Structure and Function of Blood Vessels

Question 1

Explain the principles of angiogenesis

Answer 1

The sprouting of new capillaries from pre-existing vessels

The process of angiogenesis is controlled by chemical signals in the body. Some of these signals bind to receptors on the surface of normal endothelial cells. When VEGF endothelial growth factors bind to their receptors on endothelial cells, signals within these cells are initiated that promote the growth and survival of new blood vessels. Other chemical signals, called angiogenesis inhibitors, interfere with blood vessel formation.

Question 2

List some pro-angiogenic factors

Answer 2

VEGFA
Hypoxia
FGF2
Angiopoietins
PDGF
IGF
MMPs
Intergrins

Question 3

List some anti-angiogenic factors

Answer 3

Angiostatin
Thrombospondin
PEDF
Endostatin
Soluble FLT1

Question 4

What is the importance of angiogenesis in fetal and post natal development

Answer 4

Metabolically active tissue needs to be “close” to a capillary for gaseous exchange

Question 5

Explain angiogenesis in adults

Answer 5

Limited in adults
Principally in female reproduction i.e. uterine/ovarian changes and placental development

Question 6

List pathology of angiogenesis

Answer 6

Wound healing
Skin / heart disease
Tumour development
i.e mammary tumor development

Question 7

Explain how angiogenesis occurs in repsonse to hypoxia

Answer 7

Cellular responses to hypoxia are mainly regulated by the activation of transcription factors called hypoxia-inducible factors (HIFs)

When O2 levels are low, levels of HIF increase

HIF transactivates target genes such as vascular endothelial growth factor (VEGF-A).

Endothelial cells exposed to the highest VEGF-A concentration become tip cells

Endothelial cell tip secretes proteolytic enzymes, which digest a pathway through the ECM for the developing sprout

Question 8

Define Vasculogenesis

Answer 8

Formation of blood vessels from endothelial progenitor cells
i.e. angioblasts issued from the mesoderm during embryogenesis.

Question 9

Describe the concept of a vascular tree

Answer 9

The blood is sent “in parallel”: each tissue receives fresh blood from the heart and not from other tissues

The vascular tree is split in two parts:

1- The systemic circulation
2- the pulmonary circulation

Question 10

Briefly outline the systemic circulation of the vascular tree

Answer 10

Blood leaves the left ventricle highly oxygenated and under high pressure

This blood is spread throughout the organs excluding the lungs

It is then returned to the right ventricle poorly oxygenated and under low pressure

Question 11

Briefly outline the systemic circulation of the vascular tree

Answer 11

Blood leaves the right ventricle poorly oxygenated and under high pressure

This blood is received by the lungs

The blood returns to the Left atrium highly oxygenated but under low pressure

Question 12

Explain theportal system and describe its advantage

Answer 12

Portal system:
A few organs are connected “in series” they obtain their blood “second hand” from the venous outflow of another organ. This is called portal system
i.e. Liver receiving blood from digestive tract via hepatic portal vein

Portal system main advantage
To transport a solute from one place to the other without dilution in the general circulation.

Question 13

Describe how the portal system varies between species

Answer 13

Hepatic portal vein in all vertebrate:
From the GI to the Liver.
Filter newly absorbed compounds

Renal portal vein in all vertebrate-not mammals (birds)
From the hind limbs to kidney
Resorb salt, water…

Question 14

Explain the function of anastomosis

Answer 14

Special circulations which form a bridge between arteries if one becomes blocked so that the organ is able to still receive a collateral supply of blood to mitigate necrosis

Question 15

Explain the function of anastomosis

Answer 15

Special circulations which form a bridge between arteries if one becomes blocked so that the organ is able to still receive a collateral supply of blood to mitigate necrosis

Question 16

Describe how a vascular tree can vary between species

Answer 16

Differences in aortic arches
i.e. mammals have left aortic arch whereas birds have a right aortic arch

Difference in subclavian placement
i.e. Mammals L & R branches off aorta and aortic arch whereas birds branches off of common carotid

Question 17

Outline the main functions of blood circulation

Answer 17

To transport gases to and from organs and tissues (E.g. CO2 & O2)

To deliver nutrients (e.g. digested food)

To remove waste (e.g. by products of cellular metabolisms)

To communicate (e.g. insulin)

To maintain the cardiac output pressure (e.g. erectile tissues)

To maintain a constant temperature (for warm blood animals)

Question 18

Explain how blood is distributed throughout tissues of the body according to their physiological purpose

Answer 18

Generally during rest the GI systems, kidneys and skeletal muscle and brain will receive the majority of blood (Digestion and excretion are priorities)

However, during exercise the Gi system, brain and kidneys blood supply drops significantly within similar range of the bones and skin. Whilst skeletal muscle receives a large increase in blood (80-85%) to support the metabolising tissues. Exercise is the priority

Question 19

Describe the principal components of a vascular system and their function

Answer 19

Delivery system: arteries and arterioles
- High pressure
- ~15% blood volume

Exchange system: capillaries
- Intermediate pressure
- ~5% blood volume

Return system: venules and veins
- Low pressure
- ~80% blood volume
- (reservoir of blood)

Question 20

Describe the microscopic structure of veins & arteries and describe their properties

Answer 20

Endothelium

Tunica intima (or interna)
smooth muscles, collagen, elastin

Internal elastic lamina
elastin

Tunica media
smooth muscles, collagen, elastin

External elastic lamina
elastin

Tunica adventitia (or externa)
collagen, elastin

Question 21

Outline the structure of aorta

Answer 21

High levels of elastin and collagen which damp the oscillating cardiac output when blood leaves.

Blood pressure is the highest and has the largest range within the aorta
Systolic pressure (max pressure in aorta when the heart contracts and ejects blood into the aorta from the left ventricle) 120mmHg
Diastolic pressure (min pressure in aorta when the heart is relaxing before ejecting blood into the aorta from the left ventricle) 80mmHg

Question 22

Outline the structure and function of arteries

Answer 22

Have a thick outer layer of longitudinal collagen and elastic fibers to avoid leaks and bulges. They have a thick wall, which is essential to withstand the high pressures.

They also have thick layers of circular elastic fibres and muscle fibres to help pump the blood through after each contraction of the heart.

In addition the narrow lumen maintains the high pressure inside the arteries.

Question 23

Outline the structure and function of the arterioles

Answer 23

Composed of smooth muscle cells

They contract which diminishes the radius of the arterioles (contraction allows regulation in blood pressure between arteries and capillaries).

They provide resistance to blood flow

(hypertension - too much will damage capillaries and could lead to heart failure)

Question 24

Outline the structure and function of the capillaries

Answer 24

Contain no collagen or elastin

Have relatively low blood pressure due to fragility and permeability

Three types:
-Continous
-Fenestrated
-Discontinuous sinusoid

Mechanisms of nutrient exchange:
-diffusion - O2, CO2, glucose, hormones, electrolytes – diffuse down [ ] gradients
-lipid soluble molecules can pass through cell membrane easily (drugs/therapeutics)
-water soluble molecules generally require transport mechanisms to enter/exit cells

Question 25

Describe continuous capillaries

Answer 25

Continuous lining of endothelial cells except for clefts between cells

Found in the majority of the body

Question 26

Describe fenestrated capillaries

Answer 26

Fenestration are no true holes, but rather where the cell membrane is compressed to permit greater fluid transmission

Found in glomerular capillaries in the kidney

Question 27

Describe discontinuous sinusoid’s capillaries

Answer 27

Some wider intercellular gaps permit increased exchange with surrounding tissues

Found in liver, bone marrow, lymphoid tissue, some endocrine glands

Question 28

Outline the structure and function of the veins

Answer 28

High levels of collagen but low levels of elastin

This is because blood does not flow in pulses and so the vein walls cannot help pump the blood on.

Veins also have thin walls which allows the near by muscles to press against them so that they become flat. This helps the blood to be pushed forwards towards the heart.

There is only a thin outer layer of longitudinal collagen and elastic fibres as there is low pressure inside the vein and so little chance of bursting.

A wide lumen is needed to accommodate the slow flowing blood due to the low pressure. They have valves to prevent blood pooling at extremities

Question 29

Describe the process of venous return

Answer 29

Depends on pressure difference between venules and RA (changes in RA pressure changes venous return)

Smooth muscles contraction in tunica media

Inspiration, lower diaphragm/ abdominal compression

Existence of venous valves

Skeletal muscles

Gavitations (upper parts of the body)

Question 30

Outline the structure and function of tunica intima

Answer 30

-innermost layer of veins/artery

-exposed to the blood

-made of simple squamous

-provides a smooth inner lining that normally repels platelets and blood cells.

Question 31

Outline the structure and function of tunica media

Answer 31

-the middle layer of arteries/veins.

-thickest

-made of smooth muscle, collagen,sometimes elastic tissue depending if its a vein or artery

Question 32

Outline the structure and function of tunica externa

Answer 32

-outermost layer of arteries/veins.

-consists of loose connective tissue

-anchors the vessels and provides passage for small nerves, lymphatic vessels, and smaller blood vessels.

-the inner half are nourished by diffusion from blood in the lumen.

Question 33

Explain the effect of changing vessel diameter on vascular resistance