Venous Return

Question 1

Venous return

Answer 1

-the rate of blood flow back to the heart
-under normal conditions venous return=cardiac output (because cardiac system is a closed loop)

Question 2

Major affects in preload

Answer 2

-Blood volume/venous return and venous compliance affects preload (filling of ventricle)
-changes in preload then effect stroke volume, cardiac output, and mean arterial pressure

Question 3

Systemic vs pulmonary circulation

Answer 3

-systemic circulation will contain ~84% of blood, and ¾ of that 84% will be located within the veins

Question 4

Balance between venous return and cardiac output

Answer 4

-regulated by Frank starling mechanism
-deliver more back then pump more out

Ex. venous return increased on right side, ventricular preload on right side increased, stroke volume increased on right side, which then increases pulmonary flow. An increase of pulmonary flow, increases pulmonary venous return, which increased left ventricular preload, which increases stroke volume

Question 5

How is venous return measured?

Answer 5

-assessed using central venous pressure which is the pressure where blood enters the right atrium
-venous return=right atrial preload

Question 6

Central venous pressure

Answer 6

-blood pressure in the vena cava near the right atrium
-typically ranges from 2-6 mm Hg (right atrial pressure is slightly lower which is why flow mores from vena cava to atrium)

Question 7

What variables affect Venous pressure?

Answer 7

1. systemic vascular resistance (regulates blood flow into venous system)- when high, rate of blood flow entering venous system will be low, and decrease pressure

2.cardiac output- as it increases, blood is removed more rapidly, and venous pressure decreases
Pressure and volume in venous system
-greater volume entering venous system will result in increased pressure, and therefore greater venous return

Question 8

Hemodynamic factors affecting venous return

Answer 8

1.Venous pressure (Pv)
2.Right atrial pressure (PRa)
3.Venous resistance (RV)

Question 9

Formula of hemodynamic factors and venous return

Answer 9

Venous return= (venous pressure-right atrial pressure)/venous resistance

Question 10

What results in Increased venous return?

Answer 10

increased venous pressure, decreased right atrial pressure and decreased venous resistance

Question 11

what results in decreased venous return?

Answer 11

-decreased venous pressure, increased right atrial pressure, and increased venous resistance

Question 12

What does the intraplural pressure regulate?

Answer 12

-intrapleural space- pressure within the intrapleral space between the lungs,heart and chest wall
-pressure of right atrium and vena cava

Question 13

Lung inspiration and venous return (right side of heart)

Answer 13

Inspiration: chest wall expands, diaphragm descends- intrapleural pressure becomes negative leading to expansion of lungs, vena cava and cardiac chambers.
- Expansion of the cardiac chambers results in right atrial pressure decreasing
- Expansion of vena cava results in venous resistance decreasing
- Both result in net increase in venous return

Question 14

Lung expiration and venous return (right side of heart)

Answer 14

-chest wall and diaphragm close space
-intrapleural pressure increases
-right atrial pressure increases
-venous resistance increases
THEREFORE venous return decreases

Question 15

Why do animals breath fast and hard?

Answer 15

-to increase venous return

Question 16

Inspiration and left side of heart

Answer 16

-inspiration results in increased pulmonary blood volume and therefore reduction in blood flow to left side of the heart and therefore decreased venous return

Question 17

Expiration and left side of the heart

Answer 17

-deflation of lungs forces more blood out of the lungs resulting in increase in venous return to the left side of heart

Question 18

Muscle pump and venous return

Answer 18

-peripheral veins surrounded by muscle. Muscles compress the vein when they contract (increase venous pressure) and will propel blood in venous system back to the heart
-also increases movement from arterial to venous side (by increasing pressure difference)

Question 19

Sympathetic nerves and venous return

Answer 19

-small veins embedded in organs are innervated by sympathetic nerves (also can be affected by catecholamines and angiotensin 2)
-if increase sympathetic nerve activity, reduction of venous compliance occurs, increasing venous pressure, and therefore increasing venous return

Question 20

Exercise and venous return

Answer 20

-Skeletal muscle and respiratory pump increase venous return which increases end diastolic volume
-Sympathetic nervous system increases ventricular inotropy (contractility) which decreases end systolic volume
- increased venous return, and increases inotropy result in increase in stroke volume
-increased blood pressure (CO X SVR) increases afterload which would limit stroke volume. But healthy animals have the ability to overcome this by increasing inotropy and preload. Sick animals will be less suited to deal with this.

Question 21

Why do healthy animals have the ability to overcome increase in afterload and limit to stroke volume?

Answer 21

-remodelling of the heart through exercise