ANAT2241 Cardiovascular System

From Embryology

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ANAT2241 This practical support page content is not part of the virtual science practical class and provides additional information for student self-directed learning purposes. All practical class pages are located on Moodle - ANAT2241

General Objective

Adult human cardiovascular system

To know and recognise the histological structure of blood vessels and the heart and to relate vascular structure to function.

Specific Objectives

  1. To know the basic architecture of vascular structures.
  2. To identify the tunica intima, tunica media and tunica adventitia in arteries and veins.
  3. To describe the types of capillaries and sinusoids and their light and electron microscopic features.
  4. To know the histological features of the epicardium, myocardium and endocardium.

Learning Activities

Examine the following virtual slides and in your course manual identify, draw, label and give the functions of the following structures:

Virtual Slides: Cardiovascular System


Starting at the Heart:

  • Elastic arteries (conducting) - large arteries
    • examples - aorta, brachiocephalic, common carotid, subclavian, vertebral, common iliac arteries.
    • thin wall compared to diameter.
    • tunica media more elastic fibres, less smooth muscle.
    • pressure reservoir.
  • Muscular arteries (distributing) - medium sized arteries
    • examples - axillary, brachial, radial, intercostal, splenic, mesenteric, femoral, popliteal, tibial.
    • capable of greater vasoconstriction/vasodilation.
    • adjust blood flow rate.
  • Arterioles - small arteries deliver blood to capillaries.
    • tunica interna, media (smooth muscle few elastic fibres), externa (elastic and collagen fibres)
    • regulate blod flow into capillaries.
  • Capillaries - smallest vessels.
    • endothelium and basement membrane.
    • main site of exchange (nutrients, waste).
    • connect arterioles and venules.
  • Venules - smallest veins.
    • uniting capillaries drain into veins.
    • further from capillary bed tunica media increases.
  • Veins - same 3 coats as arteries.
    • tunica interna and tunica media thinner than companion artery, tunica externa thicker.
    • many veins contain valves to prevent back flow.


Blood vessel wall

tunica intima

  • delimits the vessel wall towards the lumen of the vessel and comprises its endothelial lining (typically simple, squamous) and associated connective tissue.
  • Endothelium - simple squamous epithelium, cells organised longitudinally, joined by tight and gap junctions.
  • Internal elastic lamina (layer, membrane) not obvious due to may elastic layers.
  • the tunica intima of elastic arteries is thicker than in other arteries.

tunica media

  • layer of circumferential smooth muscle and variable amounts of connective tissue.
  • A second layer of elastic fibers, the external elastic lamina, is located beneath the smooth muscle.
  • Components
    • Elastin - fenestrated sheets or lamellae between muscle layers.
    • Smooth Muscle Cells - arranged in layers.
    • Collagen fibre and ground substance.

tunica adventitia

  • mainly of connective tissue fibres.
  • The tunica adventitia blends with the connective tissue surrounding the vessel, and the definition of the outer limit of the tunica adventitia is therefore somewhat arbitrary.
  • Thin - collagen fibres, elastic fibres (not lamellae), fibroblasts, macrophages.
  • vasa vasorum - (Latin, "vessels of the vessels") the network of small blood vessels that supply large blood vessels.


Artery histology 05.jpg Artery histology 06.jpg
Aorta overview Aorta elastin



  • elastic arteries - the tunica intima of elastic arteries is thicker than in other arteries.
  • muscular arteries
  • arterioles - smaller vessels with a diameter below 0.1 - 0.5 mm. Endothelial cells are smaller than in larger arteries, and the nucleus and surrounding cytoplasm may 'bulge' slightly into the lumen of the arteriole. Endothelium rests on a internal elastic lamina, which may also be incomplete. The tunica media consists of 1-3 concentric layers of smooth muscle cells. Difficult to identify an external elastic lamina or to distinguish the tunica adventitia from the connective tissue surrounding the vessel. Smooth muscle regulates tissue blood flow and arterioles receive both sympathetic and parasympathetic innervation.
  • capillary - arise from the final branches of arterioles, lined by a endothelial cells, each cell forms the wall around the entire circumference of a segment of the capillary. The lumen only allows 1-2 red blood cells to fit side by side in the capillary. Three main types of capillaries (continuous, fenestrated and discontinuous) can be distinguished based on features of their endothelium.

Muscular Arteries

  • Medium sized (muscular) artery and vein - (2 virtual slides)
  • Distinct layers of wall of an artery: tunica intima, internal elastic lamina, tunica media (many layers of smooth muscle), external elastic lamina (multiple layers) extending into tunica adventitia;
  • Neurovascular bundle
  • In the vein, the same 3 tunics can be seen but the tunica media is reduced, and the tunica adventitia is wider compared to the artery.
  • nervi vasorum - (nervi vascularorum) vascular nerves that innervate both arteries and veins and control vessel vasodilation and vasoconstriction.
    • The toluidine blue stains nucleic acids blue and polysaccharides purple and also increases the sharpness of histology slide images.
Artery histology 01.jpg Artery histology 02.jpg
Artery overview Artery detail
Artery histology 03.jpg Artery histology 04.jpg
Artery elastin Artery elastin detail


Vena cava

Vein histology 02.jpg Valves are absent.
  • tunica intima - contains some subendothelial connective tissue.
  • tunica media - comparatively thin with collagen and elastic fibres.
  • tunica adventitia - very wide, contains bundles of longitudinal smooth muscle and the transition to the surrounding connective tissue is gradual.


Vein valve

Small to medium-sized veins are characterised by the presence of valves.

Vein valve animation.gif

Animation shows how venous valves prevent the back flow of blood.

  • valves are formed by loose, pocket-shaped folds of the tunica intima.
  • one to three pockets form the valve.
  • Valves are absent in largest veins of the abdomen and thorax.

Vein histology 01.jpg

Venule with endothelium (simple squamous epithelium) lining.

Electron Micrograph

Capillary EM1


 ‎‎Mobile | Desktop | Original

EM Slides
Vascular Sinus EM1

Vascular Sinus EM01b.jpg

 ‎‎Mobile | Desktop | Original

EM Slides

Heart Histology

Cardiac Layers


  • Inner layer of the heart, contains blood vessels. Has 3 sublayers
    • Endothelium - innermost portion a simple squamous epithelium.
    • Smooth Muscle and Connective Tissue - middle layer of the endocardium is mix of connective tissue and smooth muscle.
    • Subendocardial Layer - outer layer of the endocardium is loose connective tissue joining the endocardium and myocardium.


  • Middle layer of the heart, thickest contains cardiomyocytes, blood vessels.
  • Muscular layer.


  • Outer layer of the heart, contains blood vessels and lymphatics.
  • Visceral layer of pericardium rather thin.

Cardiac muscle, the myocardium, consists of cross-striated muscle cells, cardiomyocytes, with one centrally placed nucleus.

  • Nuclei are oval, rather pale and located centrally in the muscle cell which is 10 - 15 µm wide.
  • Cardiac muscle cells excitation is mediated by rythmically active modified cardiac muscle cells.
  • Cardiac muscle is innervated by the autonomic nervous system (involuntary), which adjusts the force generated by the muscle cells and the frequency of the heart beat.
  • Cardiac muscle cells often branch at acute angles and are connected to each other by specialisations of the cell membrane in the region of the intercalated discs.
    • Intercalated discs invariably occur at the ends of cardiac muscle cells in a region corresponding to the Z-line of the myofibrils.
  • Cardiac muscle does not contain cells equivalent to the satellite cells of skeletal muscle.
Cardiac muscle histology.jpg

Cardiac muscle histology

Image of primate heart stained with Alizarin blue.
  • Red Blood Cells (orange cells) Cardiac Muscle Cells (blue)
  • Cardiac muscle cells are cut longitudinally.
  • At high magnification see both striations and the large nuclei of the cardiac muscle cells.
  • Follow the course of individual cardiac muscle cells and note fine, dark blue lines which seem to cross (traverse) the fibres.
  • Intercalated Discs that connect the individual muscle cells and permit the conduction of electrical impulses between the cells.
    • seen in longitudinal sections.

Heart histology 002.jpg Heart histology 004.jpg

Heart histology 003.jpg Heart histology 001.jpg

Cardiac Valves

Note for the semilunar valves not anchored to the walls of the atria by chordae tendineae.

Only the mitral and tricuspid valves, located between the atria and the ventricles, are connected to the cardiac wall by chord tendineae and papillary muscles.

Adult Heart Valves.jpg Gray0498.jpg

Unlabeled Images

Cardiac Features

Cardiac muscle (electron micrograph)
Cardiac muscle (electron micrograph)

Intercalated Discs

  • seen in longitudinal sections.
  • connect the individual muscle cells.
  • permit the conduction of electrical impulses between the cells.

Histology "step-like" appearance due to:

  • transverse part - crossing fibres at right angle to myofibrils.
  • lateral part - runs in parallel to myofibrils.

Junctional Components

  1. Fascia adherens – major portion of transverse component. Anchoring sites for actin, and connect to the closest sarcomere.
  2. Macula adherens – (desmosomes) transverse and lateral components. Bind individual myocytes to one another. stop separation during contraction by binding intermediate filaments, joining the cells together. Macula adherens junctions are also called desmosomes.
  3. Gap junctions - lateral component. Allow action potentials to spread between cardiac cells by passage of ions between cells, producing depolarization of the heart muscle. Allows muscle to act as syncytium.

Links: EM image - intercalated disc

Purkinje Fibres

Purkinje Fibres
  • modified cardiac muscle cells. Compared to ordinary cardiac muscle cells:
    • contain large amounts of glycogen.
    • fewer myofibrils.
    • thicker cells.
  • extend from the atrioventricular node, pierces the fibrous body, divides into left and right bundles, and travels, beneath the endocardium, towards the apex of the heart.
  • bundle branches contact cardiac muscle cells through specialisations similar to intercalated discs.
  • conduct stimuli faster than ordinary cardiac muscle cells (2-3 m/s vs. 0.6 m/s).
  • discovered in 1839 by Jan Evangelista Purkyně).

Links: Heart Histology | Cardiac AZB Labeled | Cardiac AZB | Cardiac label LS | Cardiac LS | Cardiac label TS | Cardiac TS | Purkinje fibres | Purkinje fibres detail | Histology


  • artifact - changes and distortions introduced to the normal tissue structure by the histological processing. Common artifacts include: folds (gives the tissue a darker appearance), tears (rips in the tissue can be seen in epithelia), shrinkage when tissues loose mainly liquid through histological processing, and cuts often used in tissue preparation.
  • elastic fibres - coloured light yellow in fresh tissues, special stains required to show in tissue sections. Composed of the protein elastin, can be stretched and return to original length.
  • elastic laminae - layer of elastic tissue in a blood vessel wall.
  • external elastic lamina - elastic tissue layer that lies within the tunica media of blood vessels.
  • internal elastic lamina - (internal elastic lamella) elastic tissue layer that forms the outermost part of the tunica intima of blood vessels.
  • nervi vasorum - (nervi vascularorum) vascular nerves that innervate both arteries and veins and control vessel vasodilation and vasoconstriction.
  • pericyte - (Rouget cells, mural cells) a contractile cell located around endothelial cells of capillaries, venules and larger lymphatic vessels. Role in regulating blood flow, phagocytosis, and signalling.
  • tunica - (Latin, "tunic") refers to a coat or coating surrounding a blood vessel.
  • tunica adventitia - (adventitia, tunica externa) the outermost layer of a blood vessel surrounding the tunica media. Anchors blood vessel to surrounding tissue and consists of many collagen fibres.
  • tunica intima - (intima) innermost layer in both arteries and veins.
  • tunica media - (media) middle layer in both arteries and veins.
  • vasa vasorum - (Latin, "vessels of the vessels") the network of small blood vessels that supply large blood vessels.
  • vena cava - the largest veins in the body, formed by the superior vena cava (SVC) and inferior vena cava (IVC).

Course Links

Moodle - ANAT2241 - 2019

Histology Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | ANAT2241 Support | Histology | Histology Stains | Embryology Glossary

Common Histology Stains  
Histology Stains - Common Stains and Their Reactions
mucins - light blue
pale pink
bright red
colloid - pinkmuscle - red
Iron Haematoxylin
Van Gieson
muscle: yellow/browncartilage - pink
Verhoeff's Elastin
elastic fibres - black
Silver Impregnation
reticular fibres - black
Methyl Green
dark green
light green
light green
Nuclear Fast Red
Gomori's Trichrome
keratin - redmuscle - purple/red
Heidenhain's Azan
deep blue
muscle - red
Osmium Tetroxide
myelin, lipids - black
Alcian Blue
mucins, - blue
Periodic acid-Schiff (PAS)
mucins, glycogen, glycocalyx - magenta
Phosphotungstic Acid-Hematoxylin (PTAH)
muscle bands - blue
Masson's Trichrome
cartilage, mucins - blue or green; muscle - red
Luxol Fast Blue
myelin - blue
Aldehyde Fuchsin
elastic fibres, mast cells - deep purple
Light Green
light green
dark blue
nucleic acids, Nissl granules - dark blue
Romanowsky (e.g. Leishman's)
acidophils - red, basophils - blue, azurophilic - purple
Aldehyde Pararosanilin elastic fibres - purple
Haematoxylin and Eosin
One of the most common staining techniques in pathology and histology. Acronym "H and E" stain. (H&E, HE).

  • Stains nuclei blue to dark-blue.
  • Stains the matrix of hyaline cartilage, myxomatous, and mucoid material pale blue.
  • Stains myelin weakly but is not noticeable if combined with eosin stain.
  • combined with Orange G (H & Or. G.) instead of eosin, specifically stains the granules of acidophilic cells of the adenohypophysis (anterior pituitary).
  • Stains cytoplasm pink to red; red blood cells are also bright red.
  • Common counterstain to hematoxylin.
  • Stain intensity varies with the formula as well as the fixative.

Practical Support

Pages can be accessed from any internet connected computer.

ANAT2241 Support Links: The Virtual Microscope | Covering and Lining Epithelia | Glandular Epithelia | CT Components | CT Types | Bone, Bone Formation and Joints | Muscle | Nervous | Blood | Eye | Cardiovascular | Respiratory | Integumentary | Gastrointestinal | Gastrointestinal Organs | Lymphatic and Immune | Endocrine | Urinary | Female Reproductive | Male Reproductive | Histology Stains | Histology Drawings | Practicals Health and Safety 2013 | Moodle - 2019

ANAT2241 This practical support page content is not part of the science practical class and provides only background information for student self-directed learning purposes.

Cite this page: Hill, M.A. (2024, February 22) Embryology ANAT2241 Cardiovascular System. Retrieved from

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G