L19. Histology of the Heart

Question 1

What are the three histological layers of the heart?

Answer 1

Epicardium (outer surface), simple squamous epithelium, thick CT, nervous tissue, blood vessels and fat
Myocardium: thickest layer, contains the myocardial (functional) layer
Endocardium (inner layer) of endothelial cells with CT and contains the specialised conduction tissue

Question 2

What are the histological features of the cardio myocytes?

Answer 2

Striated
Sarcomere arrangements
Small cells with central nuclei
Branching fibres
Connected by intercalated discs

Question 3

What are the functions of gap junctions between cardiac cells?

Answer 3

Coordinate contraction: electrically couple cells and coordinate the action potentials running through the cells

Question 4

What is meant by the SA node being the pace setters?

Answer 4

The SA node has intrinsic/spontaneous action potentials. These fire at the fastest speed compared to all other pacemaker cells in the heart (ie. the AV node)
This means that all cells of the heart follow the beat set by these cells.

Question 5

What are the Purkinje Fibres?

What do these cells look like microscopically?

Answer 5

Modified cardiac muscle cells: act as cables for the electrical signals

They are larger
Lost/limited contractile ability
Full of glycogen (stored starch as energy)
Form bundles in the SUB-ENDOCARDIUM

Question 6

What are the three layers of the blood vessels?

Answer 6

Tunica Intima
Tunica Media
Tunica Adventitia

Question 7

What are the features of the Tunica Intima?

Answer 7

In contact with the blood
Simple squamous epithelium: endothelium - elongated cells in the direction of flow
Lies on a basal lamina
Supported by thin subendothelial connective tissue layer
An ACTIVE LAYER: inhibits clotting and seceretes factors (eg. vWF, NO, endothelin)

Question 8

What are the features of the tunica media?

Answer 8

Consists mainly of smooth muscle arranged concentrically
Contraction: constricts the lumen: increases BP
SYNTHETIC CELLS: secrete connective tissue components (act like fibroblasts): collagen type III, elastin, ground substance

Question 9

What are the features of the tunica adventitia?

Answer 9

Binds the vessel to the surrounding tissue

Connective tissue with fibroblasts embedded throughout

Question 10

What is the vaso vasorum?

Answer 10

In larger vessels where the tunica adventitia layer is very thick, it requires its own blood supply called the vaso vasorum.

Question 11

How do arteries play a role in regulating pressure?

Answer 11

They regulate the pulsatile pressure of the blood coming out of the heart

Question 12

What are the elastic arteries and their characteristics?

Answer 12

Closest to the heart: aorta, carotid, bracheocephalic

Very thick and multiple layers of elastin in the t.media which store energy and compress the blood in the lumen. This is important because blood is very pulsatile coming out of the heart and the elastic arteries control this

Question 13

What are the muscular arteries and their characteristics?

Answer 13

Distribute the blood to the tissues

Little elastin in the media: only has 2 areas: internal and external elastic membrane

Contractions of the smooth muscle: under the influence of the autonomic nervous system

Question 14

What are the arterioles and their characteristics?

Answer 14

Very small diameter vessels (<0.1mm)
Have only 1-3 layers of smooth muscle
But these CONTRIBUTE THE MOST to blood pressure changes: constriction and dilation

Question 15

What are the capillaries and their characteristics?

Answer 15

Very small diameter (t have associated SM)

Question 16

What are fenestrated capillaries?

Answer 16

Found in the pancreas, intestine and endocrine glands and kidney
Inner and outer membranes fuse: single membrane with pores: very rapid exchange

Question 17

Is the diameter of veins larger or smaller than that of their corresponding arteries? Why?

Answer 17

It is larger because the flow is slower in the veins and so there is a larger blood volume in the veins

Question 18

What is the pressure in the veins compared to that in the arteries?

Answer 18

The pressure is much much smaller.

The capillaries act as a pressure filter

Question 19

What characteristics to veins generally have, compared to arteries?

Answer 19

Thinner media
Thicker adventitia: require extra help to withstand hydrostatic pressures
Contain valves: one way flow
Skeletal muscle contractions aid the flow back to the heart

Question 20

What is special about the venules?

Answer 20

Preferred site of diapedesis (inflammation)

Question 21

What are the medium and large veins and their characteristics?

Answer 21

Have very well developed subendothelial CT
Enlarged adventitia
Sometimes longitudinal smooth muscle bundles run through the adventitia

Question 22

What is the purpose of the lymphatic system?

Answer 22

Extracellular fluid leaks out into the capillaries and is carried away by the vessels
Carry it the large veins

Question 23

What are the features of the lymphatic vessels?

Answer 23

Start out very thin walled and as they get larger, begin to look like veins
Have valves for flow
Characterised by the absence of RBCs