Properties of Veins and the Venous System

Properties of veins

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 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 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, veins have:
  • Larger lumens 
  • Thinner walls
  • Less smooth muscle and elastic 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 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 are collapsed when empty but able to distend significantly. This property is known as compliance.
  • 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 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 tend to surround the artery in an irregular branching network.
  • Functions as a countercurrent heat exchange system → allows cool blood returning from the periphery to be warmed before returning to the heart
• Venous circulation is a low-pressure system:
  • Averages only 10 mm 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: 
  • When skeletal muscles contract, they squeeze the veins between them.
  • This pushes blood forward in the circuit, toward the heart, increasing preload.
• 1-way venous valves:
  • Only allow blood to move forward
  • Prevent retrograde flow

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 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
  • 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 → ↓ venous return to the heart
• Clinical relevance: The amount of venous return is directly related to preload, which is one of the major components determining the stroke volume, and thus cardiac output (CO).

Venous Function Curves

Understanding venous function curves

Venous function curves (also known as systemic vascular function curves) plot central venous pressure (CVP) against CO.

Cardiac output:

• Represents the amount of blood pumped out of the heart per minute
• CO = HR × stroke volume
  • HR: number of heartbeats per minute
  • Stroke volume: volume of blood pumped out per contraction
• CO is affected by:
  • Preload: how much the ventricles stretch prior to contraction (directly related to how much blood fills the ventricles)
  • Afterload: the force the ventricle needs to overcome to pump blood out to the body (i.e., aortic pressure)
  • 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: 

• 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)
  • Right atrial pressures
• CVP increases with:
  • ↑ In venous blood volume
  • ↓ In venous compliance 
• Normally between 0 and 12 mm Hg
• Usually plotted on the Y-axis of venous function curves

Venous function curve shape:

• CO and CVP are inversely related:
  • Linear relationship
  • As CO ↑ → CVP ↓ 
  • Increasing CO moves more blood out of venous circulation and into arterial circulation (in the short term) → ↓ venous blood volume → ↓ CVP
• Because veins can collapse completely, there is a CO at which CVP 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 recoil 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 axis (usually the Y-axis)

Factors affecting the shape/location of the curve:

• Blood volume
• Systemic vascular resistance (SVR)
• Venous compliance

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 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 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 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 is unchanged).
  • ↑ SVR means there is an ↑ afterload; this makes it harder to pump blood → ↓ SV → ↓ CO
  • Therefore, for a given CVP, 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 is unchanged).
  • ↓ SVR means there is ↓ afterload; this allows the heart to more easily pump blood → ↑ SV → ↑ CO
  • Therefore, for a given CVP, the CO will be higher.

• 

• 

How venous compliance 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 → ↓ CO

Combined Venous/Cardiac Function Curves

Understanding combined venous/cardiac function curves

• Cardiac function curves:
  • Plot CVP or left ventricular end-diastolic pressure on the X-axis.
  • Plot CO on the Y-axis.
  • Demonstrates the principles of the Frank-Starling law (i.e., how preload affects CO): 
    • Intrinsic properties of actin and myosin filaments in the cardiomyocytes allow the cells to contract more, the more they are stretched.
    • As left ventricular end-diastolic pressure increases due to increased ventricular filling, stroke volume increases as well.
    • ↑ Left ventricular end-diastolic pressure (i.e., ↑ preload) → ↑ 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 point: 
    • Represents the “steady-state” operating point for a particular set of physiologic conditions
    • Usually around a CVP of 2 mm 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 point moves down along the venous function curve, resulting in:
  • ↓ CO
  • ↑ CVP
• 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 of water):
  • ↑ Blood volume shifts the venous function curve up and to the right
  • The new equilibrium point has a higher CO (also has a higher CVP).
• 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 
  • ↓ 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 point has a higher CO
• Stronger cardiac contractions bring the CO back into a normal range, despite the lower CVP.