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Flashcards in CVS 2 Deck (19):

Describe the structure of muscular arteries.

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.


How is vasoconstriction of muscular arteries stimulated?

- 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.


What are arterioles and what is their structure?

- 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


What are metarterioles? What is special about their structure?

- 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.


How can arterioles direct blood where it is most needed?

- 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).


What is a capillary?

- 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.


Why does gas/nutrient exchange occur in capillary beds?

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.


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

- Pericytes (form branching network)
- Are capable of dividing into muscle cells or fibroblasts during angiogenesis, tumour growth and wound healing.


Describe the movement of fluid across capillary beds.

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)


Describe the structure of post-capillary venules.

- 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.


Describe fluid movement in venules.

- 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)


What are the main structural differences between veins and arteries?

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


Describe the tunica media of veins.

- 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.


Why are veins called capacitance vessels?

- 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.


Describe the presence of venous pressure gradients in the body.

- Gravity in upright human body can cause vertical gradient of venous pressures.
- This is equalised when supine.


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

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.


Name diseases associated with venous pressure.

- Calf muscle pump failure leads to VENOUS HYPERTENSION.
- Gravity effect causes increased pressure in limbs - VENOUS ULCERATION.


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

- 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.


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

- 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.