The venous system

Question 1

what are the characteristics of veins?

Answer 1

• they have thin walls
• they have **wide lumens **
• they have** high compliance** (ability to stretch or narrow in response to high or low pressure)
• they have **low bloow pressure **

Question 2

what are the** 2 main functions** of veins?

Answer 2

1. to transport blood back to the heart - right atrium (ie venous return)
2. to act as capacitance vessels - accomodating large volumes of blood - due to their structure high compliance

Question 3

what is stroke volume?

Answer 3

• the volume of blood that is ejected by the ventricles duting each contraction

Question 4

what is another name for veins?

Answer 4

capacitance vessels - they have a large capacity for blood

Question 5

what is the force that drives the blood in the veins back to the heart?

Answer 5

the pressure differenence between the peripheral veins & the right atrium

Question 6

what do the peripheral veins of the arms and legs contain?

Answer 6

valves - to prevent back flow & insure that blood only flows in the direction towards the heart

Question 7

How does** posture** influence venous pressure?

ie standing upright from a supine position

Answer 7

• movement from a supine (lying down) position to a standing position is a challenge for the CV system
• after you stand up from a supine position, gravitational forces pull venous blood to the lower limbs and 500ml of blood can be redistributed to the peripheral veins

Question 8

what happens to the veins above heart level after posture change?

Answer 8

• unsupported veins eg superficial veins (veins close to surface of body) collapse
• cranial cavity veins do not collapse as the CSF pressure falls by the same amount as the venous pressure

Question 9

what happens to the veins below heart level aftr posture change?

Answer 9

• the valves in the peripheral veins close and therefore preven backflow & ensure blood is gooing toward the heart
• venous pooling occurs - slower transit time of blood through the venous circulation back to the heart

Question 10

what is venous pooling? what effect does it have?

Answer 10

• blood is redistributed to the peripheral veins - this is called venous pooling - leads to a slower transit time of venous circulation back to the heart
• it leads to a drop in stroke volume and therefore also a drop in cardiac output

Question 11

what type of hypotension does changing posture cause?

Answer 11

postural hypotension or orthostatic hypotension

Question 12

after standing up from a supine position, what is the compensation (self protecting mechanisms) or upright posture?

Answer 12

• 500ml of blood is redistributed to the peripheral veins which causes an increase in capillary filtration
• neuroendocrine responses - certain mechanisms are activated in response to low pressure
• renal excretion of salt and water

Question 13

what effect does an** increased capillary pressure **in the lower limbs have?

Answer 13

• an increase in capillary filtration in the lower limbs reduces plasma volume
• reducing the plasma volume also reduces the arterial pressure

Question 14

what effect do the neuroendocrine responses have on the lower limbs?

Answer 14

• a reduced carotid sinus pressure triggers a baroreflex to increase presure back to normal
• then baroreflex triggers the RAAS system which then triggers vasopressin /ADH secretion which both have vasoconstriction effects to raise blood pressure

Question 15

what happens to renal excretion of salt and water in terms of compensation of upright posture?

Answer 15

• renal excretion of salt and water is reduced by an increase in sympathetic stimulation of RAAS
• ADH secretion reduces renal salt and water excretion

Question 16

what is venous return defined as?

Answer 16

• the return of blood to the heart

Question 17

what 3 factors are infuenced by venous return?

Answer 17

1. end diastolic volume EDV (amount of blood in ventricles before contraction)
   2.** stroke volume** (volume of blood pumped by the heart)
   3.** cardiac output** (volume of blood pumped per min from each ventricle)

Question 18

what are the drivers for venous return?

Answer 18

• the skeletal muscle pump
• respiratory pump
• cardiac cycle
• sympathetically - mediated venoconstriction

Question 19

how does the skeletal muscle pump drive venous return?

Answer 19

• veins are arranged parallel in between the skeletal muscle fibers
• when the muscle contracts,the veins are compressed and squeezed, which** causes an increased blood flow toward the heart** (direction of flow influenced by valves)
• due to an increase in blood flow, there is a decrease in venous pressure

Question 20

how does the respiratory pump drive vernous retrun?

Answer 20

• during inspiration, contraction of the diaphragm increases the volume of the chest & decreases the volume of the abdominal cavity
• this results in a fall in intrathoracic pressure and a** rise in abdominal cavity pressure **
• this pressure difference increases the pressure gradient which favours blood flow in the direction from the abdominal to the thoracic veins and therefore **enhances the filling of the right ventricle via the vena cava **

Question 21

how does the cardiac cycle drive venous return?

Answer 21

• cardiac contraction stimulates venous return via
• **vis a tergo **( force acting from behind) - the contraction of the left ventricle pushes blood from behind all throughout the vasculature in the body
• vis a fronte (force acting from in front) - during ventricular contraction, the atrioventricular ring is pulled down and causes the atria to expand … this creates a negative pressure in the right atrium - exerts a suction effect and draws blood into the atria

Question 22

how does** sympathetically mediated venoconstriction** drive venous return?

Answer 22

• venoconstriction drives venous return via increasing venous pressure (NA) and redcucing vein compliance