Cardiovascular System 4

Question 1

5 Major types of blood vessels

Answer 1

1. Arteries
2. Arterioles
3. Capillaries
4. Venules
5. Veins

Question 2

Describe the structure and function of Arteries

Answer 2

Elastic (conducting) arteries

• Largest diameter arteries
• Carry blood away from heart
• Middle layer contains large numbers of elastic fibers

Function

• Store elastic energy
• Helps move blood during diastole
• Pressure reservoir

Muscular (distributing) arteries

• Medium sized arteries
• Middle layer contains high proportion of smooth muscle

Function

• Distribute and regulate blood flow to muscles & internal organs (constrict, decrease blood flow & dilate, increase blood flow and diameter)
• Allows to redistribute to parts of body due to constriction and dilation
• Superficial muscular arteries form pressure points e.g. to control bleeding or measure pulse (carotid, brachial, femoral)

Question 3

Describe the structure and function of Arterioles

Answer 3

• Small, almost microscopic arteries
• Deliver blood to capillaries

Function

• Resistance vessels
• Key regulators of systemic vascular resistance, through
  
  • Vasoconstriction (decrease blood flow, sympathetic stimulation of circular smooth muscle)
  • Vasodilation (increase blood flow) (e.g. low oxygen levels)

Question 4

Describe the structure and function of Capillaries

Answer 4

• Microscopic vessels (microcirculation)
• Walls consist of only endothelium and basement membrane

Function

• Exchange of nutrients and wastes via interstitial fluid
• Exchange vessels

Question 5

Describe the structure and function of Venules

Answer 5

• Small veins formed from merging of several capillaries

Function
- Venules merge to form veins

Question 6

Describe the structure and function of Veins

Answer 6

Composed of essentially same 3 tunics as arteries

• Tunica interna thinner
• Tunica media thinner (Less smooth muscle & elastic fibers)
• Tunica external (Thickest layer - collagen & elastic fibers/Lack elastic lamina of arteries/Many contain valves to prevent backflow of blood)

Function
- Capacitance vessels (volume reservoir)

Question 7

Explain the mechanisms that regulate blood flow through arteries, capillaries and veins

Answer 7

Haemodynamics

- Blood flow = volume of blood that flows through a tissue per unit time = determined by blood pressure and resistance

Question 8

Haemodynamic variables - Blood flow

Answer 8

Blood flow

• volume of blood that flows through a tissue per unit time
• determined by blood pressure and resistance
• proportional to the pressure gradient
• Inversely proportional to resistance

Question 9

Haemodynamic variables - Pressure

Answer 9

Pressure

If blood pressure is too low (Hypotension < 90/60)

• vessels collapse
• blood flow stops
• tissues die

If blood pressure too high (Hypertension > 140/80)

• vessel walls stiffen
• capillary beds may rupture
• Blood Pressure (mmHg) - arterial pressure (120mmHg at aorta → 35mmHg at start of capilaries)
• Capillary Hydrostatic Pressure (CPH) - pressure on capillary walls (35mmHg → 18mmHg)
• Venous pressure - venous system (low: around 18mmHg)
• Total Pressure = circulatory pressure (around 100mmHg)

For flow to occur circulatory pressure must be more than (>) total Peripheral Resistance (= resistance in entire CVS)

Question 10

Haemodynamic variables - Resistance

Answer 10

Opposition to blood flow due to friction between blood and vessel wall

Depends on:

• Vessel length
  - Increased length increases resistance
  - In adults normally fixed length
• Vessel diameter
  - Decreasing diameter increases resistance
  - If dilate, increase diameter, decrease resistance
  - Resistance inversely proportional to 4th power of radius of lumen

Peripheral resistance highest in arterioles; actively controlled – vasoconstriction & vasodilation

Question 11

Describe the relationship among vessel diameter

Answer 11

Downwards curve (middle at Arteries decreasing to Capilaries (smallest) increasing highest to Venae cavae)

Elastic arteries - Around 2.5cm
Muscular arteries - Almost 1cm
Arterioles - 0.005cm 
Capilaries - Smallest (Almost 0cm)
Venules - 0.01cm
Veins - 1.5cm
Venae cavae - Biggest (Higher than 3cm)

Question 12

Describe the relationship among cross-sectional area

Answer 12

Increasing upwards curve (Lowest at arteries, increasing and highest at Capilaries and decrease at Venae cavae)

Elastic arteries - Lowest
Muscular arteries 
Arterioles 
Capilaries - Highest
Venules 
Veins 
Venae cavae - Lowest

Question 13

Describe the relationship among blood pressure

Answer 13

High to low (Highest at arteries, lowest at Venae cavae)

Elastic arteries - Almost 100-120 mmHg
Muscular arteries - 60-100 mmHg
Arterioles - 40-60 mmHg
Capilaries - 25-40 mmHg
Venules - 15-25 mmHg
Veins - 10-15 mmHg
Venae cavae - Around 10 mmHg

Question 14

Describe the relationship among blood velocity

Answer 14

Fastest to Slowest then increasing curve (fastest at Arteries decreasing to Capilaries (slowest) increasing highest to Venae cavae)

Elastic arteries - 21-32 cm/sec
Muscular arteries - 7-21 cm/sec
Arterioles - 3-5 cm/sec
Capilaries - 2 cm/sec (Slowest)
Venules - 2.5-4 cm/sec
Veins - 4-10 cm/sec
Venae cavae - 10-18 cm/sec