Introduction and histology of the CVS

Question 1

Describe the factors influencing exchange of substances between the blood in capillaries and the surrounding tissue

Answer 1

Area available for exchange, Diffusion resistance and concentration gradient

• Determined by the Capillary Density
  Diffusion Resistance – The difficulty of movement through the barrier
• Determined by the nature of the barrier and the molecules that are diffusing. A major component of diffusion resistance is the distance over which diffusion must occur, the path length.
  Concentration gradient
• Concentration gradients drive diffusion, the relevant gradient being between the capillary contents and the nearby cells.

Question 2

List the typical bloody flow to the major organs in the body

Answer 2

Brain: 0.75-0.75
Heart: 0.3-1.2
Kidneys: 1.2-1.2
Gut: 1.4-2.4
Muscle: 1.0-16.0
Skin: 0.2-2.5

Question 3

Describe the major functional components of the circulation

Answer 3

A Pump
-	The Heart
Distribution Vessels
-	The Arteries
Flow Control
-	Cardiac output must be distributed appropriately by restricting flow to those parts of the body that are easy to perfuse, so as to drive blood to parts that are not so easy to perfuse. Flow control is via resistance vessels, the arterioles and pre capillary sphincters
Capacitance
-	Capacitance is the ability to cope with changes in the cardiac output. This is a store of blood that can be called upon to cope with temporary imbalances between the amount of blood returning to the heart and the amount that it is required to pump out. This store is the veins.

Question 4

Describe the distribution of blood volume over major parts of the circulation

Answer 4

At rest, the blood volume of ~5 Litres is distributed:

11% (0.55 L) in the Arteries and Arterioles
5% (0.25 L) in the Capillaries
17% (0.85 L) in the Heart and Lungs
67% (3.35 L) in the Veins

Question 5

Describe how arteries are named

Answer 5

Arteries carry blood away from the heart. Different arteries contain varying amounts of elastic fibres and smooth muscle fibres in their walls, so are named Elastic (conducting) and Muscular (distributing) arteries.

Muscular Arteries – Branch into Arterioles, the function of which is to regulate the amount of blood reaching an organ or tissue and regulating blood pressure. The diameter of arteries and arterioles is controlled by the Autonomic Nervous System. The arterioles branch into smaller vessels

Question 6

Describe how capillaries are named

Answer 6

A capillary wall is mostly one cell thick and allows exchange of substances between blood and tissues. The capillary wall may be continuous or fenestrated (openings in the structure). Both of these types of capillaries may be surrounded partially by Pericytes.
There is a final category of capillaries, Sinusoids, which are general larger in diameter and may contain special lining cells and an incomplete basal lamina. Sinusoids are found in the liver, spleen and bone marrow.

Question 7

Describe how veins are named

Answer 7

Capillaries merge into large vessels called the Venules, which merge to form even larger vessels called the Veins. These vessels carry blood towards the heart.

The construction of a vein is similar to that of an artery, except that its wall is thinner and its lumen wider and irregular. The veins usually contain semilunar, paired valves that permit blood to flow in only one direction. However, those veins that are narrower than 1mm in diameter and those in the thoracic and abdominal cavities do not have valves.

The veins collapse if blood pressure is not maintained. Blood flow in veins is to a great extent determined by ‘muscle-pump’ action in the leg and pressure factors in the abdominal and thoracic cavities.

Question 8

Describe the structure of elastic arteries

Answer 8

The walls of arteries and veins have three layers:

• Tunica Intima – Endothelial cells with long axes
• Tunica Media – MAIN FEATURE – 40-70 fenestrated elastic membranes, with smooth muscle cells and collagen between these lamellae.
• Tunica Adventitia – The layer of fibroelastic connective tissue containing vasa vasorum

Question 9

What are the three layers in the walls of arteries and veins?

Answer 9

Tunica Intima – Next to the lumen
Tunica Media – Intermediate
Tunica Adventitia – Outer layer

Question 10

Describe the structure of muscular arteries

Answer 10

• Tunica Intima – Endothelium, sub-endothelial layer, thick internal elastic lamina.
• Tunica Media – MAIN FEATURE – 40 layers of smooth muscle cells (These cells are connected by gap junctions for coordinated contraction), prominent external elastic lamina.
• Tunica Adventitia – Thin layer of fibroelastic connective tissue, containing vasa vasorum

Innervated by sympathetic - NA release

Question 11

Describe the structure of arterioles

Answer 11

• Arteries with a diameter of less than 0.1mm are considered to be arterioles. Arterioles have one to three layers of smooth muscle in their tunica media.
• The Thin Internal Elastic Lamina (IEL) is present in larger arterioles only.
• In small arterioles, the tunica media is composed of a single smooth muscle cell that completely encircles the endothelial cells.
• The External Elastic Lamina (EEL) is absent. The tunica adventitia is scant.
• As arteries diminish in diameter, the number of smooth muscle layers in the tunica media (TM) decreases.

Question 12

Describe the structure of metarterioles

Answer 12

Arteries that supply blood to capillary beds are called metarterioles. They differ from arterioles in that the smooth muscle layer is not continuous. Rather, the individual muscle cells are spaced apart and each encircles the endothelium of a capillary arising from the metarteriole. This is a precapillary sphincter.

Question 13

Describe the structure of capillaries

Answer 13

Capillaries are made of a single layer of endothelium with basement membrane and hold ~ 5% of total blood volume. However, the present by far the largest surface area for gas and nutrient exchange (estimated ~600m2). Passing RBCs fill almost the entire lumen, minimising the diffusion path to adjacent tissues.

capillaries are 7-10m in diameter

Question 14

What are continuous capillaries, fenestrated capillaries and sinusoid capillaries?

Answer 14

• Continuous: Pericytes form a branching network on the outer surface of the endothelium. These cells are capable of dividing into muscle cells, or fibroblasts during angiogenesis, tumour growth and wound healing.
• Fenestrated: Pericytes still present, but with Fenestrations (pores) in the endothelium
• Sinusoidal capillaries are fenestrated capillaries that have larger openings, allowing RBC/WBCs to pass using a process aided by discontinuous basal lamina. Primarily located in the bone marrow, lymph nodes and adrenal gland.

Question 15

What are post capillary venules?

Answer 15

Post capillary venules receive blood from capillaries, have a diameter of 10-30m and are even more permeable than capillaries.
Their wall is similar to that of capillaries (endothelial lining with associated pericytes).
Because their pressure is lower than that of capillaries or the surrounding tissue, fluid tends to drain into them

Question 16

What are venules?

Answer 16

• As the diameter of merging venules increases to more than 50m, smooth muscle fibres begin to be associated with the endothelium (i.e. a tunica media begins to appear).
• Venules can have a diameter of up to 1mm.