 mm Hg, which can peak at 20 to 30 mm Hg under abnormal conditions (e.g., serious heart failure).
Can decrease to about -3 to -5 mm Hg under exceptional circumstances.
Venous Resistance
Large veins have minimal resistance when distended (approaching zero).
Compression occurs in veins by surrounding tissues, especially within the thorax.
Example of Venous Resistance
Veins from Arms: Compressed over the first rib, leading to low neck vein pressure (possibly collapsing under atmospheric pressure).
Veins in the Abdomen: Compressed by organs increasing resistance to flow.
Effects of Gravity on Venous Pressure
Standing still can cause high pressures (up to 90 mm Hg) in veins of the feet due to gravity, while right atrium pressure remains at 0 mm Hg.
Venous Pump
Mechanism: Muscle movement compresses veins, directing blood to heart, aided by valves preventing backflow.
Consequential Effects of Inactivity: Lack of movement causes venous pressure in legs to rise, leading to swelling and reduced blood volume in circulation.
Blood Reservoir Function of Veins
Characteristics:
Veins generally contain over 60% of blood volume at any given time; they hold a significant variable reservoir function.
Specific Blood Reservoirs:
Spleen
Liver
Large abdominal veins
Venous plexus beneath the skin
Heart and lungs.
Varicosities
Definitions:
Varicosities: Abnormally dilated and tortuous superficial veins measuring > 3 mm in diameter.
Reticular Veins: Abnormally dilated skin veins measuring 1-3 mm in diameter.
Clinical Presentation of Varicose Veins
Dilated, tortuous, elongated, and nodular veins in lower extremities.
May be asymptomatic or lead to clinical effects.
Associated conditions include intraluminal thrombosis and valvular deformities, potentially presenting heavy sensations or pain.
Edema and skin changes may occur.
Clinical Effects
Blood pooling, potential thrombosis, and emboli.
Haemosiderin staining and lipodermatosclerosis.
Venous stasis resulting in various issues like dermatitis, eczema, and ulcers.
Local and Humoral Control of Blood Flow
Local Control: Each tissue regulates its blood flow based on metabolic needs, maintaining minimal necessary levels.
Phases of Local Blood Flow Control
Acute Control:
Rapid changes via local vasodilation or vasoconstriction of arterioles, occurring quickly (seconds to minutes).
Long-term Control:
Changes over days/weeks, using adaptations in blood vessel size and number for better regulation.
Acute Control Mechanisms
Theories Explaining Local Blood Flow:
Vasodilator Theory:
Increased metabolism or oxygen deficiency stimulates release of vasodilator substances like adenosine and carbon dioxide resulting in vasodilation.
Oxygen Lack Theory:
Inadequate nutrients (oxygen in particular) lead to open sphincters allowing increased blood flow as smooth muscle relaxes.
Examples of Acute Control
Reactive Hyperemia:
Followed after a temporary block of blood supply; results in a 4-7x increase in blood flow upon restoration.
Active Hyperemia:
Increased blood flow during tissue activity (e.g., exercise or secretion).
Autoregulation of Blood Flow
Mechanism where tissue blood flow returns to normal levels out of proportion to maintained elevated arterial pressures, using:
Metabolic Mechanisms: Excess nutrients lead to constriction.
Myogenic Mechanisms: High arterial pressure stretches vessels leading to reactive vasoconstriction.
Long-term Blood Flow Regulation
Develops over longer periods (days, weeks) involving physical restructuring of tissue vasculature to meet metabolic needs.
Influence of Oxygen
Low oxygen conditions drive increased vascularity; observed in animals at altitude or during specific development processes.
Development of Collateral Circulation
Formation of new vessels occurs in cases of arterial or venous blockage for supply restoration, initially through dilation of existing loops.
Humoral Control of Circulation
Control via hormones and substances into body fluids.
Hormones have systemic effects or local influences.
Vasoconstrictor Agents
Norepinephrine & Epinephrine:
Norepinephrine is a powerful vasoconstrictor; epinephrine may cause vasodilation in certain tissues (e.g., coronary arteries during increased activity).
Angiotensin II: Increases total peripheral resistance through arteriolar constriction.
Vasopressin: Acts as a vasoconstrictor and promotes water reabsorption in kidneys.
Vasodilator Agents
Bradykinin: Triggers vasodilation and increases capillary permeability.
Histamine: Released during damage/inflammation; causes arteriole vasodilation and increased capillary permeability.
Ionic and Chemical Factors in Vascular Control
Chemical factors can impact vascular tone:
Calcium Ion: Constricts vessels.
Potassium Ion: Causes vasodilation.
Magnesium Ion: Produces powerful vasodilation.
Hydrogen Ion: Lower pH induces dilation; slight increases can constrict.
The Microcirculation
Definition: Essential for nutrient transport to tissues and waste removal.
Capillary Structure: Comprises a single-layer, highly permeable endothelial cell layer.
Structure of Microcirculation
Nutrient Arteries: Branch into arterioles, proceeding to capillaries, with arterioles having muscular walls.
Metarterioles: Intermittent smooth muscle fibers act as precapillary sphincters that regulate capillary access.
Venules: Larger than arterioles; have weaker muscular coats.
Capillary Wall Structure
Composed of uni-cellular layers with intercellular clefts, allowing selective permeability.
**Variations in Permeability:
Brain:** Extremely tight junctions permit passage of small molecules (e.g., water).
Liver: Wider clefts allow significant substance transfer.
Gastrointestinal Tract: Intermediate permeability compared to muscles and liver.
Kidneys: Specialized structures (fenestrae) enable high substance permeability.
Blood Flow in Capillaries
Intermittent blood flow due to metarteriole and precapillary sphincter contractions governed by tissue oxygen levels.
Material Exchange at Capillaries
Primary Mechanism: Diffusion, particularly for lipid-soluble substances.
Lipid-insoluble substances pass through intercellular pores.
Fluid Filtration Across Capillaries
Determined by four forces:
Capillary Hydrostatic Pressure.
Interstitial Fluid Hydrostatic Pressure.
Capillary Plasma Colloid Osmotic Pressure.
Interstitial Fluid Colloid Osmotic Pressure.
Positive net filtration leads to fluid movement out of capillaries; negative results in absorption back into capillaries.
The Lymphatic System
Provides a route for fluid from interstitial spaces to circulate back into blood.
Lymphatics facilitate transport of large molecular weight substances.
Structure of Lymphatic Capillaries
Comprised of endothelial cells with overlapping edges forming valves.
Formation of Lymph
Derived from interstitial fluid entering lymphatics; composition closely resembles interstitial fluid.
Lymphatic Pump and Flow
Stretching of lymph channels causes smooth muscle contraction, pumping lymph through vessels.
External factors such as muscle contractions and body movements also enhance lymph flow.
Factors Influencing Lymph Flow
Elevated interstitial fluid pressure, decreased plasma colloid osmotic pressure, increased interstitial fluid colloid osmotic pressure, and heightened capillary permeability promote lymph flow.
Lymphatic Channels of the Body
Most body tissues feature lymphatic drainage, with exceptions including superficial skin layers and the CNS.
Major Lymph Drainage Pathways
Lymph vessels from the lower body drain into the thoracic duct.
Left head/arm/part of chest lymph also enters the thoracic duct.
Right head/arm/right chest lymph drains into the right lymph duct.
Functions of Lymphatic Circulation
Return of proteins to systemic circulation.
Transport long-chain fatty acids/cholesterol via chylomicrons.
Lymph nodes filter bacteria and particulates.
Lymphocytes and neutrophils in lymph contribute to immune defense.
Maintenance of blood volume in relation to blood volume fluctuations.