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Flashcards in CVS 2 Deck (19)
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1
Q

Describe the structure of muscular arteries.

A
  1. Tunica intima
    - endothelium
    - subendothelial layer
    - thick internal elastic lamina
  2. Tunica media
    - 40 layers of smooth muscle cells (connected by gap junctions for coordinated contraction)
    - prominent external elastic lamina
  3. Tunica adventitia
    - thin layer of fibroelastic CT containing vasa vasorum, lymphatic vessels and nerve fibres.
2
Q

How is vasoconstriction of muscular arteries stimulated?

A
  • Outer tunica adventitia contains unmyelinated sympathetic nerve endings.
  • NA released at nerve endings diffuses through fenestrations in external elastic lamina into external tunica media to depolarise some of the superficial smooth muscle cells.
  • Depolarisation is propagated to all cells via gap junctions.
3
Q

What are arterioles and what is their structure?

A
  • Arteries with diameter of <0.1 mm.
  1. Tunica intima
    - layer of endothelial cells
    - very thin layer of subendothelial CT
  2. Tunica media
    - 1-3 layers of smooth muscle cells
  3. Tunica adventitia
    - Layers of fibroblasts
4
Q

What are metarterioles? What is special about their structure?

A
  • Arteries that supply blood to capillary beds.
  • The individual muscle cells are spaced apart and each encircles the endothelium of a capillary arising from the metarteriole - precapillary sphincter.
5
Q

How can arterioles direct blood where it is most needed?

A
  • Precapillary sphincters can control blood flow into the capillary bed upon contraction.
  • Most arterioles can dilate to 60-100% of their resting diameter, or maintain up to 40% constriction for a long time (e.g. Arterioles to skeletal muscle dilate during exercise).
6
Q

What is a capillary?

A
  • Blood vessel just large enough to allow the passage of blood cells one at a time.
  • Made up of a single layer of endothelium and its basement membrane.
7
Q

Why does gas/nutrient exchange occur in capillary beds?

A
  1. Largest surface area.
  2. Passing RBCs fill entire capillary lumen - minimised diffusion distance to adjacent tissue.
  3. Blood velocity is at its lowest (0.3 mm/sec) - allows time for exchange.
8
Q

Which cells are found on the outer surface of the capillary endothelium?

A
  • Pericytes (form branching network)

- Are capable of dividing into muscle cells or fibroblasts during angiogenesis, tumour growth and wound healing.

9
Q

Describe the movement of fluid across capillary beds.

A

Arterial end - fluid filtration
~ capillary hydrostatic pressure (35 mmHg) > blood osmotic pressure (25 mmHg)

Mid capillary - no net fluid movement
~ capillary hydrostatic pressure (25 mmHg) = blood osmotic pressure (25 mmHg)

Venous end - fluid reabsorption
~ capillary hydrostatic pressure (15 mmHg) < blood osmotic pressure (25 mmHg)

10
Q

Describe the structure of post-capillary venules.

A
  • Wall is similar to that of capillaries (endothelial lining + associated pericytes) but are even more permeable.
  • Contain valves - thin, intimal extensions that press together to restrict retrograde transport of blood.
11
Q

Describe fluid movement in venules.

A
  • Fluid tends to drain into venules as:
    • are very permeable
    • pressure is lower than in capillaries or surrounding tissues
  • Except during inflammatory response: fluid and leukocytes emigrate (preferred site for leukocyte emigration)
12
Q

What are the main structural differences between veins and arteries?

A

Veins have:

  1. Larger diameter
  2. Thinner walls
  3. Walls have more CT and less elastic/muscle fibres
13
Q

Describe the tunica media of veins.

A
  • Thin (only 2 or 3 layers of smooth muscle in small and medium veins).
  • Exception in large superficial veins of legs - have well defined muscular wall, possibly to resist distension caused by gravity.
14
Q

Why are veins called capacitance vessels?

A
  • Capacitance = ability of a blood vessel to increase the volume of blood it holds without a large increase in pressure. Inversely proportional to elasticity.
  • Because veins have thin, non-elastic walls, they can stretch a great deal.
15
Q

Describe the presence of venous pressure gradients in the body.

A
  • Gravity in upright human body can cause vertical gradient of venous pressures.
  • This is equalised when supine.
16
Q

How is an increased venous pressure in lower limbs due to gravity prevented when upright?

A
  1. Calf muscle pump: skeletal muscle contraction (system only active when moving).
  2. Thoracic pump: lowering of diaphragm during breathing lowers pressure - sucking effect attracts blood.
17
Q

Name diseases associated with venous pressure.

A
  • Calf muscle pump failure leads to VENOUS HYPERTENSION.

- Gravity effect causes increased pressure in limbs - VENOUS ULCERATION.

18
Q

What does the Frank-Starling Law of the Heart state?

A
  • The stroke volume of the heart increases in proportion to the left ventricular end diastolic volume.
  • I.e. As a larger volume of blood flows into the ventricle, the heart walls will stretch causing a greater expansion during diastole, in turn increasing the force of contraction and the quantity of blood pumped into the aorta during systole.
19
Q

Why does an increased ventricular blood volume increase the contraction force and cardiac output?

A
  • Extra blood entering heart (increased venous return) increases load experienced by each muscle fibre.
  • Stretching of muscle fibres (increased pre-load) increases the calcium sensitivity of the myofibrils, causing a greater number of actin-myosin cross-bridges to form within the muscle fibres.
  • Augments cardiac muscle contraction.