Histology of the Cardiovascular System

• Curriculum : Phase 1/Semeser 3/Cardiovascular/Session 1

2020/20201

• Lecturer : Dr. Rajaa Ali Al- Taee

• Msc. PhD. Histology

• Hammurabi Medical Collage / Babylon University

Histology of the Cardiovascular System

• Objectives :

By the end of this lecture and following completion of the self study you should be able to:

describe how blood vessels (arteries, arterioles, capillaries, venules and veins)

are named.

describe the structure of different types of blood vessels in relation to their

function in supplying blood to and from the tissues of the body.

You should also revise the structure of cardiac muscle covered in the Tissues

of the Body module.

References:

Histology Textbooks ‘Basic Histology’, Junqueira,13 th

Edition chapter 1,2,3.pp:1-72

‘Colour Atlas of Histology’ Gartner and Hiatt 5 th Edition.

Blood Vessels

Arteries

Distribute blood to tissues

Capillaries

Permit exchange

between blood and tissue

Veins

Return the blood to the Heart

Types of Blood Vessels

Structural Plan of Blood Vessels

1.The tunica intima :

(layer of endothelial cells ,a thin

subendothelial layer of loose con.

T. with occasional smooth muscle

cells). In arteries, the intima is separated

from the media by an internal elastic

lamina

2.The tunica media:

(consists of concentric layers of smooth

muscle cells and amounts of elastic

fibers and reticular fibers of collagen

type III, proteoglycans, and

glycoproteins)

In arteries, the media has a thinner

external elastic lamina

3.The tunica adventitia

:

consists of :

• connective tissue.

• Lymphatic

capillaries, vasa

vasorum, and

nerves.

Figre : summarized the comparison between veins & arteries

Large Elastic Arteries :

(Aorta and its large branches )

The tunica intima: thicker than the muscular

artery.

The tunica media : predominat elastic fibers,

concentrically arranged, perforated elastic

laminae {number increases with age (40 in the

newborn, 70 in the adult)} & smooth muscle

cells.

The tunica adventitia: is relatively

underdeveloped

The muscular arteries:1.The intima :

• a very thin subendothelial layer

• internal elastic lamina,

2.The tunica media :

• up to 40 layers of more prominent

smooth muscle

• An external elastic lamina

(is present only in the larger

muscular arteries.)

3.The adventitia: consists of :

• connective tissue.

• Lymphatic capillaries, vasa

vasorum, and nerves.

Arterioles

• Less than 0.5 mm in diameter.

• The subendothelial layer is very thin.

• Elastic laminae are absent .

• Tunica media is composed of

circularly arranged smooth muscle

cells.

• In both small arteries and arterioles,

the tunica adventitia is very thin.

? Identify the characteristic tunica of arteries?

Capillaries

• permit metabolic exchange between

blood and tissues.

• single layer of endothelial cells

• diameter varies (5 to 10 micro m ).

• their individual length is not more

than 50 micro m.

• comprise over 90% of all blood

vessels in the body.

Type of capillaries

1. The continuous

(somatic) capillaries.

2. The fenestrated

(visceral) capillaries.3. The discontinuous

sinusoidal capillaries.

●Absence of fenestrae in their wall.

●Found in all type of muscular

tissue, C.T., exocrine gland and

nervous tissue.

●Transport of macromolecules

across the endothelial cytoplasm by

pinocytotic vesicle.

●Presence of fenestrae (channels)

obliterated by a diaphragm .

●basal lamina is continuous.

●In kidney.

➢The endothelium (discontinuous

layer).

➢Multiple fenestrations without

diaphragms.

➢The basal lamina is

discontinuous.

➢In liver and hematopoietic

organs (Bone Marrow and spleen).

Pericytes:

Along capillaries and postcapillary

venules.

Long cytoplasmic processes surround

the endothelial cells.

Function:

• After tissue injuries, proliferate

and differentiate to form new

blood vessels and C.T. cells.

• Contractile function.

? Memorize types of capillaries.

Postcapillary Venules

• T. intima:

– endothelium

– a very thin subendothelial layer.

• T. media : may contain:

only contractile pericytes.

Small & Medim sized Veins : are characterized by:

• Intimal layer I (containing sub-endothelial tissue)

• Tunica media M : (small amount of elastic tissue & smoot muscle .

• Tunica adventitia A : thicker than the media ( collagen fiber, small amouts of smooth

muscle & elastic fibers)

The larger vessels contain valves (intimal extensions & ensure unidirectional blood flow)

Large Veins: (ex: vena cava)

• Well developed intimal ,medial

and adventitial tunics.

• T. media: (smooth muscule

,reticular & collagen fiber).

• T. Adventitia well-developed

layer (longtitudinally

arranged collagen & smooth

muscle fibers, adipose A and

vessels V =vasa vasorum)

• Contain valves.

A Comparison of a Typical Artery and a Typical Vein

Two features of microcirculation are important in the amount of

blood perfusing the capillary beds:

The pre-capillary sphincter (PS), can restrict the flow into the

capillaries.

Thoroughfare channels (TC) can provide a route for blood to

enter the venous system without traversing the capillary bed.

vasa vasorum (vessels of vessels).

Amicrovasculature lie in the tunica adventitia

of large blood vessels ( ex. Aorta ).

function: to bring O2 and nutrients to local

cells too far from the lumen to be nourished by

blood there.

arterioles (A),capillaries and venules (V)

constitute the vasa vasorum (vessels of

vessels).

? Can you diffrentiate between veins & arteries?

Heart:

• Muscular organ , when

contracts, pumping the

blood through the

circulatory system.

• Producing a hormone called

atrial natriuretic factor.

The walls of all four heart chambers consist

of three tunics:

• Endocardium

• Myocardium

• Epicardium

Structure of heart1. superior vena cava

2. pulmonary semilunar

3. right atrium

4. tricuspid valve

5. right ventricle

6. inferior vena cava

7. septum

8. left ventricle

9. bicuspid) or mitral valve

10. aortic semilunar valve

11. left atrium

12. aorta

Endocardium

• Consists of:

➢ squamous endothelial cells.

➢ subendothelial layer of loose C. T.

(contains elastic, collagen fibers, smooth m.

, veins, nerves, and branches of the

impulse – conducting system of the heart

(Purkinje cells).

Myocardium• Thickest of the tunics of the heart.

• Consists of cardiac muscle cells surround

the heart chambers.

Pericardium:• Consist of:

1. Epicardium :

• Parietal layer.

• Visceral layer

Is mesothelium (simple sq.epi).

2. Subepicardial layer of loose C.T.

contains:

veins, nerves, ganglia and adipose tissue.

The heart has a specialized system to generate a

rhythmic stimulus (spread to the entire myocardium)

consist of:

➢ sinoatrial node (pacemaker). modified cardiac

muscle cells (fusiform),

with fewer myofibrils

➢ atrioventricular node: similar to those of the

sinoatrial node, but their cytoplasmic projections

branch ,forming a network.

➢ atrioventricular bundle

(bundle of His): formed by cells similar to those

of the atrioventricular node.

➢ Purkinje fibers:

• End of atrioventricular bundle

• 1-2 central nuclei.

• few myofibrils

The Lymphatic Vascular System:

Lymphatic capillaries:

In the various tissues as thin, closed-ended

vessels , consist of:

• A single layer of endothelium

• An incomplete basal lamina.

The larger lymphatics :similar to veins

except:

• They have thinner walls and lack

separation between tunics.

• More numerous internal valves