The Structure of blood vessels - 2

Question 1

When your are cold, where is the blood directed to?

Answer 1

The core.

Question 2

The microcirculation comprises of…

Answer 2

Small arterioles, the capillary bed and post capillary venules.

Question 3

Since capillarys cannot control their diameter, what UPSTREAM instead controls flow

Answer 3

Arterioles
Terminal arterioles (single SM layer)
Metarterioles (incomplete SM layer)
Precapillary sphincter (can con/relax >20x per min)

Question 4

When is blood allowed to “flush” through the capillaries?

Answer 4

When all 4 layers are relaxed

Question 5

What happens specifically in the precapillary sphincter closes. Why would this happen?

Answer 5

Blood is redirected via a ‘thoroughfare channel’ to a venule, so capillary is bypassed.

If the tissue doesn’t have a high demand this occurs

Question 6

Arteriovenous anastomoses

Answer 6

When arterioles lead directly to venules, if
Relaxed: shunt directly from the arteriole to venous system.

Contracted: Blood forced into metarterioles nearby > capillary beds

Question 7

Cell coat of venules?

Answer 7

pericytes

Question 8

Examples

Answer 8

Digestion : inc blood flow, precapillary sphincters open, blood to capillaries

Cold: Increased shunting to core

Question 9

Capillaries

Answer 9

Internal diameter: 8-10 micrometres (one RBC thick)

Walls: endothelium with BM, no CT or muscle.

Many Different types; continuous, fenestrated, sinusoid

Question 10

Continuous Capillary types. How do they transfer particles

Answer 10

1) Continuous Capillary with closed intercellular clefts: in CNS, tight-junctions make ‘complete seal’ > blood-brain barrier (V SELECTIVE)
2) Continuous capillaries with open intercellular clefts: muscle, CTs, Lungs. 6nm cleft allow water, ion, and other small molecules but not plasma proteins.

via pinocytosis

Question 11

Fenestrated Capillary Types. How do they transfer particles

Answer 11

1) Fenestrated Capillaries w closed perfusions: fenestrae ~60nm but closed by a thin non-membranous diaphragm. Common in intestine.
- fluid exchange
- ions
- peptides

2) Fenestrated Capillaries w open perfusions: endocrine glands and kidney glomeruli (important fluid exchange)

via holes

Question 12

Sinusoids

Answer 12

Capillaries with gaps 100-1000nm wide between the edges of adjacent endothelial cells.
allow passage of large molecules and whole cells easily.

Bone marrow, spleen(where RBCs leave bloodstream) and liver.

Question 13

How is exchange maximised between sinusoids and tissue fluid?

Answer 13

Intracellular Clefts

Wide lumen= slow flow

Question 14

Post Capillary Venules

Answer 14

Diameter: 10-25 micrometes

Lack SM but have pericytes (very thin walls)

Question 15

Post Capillary Venules during inflammation or an allergic Reaction

Answer 15

Respond to histamine and serotonin with increased leakage of blood pllasma into surrounding tissue fluid.
>oedema and migration of neutrophils into the CT of vessel wall

Later followed by monocytes /macrophages and lymphocytes

Question 16

Muscular Venules

Answer 16

Larger, may have upto 2 layers SM in the media

Thin walled in relation to their diameter.

Have endothelial nuclei which bulge into lumen

Question 17

Veins. Describe structure and compare to muscular arteries

Answer 17

Thin wall & large diameter = low-pressure vessels.

Structure resembles muscular arteries, except
-All 3 tunics reduced, especially media

• NEVER a well-developed IEL
• Post death/loss of BP veins tend to collapse, whereas arteries hold their shape (due to muscular media)
• Veins have vaves to prevent backflow

Question 18

Vein valves

Answer 18

Bicuspid and in the tunica intima

Question 19

Venous pressure at feet standing still vs walking.

Answer 19

Still: 100mmHg due to gravity (base of 136cm column)

Walking: much less. This is due to whilst walking, the venous valves break the column of blood into segments, an each short segment experiences only the gravitational pressure proportional to its height, Skeletal muscle surrounding the veins compresses them at each stride, so the leg veins act as muscle pumps returning blood to the right heart

Question 20

Varicose Veins:

Answer 20

Occur when SF leg veins become dilated sufficiently so that venous valves don’t meet.
Valves become incompetent and the veins swollen and tortuous.
Condition is estimated to affect 10-20% population.
More common in women

Question 21

Venous Thrombosis

Answer 21

Formation of a blood clot, usually in deep veins of lower leg.
Caused by anything that
-slows blood-flow
- increases coagulability
-Damages endothelium

“virchow’s triad”
eg) surgery, childbirth, trauma, the pill

1/3 patients under 40 who have undergone surgery or had an MI develop it.

If part of the clot breaks loose > embolus, likely to pass through right heart and lodge in pulmonary arterial tree.

Small emboli= little-no symptoms
Large Emboli= fatal

Question 22

Compression socks

Answer 22

can increase venous pressure by 15-30 mmHg

Question 23

How much is lost from blood in capillaries to interstitial fluid?

Answer 23

3L water

120g plasma proteins

Question 24

Lymphatic capillaires

Answer 24

Smallest Lymphatic vessels

• Blind end endothelial cells
• tethered to surrounding CT by delicate filaments(pulled when tissue swollen, becomes a drainage vessel)
• Lacks BM, so increased permeability
• Large gaps between adjacent lymphatic endothelial cells.

Question 25

Lymphatic capillaries drain into

Answer 25

Collecting lymphatics which resemble being except their walls are thinner and have more valves.
Valves prevent backflow so lymph can be propelled through the vessels by any compression
-Contraction of SM
-repeated compressioin due to walking, breathing movements etc

Question 26

Where are lymphatic vessels present?

Answer 26

Most tissues, only absent in

• CNS
• Cartilage
• Bone
• Bone marrow
• thymus
• placenta
• cornea
• teeth

Question 27

How does lymph eventually enter blood stream.

Answer 27

THoracic duct (tibutary of left subclavian vein)

Right lymphatic duct