Cardiac Output, Venous Return, and Their Regulation

Calculating venous return

(mean systemic filling pressure) - (right atrial pressure) / Resistance to venous return (2/3 in veins and 1/3 in arteries, arterioles) in healthy adult 7-0/1.4 mm Hg per L/min of blood flow

common feature among all conditions of High Cardiac Output

* they all result from chronically reduced total peripheral resistance 1) Beriberi - caused by insufficient quantity of thiamine (vit. B1) in the diet. Lack of this causes diminished ability of the tissues to use some cellular nutrients, and the local tissue flow blood mechanisms in turn cause marked compensatory peripheral vasodilation (sometimes to as little as one half of normal) -->long term levels of venous return and cardiac output also increase to twice the normal 2) Arteriovenous fistula (shunt) -from major artery to vein --> greatly decreases total peripheral resistance, etc. 3) Hyperthyroidism - increased metabolism in tissues --> reduced peripheral resistance from vasodilation 4) Anemia - reduced viscosity of the blood resulting from decreased concentration of RBCs. also DIMINISHED delivery of oxygen to the tissues

Average cardiac output in adults estimation

-5 L/min -plateau for normal cardiac function curve is 13 L/min

A stretched Right atrium initiates ___

-Bainbridge reflex; passing first to the vasomotor center of the brain and then back to the heart by way of the sympathetic nerves and vagi, also to INCREASE the heart rate

Venous Return curve

-also relates venous return to right atrial pressure

Blocking the sympathetic nervous system

-by total spinal anesthesia -drugs (hexamethonium) - blocks transmission of nerve signals through the autonomic ganglia 1) the mean systemic filling pressure falls from 7 to 4 2) effectiveness of heart as a pump decreases to about 80% of normal

Cardiac Index

-cardiac output per square meter of body surface area

Hypereffective heart

-caused by 1) nervous stimulation and 2) hypertrophy of the heart muscle 1) combo of sympathetic stimulation and parasympathetic inhibition --> greatly increases heart rate and increases strength of heart contraction (contractility) to twice its normal rate. **combining these two effects --> raises output up to 25 L/min (almost twice the plateau of a normal curve 2) Hypertrophy -> increased pumping effectiveness **combining the two effects you can increase output to 30-40 L/min (2 and a half times the level in a normal person)

Net effect of opening a large arteriovenous fistula

-increase in cardiac output from 5 L/min --> 13 L/min and an increase in right atrial pressure to about +3 mm Hg (the fistula allows the blood to avoid most of the peripheral resistance and LOWERS ARTERIAL PRESSURE) - ~ a minute later, sympathetic nerve reflexes restore arterial pressure almost to normal and 1) increase the mean systemic filling pressure (b/c of constriction of all veins and arteries) from 7 --> 9 (increases venous return --> increased cardiac output and the right atrial pressure increases to about 4 mm Hg -after several weeks blood volume increases (b/c of reduced kidney output due to the slight reduction in arterial pressure). increased systemic filling pressure and venous return -->hypertrophy of heart and increased cardiac output (up to 20 L/min)

Hypoeffective Heart

-increased arterial pressure (hypertension) -inhibition of nervous excitation -pathological abnormal heart rhythms -coronary artery blockage -valvular heart disease -congenital heart disease -Myocarditis -cardiac hypoxia

Increasing arterial pressure during exercise

-intense metabolism leads to arteriole mm. relaxation to allow adequate oxygen and other nutrients needed to sustain muscle contraction --> decreases peripheral resistance which would also normally decrease arterial pressure **Nervous system immediately compensates by: 1) causing large vein constriction 2) increased heart rate 3) increased contractility

The normal external pressure on the heart is equal to the ____

-normal intrapleural pressure of -4 mm Hg

Venous Return

-quantity of blood flowing from the veins into the right atrium each minute **the primary controller of cardiac output b/c the heart will (most of the time) pump whatever amount of blood flows into the right atrium. Once this is exceeded the heart becomes the limiting factor *The venous return and cardiac output must equal each other except for a few heartbeats at a time when blood is temporarily stored in or removed from the lungs and heart

Cardiac output

-quantity of blood pumped into the aorta each minute by the heart young healthy men = 5.6 L/min women = 4.9 L/min factors affecting: 1) body metabolism 2) exercising 3) age 4) size of the body

Decreased cardiac output caused by cardiac factors

-severe coronary blood vessel blockage and subsequent MI -severe valvular disease -myocarditis -cardiac tamponade -cardiac metabolic derangements

The greater the difference b/w the mean systemic filling pressure and the right atrial pressure ---> ____

-the greater becomes the venous return

mean systemic filling pressure

-when all flow in the systemic circulation ceases (both the arterial and venous pressures come to equilibrium) * 7 mm Hg (5000 mL of blood) this pressure goes up linearly as you add blood volume

Very negative pressures in the Right Atrium

-when the right atrial pressure falls below zero (below atmospheric pressure) further increase in venous return ceases -> because of collapse of veins entering the chest (the negative pressure in the right atrium sucks the walls of the veins together where they enter the chest **therefore even very negative pressures in the right atrium cannot increase venous return significantly above that which exists at a normal atrial pressure of 0 mm Hg

Stretch of the ____ has a direct effect on the rhythmicity of the node itself to increase heart rate as much as 10-15%

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Decreased Venous return

**decreased cardiac output 1) from decreased blood volume (most common) resulting most often from hemorrhage. Not enough blood in the peripheral vessels to create peripheral vascular pressures high enough to push blood back to the heart 2) Acute Venous dilation - when the sympathetics become inactive. FAINTING often results from sudden loss of sympathetic nervous system activity 3) Obstruction of the large veins 4) Decreased tissue mass, especially decreased skeletal muscle mass - not as much oxygen is needed (especially with aging) so the heart does not need to pump as much blood to these muscles 5) Decreased metabolic rate of tissues - happens in skeletal muscle during prolonged bed rest. Also HYPOTHYROIDISM. ***if the cardiac output falls below that level required for adequate nutrition of the tissues --> person is said to suffer from circulatory shock

3 factors that affect venous return

1) Right atrial pressure 2) Degree of systemic filling (mean systemic filling pressure) measured when all blood flow is stopped 3) Resistance to blood flow - between the peripheral vessels and the right atrium

Compensatory effects initiated in response to increased blood volume

1) increased capillary pressure forces fluid out -->returning blood volume toward normal 2) The increased pressure in the veins causes them to keep distending gradually by a mechanism called stress-relaxation (causes venous blood reservoirs (liver and spleen) to distend and reduce mean systemic filling pressure 3) autoregulation causing an increase in peripheral vascular resistance --> increasing resistance to vascular return and ultimately returning cardiac output and mean systemic filling pressure to normal

Cardiac output formula

= arterial pressure/Total Peripheral Resistance **if arterial pressure is constant

Right atrial pressure vs. $cardiac output curve$

Right shifts =takes more right atrial pressure to fill the heart chambers with blood. Occurs in situations where external pressure is increased: -During cyclical changes in breathing (as much as +/- 50 mm Hg during heavy exercise -positive pressure breathing -opening the thoracic cage -cardiac tamponade *breathing against a negative pressure shifts the curve to the left