CR2.1 Blood Vessels Structure and Function

Question 1

List the five determinants of resistance to blood flow.

Answer 1

1. Blood vessel diameter / radius and length
2. Blood viscosity
3. Organisation of the vascular network (series or parallel)
4. Whether blood flow is laminar or turbulent
5. Effects of extravascular mechanical forces

N.B. (1) and (2) are Poiseuille’s Law

Question 2

Explain Poiseuille’s Law and the assumptions underlying its use to model blod flow.

Answer 2

Poiseuille’s Law: R = 8Ln/πr⁴

For a given blood vessel segment length and viscosity and π are constant. Therefore, expressed as R = 1/r⁴

N.B.

• Pulmonary circulation shorter vessel length than systemic circulation
• Viscosity of blood can change (e.g. anemia, dehydration, blood doping)

Question 3

Give examples of series and parallel resistance within the body.

Answer 3

Series: higher resistance –> Glomerular and peritubular capillaries in the kidneys

Parallel: lower resistance –> Splenic and mesenteric capillary beds into the hepatic capillary bed in the liver

Question 4

Give three examples of increased resistance with extravascular compression.

Answer 4

1. Reduced coronary artery blood flow during systole due to compression (i.e. cardiac perfusion is greatest during diastole
2. Forceful skeletal muscle contractions (especially static) compress the vasculature e.g. heavy resistance training
3. Emergency tourniquet –> occluding blood vessels to stop blood loss

Question 5

Describe the major categories of arteries.

Answer 5

1. Large or elastic arteries including the aorta, its large branches (e.g. brachiocephalic, subclavian, common carotid, iliac) and pulmonary arteries include large amounts of elastic fibres
2. Medium-sized or muscular arteries comprising other branches of the aorta (e.g. coronary and renal arteries) smooth muscle regulates perfusion
3. Small arteries (i.e. <2mm in diameter)
4. Arterioles (~20 to 100 um in diameter) found within tissues and organs and the greatest relative wall thickness and major resistance vessels

Question 6

The Extracellular Matrix (ECM) is an important determinant of blood vessel structure and function. List the three main components of the ECM and the four functions.

Answer 6

Components:

1. Collagen –> functionally, structurally and spatially distinct isoforms with small amounts of stretch under physiological conditions
2. Elastin –> the key protein in elastic fibers and >100% stretch under physiological conditions –> elastin fibers attached to fibrillin (i.e. ECM glycoprotein) –> See Marfan syndrome
3. Proteoglycans

Functions:

1. Scaffold and structural organisation –> provides integrity
2. Force transmission –> e.g. elastic recoil of aorta
3. Cell signalling either directly or though remodelling
4. Binding of groth factors and cytokines –> angiogensis

Question 7

Explain the regulation of ECM composition and how this can lead to pathological remodelling.

Answer 7

ECM composition must be tightly regulated for optimal tissue (e.g. blood vessel) function. The ECM is maintained through a balance of ECM protein synthesis (e.g. collagen, elastin and proteoglycans) and ECM protein degradation. ECM proteases degrade ECM proteins (e.g. MMPs, plasmin, ADAMTS). Three regulatory mechanisms of degradation are local activation, confinement by cell-surface receptors and secretion of inhivitors (e.g. TIMPs and serpins). Abnormal syntheis and remodelling is associated with pathology such as atherosclerosis, AAA, pulmonary hypertension.