Venous Function

Properties of Veins and the Venous System

Properties of veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology

Veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology are tubular collections of cells that transport deoxygenated blood and waste products from capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries: Histology in the periphery of the body back to the heart. 

• Compared to arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology, veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology have:
  • Larger lumens 
  • Thinner walls
  • Less smooth muscle and elastic Elastic Connective Tissue: Histology tissue
  • Lower pressures
• Veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology are capacitance vessels:
  • Capacitance: how much a vessel can stretch without significantly increasing pressure
  • Veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology are collapsed when empty but able to distend significantly. This property is known as compliance Compliance Distensibility measure of a chamber such as the lungs (lung compliance) or bladder. Compliance is expressed as a change in volume per unit change in pressure. Veins: Histology.
  • The venous system can hold up to 60%–80% of the blood volume at rest.
• Veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology often accompany an artery:
  • Veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology tend to surround the artery in an irregular branching network.
  • Functions as a countercurrent heat Heat Inflammation exchange system → allows cool blood returning from the periphery to be warmed before returning to the heart
• Venous circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment is a low-pressure system:
  • Averages only 10 mm MM Multiple myeloma (MM) is a malignant condition of plasma cells (activated B lymphocytes) primarily seen in the elderly. Monoclonal proliferation of plasma cells results in cytokine-driven osteoclastic activity and excessive secretion of IgG antibodies. Multiple Myeloma Hg
  • The pressure is affected by gravity.
  • The closer the vessel is to the heart, the lower the pressure.

Overcoming gravity: valves and muscle pumps

Pressure in the venous system is too low to spontaneously push blood against gravity; moving blood against gravity up to the heart requires:

• Skeletal muscle pump Pump ACES and RUSH: Resuscitation Ultrasound Protocols: 
  • When skeletal muscles Skeletal muscles A subtype of striated muscle, attached by tendons to the skeleton. Skeletal muscles are innervated and their movement can be consciously controlled. They are also called voluntary muscles. Muscle Tissue: Histology contract, they squeeze the veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology between them.
  • This pushes blood forward in the circuit, toward the heart, increasing preload Preload Cardiac Mechanics.
• 1-way venous valves:
  • Only allow blood to move forward
  • Prevent retrograde flow Retrograde flow Veins: Histology

Regulating capacitance

• Veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology have smooth muscle in their walls:
  • Much less than similarly sized arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology
  • However, still have the ability to constrict and dilate somewhat
• Sympathetic stimulation → venoconstriction
• Venoconstriction → ↓ capacitance → forces blood forward through the venous circuit → ↑ venous return to the heart 
• Venodilation → ↑ capacitance → more blood can be held in venous circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment → ↓ venous return to the heart
• Clinical relevance: The amount of venous return is directly related to preload Preload Cardiac Mechanics, which is one of the major components determining the stroke volume Stroke volume The amount of blood pumped out of the heart per beat, not to be confused with cardiac output (volume/time). It is calculated as the difference between the end-diastolic volume and the end-systolic volume. Cardiac Cycle, and thus cardiac output Cardiac output The volume of blood passing through the heart per unit of time. It is usually expressed as liters (volume) per minute so as not to be confused with stroke volume (volume per beat). Cardiac Mechanics (CO).

Venous Function Curves

Understanding venous function curves

Venous function curves (also known as systemic vascular function curves) plot central venous pressure Central venous pressure The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter ( CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter) against CO.

Cardiac output Cardiac output The volume of blood passing through the heart per unit of time. It is usually expressed as liters (volume) per minute so as not to be confused with stroke volume (volume per beat). Cardiac Mechanics:

• Represents the amount of blood pumped out of the heart per minute
• CO = HR × stroke volume Stroke volume The amount of blood pumped out of the heart per beat, not to be confused with cardiac output (volume/time). It is calculated as the difference between the end-diastolic volume and the end-systolic volume. Cardiac Cycle
  • HR: number of heartbeats per minute
  • Stroke volume Stroke volume The amount of blood pumped out of the heart per beat, not to be confused with cardiac output (volume/time). It is calculated as the difference between the end-diastolic volume and the end-systolic volume. Cardiac Cycle: volume of blood pumped out per contraction
• CO is affected by:
  • Preload Preload Cardiac Mechanics: how much the ventricles stretch prior to contraction (directly related to how much blood fills the ventricles)
  • Afterload Afterload Afterload is the resistance in the aorta that prevents blood from leaving the heart. Afterload represents the pressure the LV needs to overcome to eject blood into the aorta. Cardiac Mechanics: the force the ventricle needs to overcome to pump Pump ACES and RUSH: Resuscitation Ultrasound Protocols blood out to the body (i.e., aortic pressure Aortic pressure Cardiac Mechanics)
  • Inotropy (also called contractility): how hard the heart contracts
• Normally approximately 5–7 L/min
• Usually plotted on the X-axis of venous function curves

Central venous pressure Central venous pressure The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter: 

• Represents the pressure in the vena cava near the right atrium
• Used to assess:
  • Venous return to the heart (the major determinant of atrial filling pressure, and thus preload Preload Cardiac Mechanics)
  • Right atrial pressures Atrial pressures The pressure within the cardiac atrium. It can be measured directly by using a pressure catheter. It can be also estimated using various imaging techniques or other pressure readings such as pulmonary capillary wedge pressure (an estimate of left atrial pressure) and central venous pressure (an estimate of right atrial pressure). Cardiac Cycle
• CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter increases with:
  • ↑ In venous blood volume
  • ↓ In venous compliance Compliance Distensibility measure of a chamber such as the lungs (lung compliance) or bladder. Compliance is expressed as a change in volume per unit change in pressure. Veins: Histology 
• Normally between 0 and 12 mm MM Multiple myeloma (MM) is a malignant condition of plasma cells (activated B lymphocytes) primarily seen in the elderly. Monoclonal proliferation of plasma cells results in cytokine-driven osteoclastic activity and excessive secretion of IgG antibodies. Multiple Myeloma Hg
• Usually plotted on the Y-axis of venous function curves

Venous function curve shape:

• CO and CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter are inversely related:
  • Linear relationship
  • As CO ↑ → CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter ↓ 
  • Increasing CO moves more blood out of venous circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment and into arterial circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment (in the short term) → ↓ venous blood volume → ↓ CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter
• Because veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology can collapse completely, there is a CO at which CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter simply drops to 0.

Mean systemic filling pressure (mean circulatory pressure)

• Pressure in the venous system if the heart is not pumping
• Represents the elastic Elastic Connective Tissue: Histology recoil Recoil Vessels can stretch and return to their original shape after receiving the stroke volume of blood ejected by the left ventricle during systole. Arteries: Histology potential stored in the walls of the systemic vasculature caused by the presence of blood sitting in the tubes
• On venous function curves: the point where the curve meets the CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter axis (usually the Y-axis)

Factors affecting the shape/location of the curve:

• Blood volume
• Systemic vascular resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing (SVR)
• Venous compliance Compliance Distensibility measure of a chamber such as the lungs (lung compliance) or bladder. Compliance is expressed as a change in volume per unit change in pressure. Veins: Histology

How blood volume affects venous function curves

• Increase in blood volume:
  • Shifts the curve up and to the right
  • At a given CO, the CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter will be higher (and vice versa).
  • Mean systemic filling pressure is increased because there is more fluid in the venous system.
• Decrease in blood volume:
  • Shifts the curve down and to the left
  • At a given CO, the CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter will be lower (and vice versa).
  • Mean systemic filling pressure is decreased because there is less fluid in the venous system.

How systemic vascular resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing affects venous function curves

• Increased SVR:
  • The slope of the curve steepens.
  • Mean systemic filling pressure remains unchanged (because the total volume in the circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment is unchanged).
  • ↑ SVR means there is an ↑ afterload Afterload Afterload is the resistance in the aorta that prevents blood from leaving the heart. Afterload represents the pressure the LV needs to overcome to eject blood into the aorta. Cardiac Mechanics; this makes it harder to pump Pump ACES and RUSH: Resuscitation Ultrasound Protocols blood → ↓ SV → ↓ CO
  • Therefore, for a given CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter, the CO will be lower.
• Decreased SVR:
  • The slope of the curve flattens out.
  • Mean systemic filling pressure remains unchanged (because the total volume in the circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment is unchanged).
  • ↓ SVR means there is ↓ afterload Afterload Afterload is the resistance in the aorta that prevents blood from leaving the heart. Afterload represents the pressure the LV needs to overcome to eject blood into the aorta. Cardiac Mechanics; this allows the heart to more easily pump Pump ACES and RUSH: Resuscitation Ultrasound Protocols blood → ↑ SV → ↑ CO
  • Therefore, for a given CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter, the CO will be higher.

How venous compliance Compliance Distensibility measure of a chamber such as the lungs (lung compliance) or bladder. Compliance is expressed as a change in volume per unit change in pressure. Veins: Histology affects venous function curve changes

Venoconstriction:

• Occurs in conjunction with ↑ in SVR due to sympathetic stimulation
• The slope of the curve steepens.
• Mean systemic filling pressure increases (because more blood is being forced back into the heart).
• Venoconstriction almost always occurs in conjunction with an ↑ SVR → there is an ↑ afterload Afterload Afterload is the resistance in the aorta that prevents blood from leaving the heart. Afterload represents the pressure the LV needs to overcome to eject blood into the aorta. Cardiac Mechanics → ↓ CO

Combined Venous/Cardiac Function Curves

Understanding combined venous/cardiac function curves

• Cardiac function curves:
  • Plot CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter or left ventricular end-diastolic pressure Left ventricular end-diastolic pressure Cardiac Mechanics on the X-axis.
  • Plot CO on the Y-axis.
  • Demonstrates the principles of the Frank-Starling law (i.e., how preload Preload Cardiac Mechanics affects CO):
    • Intrinsic properties of actin Actin Filamentous proteins that are the main constituent of the thin filaments of muscle fibers. The filaments (known also as filamentous or f-actin) can be dissociated into their globular subunits; each subunit is composed of a single polypeptide 375 amino acids long. This is known as globular or g-actin. In conjunction with myosins, actin is responsible for the contraction and relaxation of muscle. Skeletal Muscle Contraction and myosin Myosin A diverse superfamily of proteins that function as translocating proteins. They share the common characteristics of being able to bind actins and hydrolyze mgATP. Myosins generally consist of heavy chains which are involved in locomotion, and light chains which are involved in regulation. Within the structure of myosin heavy chain are three domains: the head, the neck and the tail. The head region of the heavy chain contains the actin binding domain and mgATPase domain which provides energy for locomotion. The neck region is involved in binding the light-chains. The tail region provides the anchoring point that maintains the position of the heavy chain. The superfamily of myosins is organized into structural classes based upon the type and arrangement of the subunits they contain. Skeletal Muscle Contraction filaments in the cardiomyocytes allow the cells to contract more, the more they are stretched.
    • As left ventricular end-diastolic pressure Left ventricular end-diastolic pressure Cardiac Mechanics increases due to increased ventricular filling Ventricular filling Cardiac Cycle, stroke volume Stroke volume The amount of blood pumped out of the heart per beat, not to be confused with cardiac output (volume/time). It is calculated as the difference between the end-diastolic volume and the end-systolic volume. Cardiac Cycle increases as well.
    • ↑ Left ventricular end-diastolic pressure Left ventricular end-diastolic pressure Cardiac Mechanics (i.e., ↑ preload Preload Cardiac Mechanics) → ↑ myofilament stretching → stronger contraction → ↑ SV → ↑ CO
• Cardiac function curves can be “laid over” venous function curves.
  • Venous function curve axes are flipped.
  • The 2 curves will cross each other at an equilibrium Equilibrium Occurs when tumor cells survive the initial elimination attempt These cells are not able to progress, being maintained in a state of dormancy by the adaptive immune system. In this phase, tumor immunogenicity is edited, where T cells keep selectively attacking highly immunogenic tumor cells.This attack leaves other cells with less immunogenicity to potentially develop resistance to the immune response. Cancer Immunotherapy point:
    • Represents the “steady-state” operating point for a particular set of physiologic conditions
    • Usually around a CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter of 2 mm MM Multiple myeloma (MM) is a malignant condition of plasma cells (activated B lymphocytes) primarily seen in the elderly. Monoclonal proliferation of plasma cells results in cytokine-driven osteoclastic activity and excessive secretion of IgG antibodies. Multiple Myeloma Hg and a CO of 5L/min (normal values)

How changes in inotropy affect combined venous/cardiac function curves

Clinical scenario #1: MI MI MI is ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation, usually from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Clinical presentation is most commonly with chest pain, but women and patients with diabetes may have atypical symptoms. Myocardial Infarction leading to a decrease in inotropy

• A ↓ in inotropy:
  • Weaker heart contraction → ↓ SV → ↓ CO
  • Flattens the cardiac function curve 
• If venous function remains the same, the equilibrium Equilibrium Occurs when tumor cells survive the initial elimination attempt These cells are not able to progress, being maintained in a state of dormancy by the adaptive immune system. In this phase, tumor immunogenicity is edited, where T cells keep selectively attacking highly immunogenic tumor cells.This attack leaves other cells with less immunogenicity to potentially develop resistance to the immune response. Cancer Immunotherapy point moves down along the venous function curve, resulting in:
  • ↓ CO
  • ↑ CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter
• This ↓ CO may be too low to sustain life, so the body needs to compensate for the decreased inotropy in other ways.
• It can do this by increasing blood volume (by increasing renal absorption Absorption Absorption involves the uptake of nutrient molecules and their transfer from the lumen of the GI tract across the enterocytes and into the interstitial space, where they can be taken up in the venous or lymphatic circulation. Digestion and Absorption of water):
  • ↑ Blood volume shifts the venous function curve up and to the right
  • The new equilibrium Equilibrium Occurs when tumor cells survive the initial elimination attempt These cells are not able to progress, being maintained in a state of dormancy by the adaptive immune system. In this phase, tumor immunogenicity is edited, where T cells keep selectively attacking highly immunogenic tumor cells.This attack leaves other cells with less immunogenicity to potentially develop resistance to the immune response. Cancer Immunotherapy point has a higher CO (also has a higher CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter).
• Volume overloading the heart allows the body to increase CO into a range capable of life-sustaining perfusion.

How changes in blood volume affect combined venous/cardiac function curves

Clinical scenario #2: hemorrhage

• A ↓ in blood volume shifts the venous function curve down and to the left
• ↓ Blood volume, means:
  • ↓ CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter 
  • ↓ CO (may be too low to sustain perfusion)
• The body can compensate by increasing inotropy (heart pumps more strongly)
  • Steepens the cardiac function curve
  • New equilibrium Equilibrium Occurs when tumor cells survive the initial elimination attempt These cells are not able to progress, being maintained in a state of dormancy by the adaptive immune system. In this phase, tumor immunogenicity is edited, where T cells keep selectively attacking highly immunogenic tumor cells.This attack leaves other cells with less immunogenicity to potentially develop resistance to the immune response. Cancer Immunotherapy point has a higher CO
• Stronger cardiac contractions bring the CO back into a normal range, despite the lower CVP CVP The blood pressure in the central large veins of the body. It is distinguished from peripheral venous pressure which occurs in an extremity. Central Venous Catheter.