Circulatory System Histology

Question 1

Circulatory systems of the body?

Answer 1

CVS - closed system consisting of the heart, arteries, capillaries, veins
Lymphatic vascular system

Question 2

Roles of the CVS?

Answer 2

Transport of O2 and nutrients to tissues and CO2 and other metabolic waste away from tissues

Temp regulation

Distribution of hormones and immune cells

Reproductive function in males; penile erection

Question 3

At any one moment, where is the majority of the blood located?

Answer 3

Peripheral veins (capacitance vessels)

Question 4

3 layers of the blood vessels?

Answer 4

Tunica intima (inner layer) - single layer of squamous epithelial cells (endothelium) supported by a basal lamina and a thin layer of CT

Tunica media (middle layer) - predominantly muscle (in some areas, there is a lot of elastic tissue); layer varies a lot between blood vessels and, sometimes, can completely disappear (capillaries)

Tunica adventitia (outer layer) - made up of supporting connective tissue

Question 5

What are the elastic arteries and why?

Answer 5

Very largest arteries, e.g: aorta, as they have many sheets of elastic fibres in their tunica media to provide elastic recoil

Question 6

Why do some vessels have their own vascular supply?

Answer 6

Called vasa vasorum:
In very large arteries, only the inner half of the wall can obtain nutrients from the lumen, thus these vessels must have their own blood supply

Question 7

Structure of arterial walls?

Answer 7

Tunica intima extends to an internal elastic membrane
Tunica media
Tunica adventitia…separating media and adventitia is an internal elastic membrane

Question 8

Structures of arteriolar walls?

Answer 8

Typical diameter - 30-200μm

Have only 1 or 2 layers of smooth muscle in their tunica media and almost no adventitia; there is still an internal elastic membrane

Question 9

Structure of capillaries?

Answer 9

Composed of endothelial cells and a basal lamina (diameter of 4-8μm) - often form capilarry networks

Often have pericytes at intervals, just outside the basal lamina - they are CT cells that have contractile properties

Question 10

Types of capillary and where they are found?

Answer 10

Continuous - most common and have a continuous endothelial coat; found in muscle, CT, lung, skin and nerves

Fenestrated - have ~50nm pores in their walls; found in mucosa of gut, endocrine glands, glomeruli of the kidney

Sinusoidal/discontinuous - lack a basal lamina and have large gaps through which macromolecules, and even cells, can pass; found in liver, spleen, and bone marrow (for easy access)

Question 11

Structure of post-capillary venules?

Answer 11

10-30μm diameter - important sites for exchange, e.g: cells into inflamed tissue

Endothelial cells associated with pericytes or occasional smooth muscle cells

Question 12

What is microvasculature?

Answer 12

Small arteriole connected to a post-capillary venule, through a network of metarterioles (smallest arterioles), thoroughfare channels and capillaries

Question 13

Flow in microvasculature?

Answer 13

Precapillary sphincters, at the beginning of the artery help control flow through the network

Question 14

Structure of veins?

Answer 14

Relatively thin tunica media relative to the size of the lumen (compared to a muscular artery, as blood is at low pressure)

Contain valves - invaginations of tunica intima

Question 15

Layers of the heart?

Answer 15

Endocardium - inner layer
Myocardium - thick, middle layer
Epicardium (visceral serous pericardium) - outer layer

Question 16

What is the endocardium?

Answer 16

Inner layer that lines entire inner surface of the heart, inc. valves

Question 17

Structure of the endocardium?

Answer 17

Endothelium
Basal lamina
Thin later of collagen fibres
Layer of denser CT

In some areas, there is a SUBENDOCARDIUM (between endocardium and myocardium) of loose CT, containing small blood vessels and nerves and branches of the impulse conducting system

Question 18

Structure of myocardium?

Answer 18

Bundles and layers of contractile cardiac muscle fibres

Individual muscle fibres are surrounded by delicate, collagenous CT with a rich network of capillaries

Question 19

Differences between cardiac muscle cells and skeletal muscle cells?

Answer 19

Cardiac muscle cells have a single, central nucleus, whereas skeletal muscle cells have a nucleus at the cell periphery

Cardiac muscle cells also have intercalated discs (desmosomes and adherent junctions) passing across the fibres at regular intervals - these act to attach cells and also allow electrical activity (gap junctions)

Question 20

Structure of epicardium?

Answer 20

On the surface of the heart - a single layer of flattened epithelium, called MESOTHELIUM

Basal lamina

Fibroelastic CT and, in some places, adipose tissue (large branches of coronary vessels are found in epicardium)

Question 21

Parts of the pericardium?

Answer 21

Fibrous - sac of tough fibrocollagenous CT

Serous pericardium - later of simple squamous epithelium, backed by a basal lamina and CT:
Parietal serous pericardium - lines inner surface of fibrous pericardium
Visceral serous pericardium (epicardium) - covers heart surface

Question 22

What is the pericardial cavity?

Answer 22

Two mesothelial (serous) layers are apposed to each other and separated by a thin pericardial cavity, containing a small amount of serous fluid (lubrication) for heart movement

Question 23

Structure of fibrous “skeleton” of heart and location?

Answer 23

Formed by thick bands of fibrous CT around the heart valves, between the atria, and between the ventricles

Question 24

Functions of fibrous “skeleton” of heart?

Answer 24

CT supports valves

Also provides attachment for cardiac muscle fibres

Electrically isolates atria from ventricles as fibrous skeleton is non-conducting

Question 25

Heart valve structures?

Answer 25

Outer ENDOTHELIAL layer with basal lamina
Layer of elastic and collagen fibres
Core of dense CT, called the LAMINA FIBROSA (in continuity with the fibrous skeleton)

Leaflets of the valves separating the atria from the ventricles (mitral and tricuspid) are anchored to papillary muscle in the wall of the ventricle by chordae tendinae (collagenous strands that merge with the lamina fibrosa)

Question 26

Types of cardiac muscle cells?

Answer 26

Contractile cells (99%)

Modified muscle cells:
Pacemaker cells - highly specialised muscle cells (heart does not beat due to innervation)
Conducting cells

Question 27

Structure of pacemaker cells?

Answer 27

Diameter - 4-8µm (considerably smaller than contractile cardiac myocytes)

Are embedded in a more extensive matrix of CT

Histologically, appear PALE (P cells) due to paucity of organelles within 
Few myofibrils (which are irregularly arranged), little glycogen and no proper T-tubule system (as their main function is not contraction)

Question 28

Structure of Purkinje fibres?

Answer 28

Larger than normal cardiac muscle cells (unlike pacemaker cells)

Found in SUBENDOCARDIAL LAYER, deep to endocardium

Have abundant glycogen, no T-tubules, no intercalated discs and sparse actin and myosin filaments (tend to be found at cell periphery); thus, histologically, appear PALE and often have a very pale/clear centre

Question 29

What is the lymph vascular system?

Answer 29

Consists of lympathic vessels that drain excess tissue fluid (most returns to venules), eventually returning it to veins in the vase of the neck

Question 30

Structure of lymphatic capillaries?

Answer 30

Begin as BLIND SACS and consist of, essentially, endothelial cells

Question 31

Functions of lymphatic system?

Answer 31

Drain excess tissue fluid

Vessels pass through lymph nodes, where IMMUNOLOGICAL SURVEILLANCE occurs

Question 32

How is flow achieved in lymphatic vessels?

Answer 32

No central pump
Flow is achieved by:
Smooth muscle in walls
Hydrostatic pressure in the tissue
Compression of vessels by voluntary muscle
Valves in the vessels ensure unidirectional flow