Anatomy and overview of the circulatory system

Question 1

Functions of the cardiovascular system

Answer 1

TISSUE PERFUSION
Immune system
Hormones
Waste
Temperature control
pH control
Carbon dioxide
Oxygen
Nutrients

Question 2

Blood volume distribution in humans at rest

Answer 2

Small veins 45%
Large veins 15%
Capillaries 6%
Systemic arteries 12%
Heart 10%
Lungs 12%

Question 3

Pressure and pulsation from heart to tissue

Answer 3

Flow always goes from high pressure to low pressure
In arterioles pulsatile flow becomes laminar
Capillaries have peristalsis to make pulsatile flow, nothing to do with heart
Resistance and flow causes pressure to decrease

Question 4

Why does pressure and pulsation decrease?

Answer 4

Pressure falls because of resistance to flow

Flow is less pulsatile due to increased blood vessel compliance (pulsation is damped by movement of blood vessel wall)

Question 5

Foetal circulation

Answer 5

Maternal circulation supplies nutrients and oxygen
Lungs not required for gaseous exchange
Digestion not required

Question 6

Resting cardiac output and adjustment

Answer 6

Cardiac output= stroke volume x heart rate
Resting cardiac output is 4-7L/min
Continuously adjusted: Sleep reduced by 10%
Standing reduced by 20%
Excitement/stress increased by 20-30%
Pregnancy increased by 40%
Heavy exercise increased by 400-600%

Question 7

Factors affecting stroke volume

Answer 7

Diastolic stretch- depends on venous filling pressure
Force of contraction
Arterial pressure

Question 8

Diastolic stretch- Frank-Starling’s law of the heart

Answer 8

The volume entering at the beginning of a contraction cycle is the volume ejected at the end of the cycle

Question 9

Venous return

Answer 9

Blood returns to the heart via the vena cava
The rate of VR determines the cardiac output
The circulation is a closed loop [therefore CO=VR]

Question 10

Factors affecting VR

Answer 10

Skeletal muscle activity
Gravity
Sympathetic nervous activity
Breathing or compression of thoracic vena cava

Question 11

Skeletal muscle activity/pump

Answer 11

Veins contain non-return valves
Contraction displaces blood into next compartment
Mechanism very important for extremities

Question 12

Gravity- postural hypotension

Answer 12

Standing causes pooling of blood in the feet and lower legs
Without compensation would lead to precipitous fall in BP
Compensation mechanisms: baroreceptor reflex- vasoconstriction of veins
Local myogenic response- vasoconstriction of veins
Skeletal muscle pumping
Breathing

Question 13

Autonomic sympathetic effect on venous return

Answer 13

Veins are normally very compliant
Activation of the sympathetic nervous system causes a decrease in venocompliance
A decrease in compliance leads to an increase in venous pressure and hence venous return
Increased venous return leads to increased cardiac output

Question 14

Breathing- control of VR

Answer 14

Breathing cycle affects intra-thoracic pressure and thus VR
Inspiration leads to a decrease intra-thoracic pressure
Decreases right atrial pressure
Decreases thoracic vena cava pressure
Increases abdominal vena cava pressure
Leads to venous return