Lecture 18: Cardiac and Circular Microanatomy (Hearts and Vessels)

What are the three layers of the heart wall?

Endocardium, myocardium, epicardium

what are the layers of the endocardium?

Endothelium, subendothelial layer, subendocardial layer

What is the thickest layer of the heart wall?

Myocardium

What structure is the epicardium continuous with?

The visceral pericardium

What is the endothelium of the endocardium made up of?

Simple squamous epithelium, continuous with the circulatory endothelium

What is the subendothelial layer of the endocardium made up of?

Loose connective tissue, small vessels, nerves

What layer of the endocardium may contain purkinje fibers, specifically in the ventricles?

Subendocardial layer

What is the primary function of the endocardium?

Provides a non-thrombogenic, non-turbulent surface for blood flow

Is the endocardium thicker in the right or left atrium?

Left

What are purkinje fibers?

Modified cardiac myocytes

What function do the purkinje fibers serve?

Rapid conduction to coordinate ventricular contraction

Why do purkinje fibers need to have a large diameter?

It reduces resistance

Why do purkinje fibers need abundant gap junctions?

Aids in rapid cell-to-cell conduction

Why do purkinje fibers need less myocytes than they do glycogen?

Allows for more space for conduction machinery

Why do purkinje fibers have a high Na+ channel density?

Vital for rapid depolarization

What makes up the myocardium?

Primarily cardiac muscle cells

What connects adjacent cardiac myocytes?

Intercalated discs

What is the function of the myocardium?

Generates contractile force to pump blood

What makes up the epicardium?

Mesothelium, connective tissue, and adipose tissue, coronary vessels and nerves

What type of cells make up the mesothelium of the epicardium?

Simple squamous

Why does the epicardium have a lot of connective tissue and adipose tissue?

For cushion and support

What is the primary function of the epicardium?

It serves as the protective outer covering of the heart

What are cusps/leaflets made up of?

An endocardium covering a fibrous connective tissue core

What is the function of heart valves?

Ensure unidirectional blood flow

Where are AV valves located?

Between the atria and the ventricles

Where are semilunar valves located?

Between the ventricles and the aorta/pulmonary artery (depending on the side of the heart)

What are chordae tendineae?

The small tendons connecting the AV valve flaps to the papillary muscle

What heart valves do not have chordae tendineae?

Semilunar valves

What is the cardiac skeleton?

A dense, fibrous connective tissue framework that forms the heart's structural core

What are fibrous annuli?

The dense connective tissue rings that join to create the cardiac skeleton

What do fibrous annuli do?

Support AV & semilunar valves

What are the primary functions of the cardiac skeleton?

-Provides structural support for valves & myocardium

-Acts as an electrical insulator between atria & ventricles in such a way that there can only be conduction through the AV node

What are the three layers of a typical vessel wall?

Tunica intima, tunica media, tunica adventitia

What layer is thicker in arteries than veins?

The tunica media

What layer is thicker in veins than arteries?

The tunica adventitia

What are the three layers of the tunica intima?

Endothelium, subendothelial layer, internal elastic lamina

What is the function of the endothelium in the tunica intima?

It forms a smooth, non-thrombogenic surface

What makes up the subendothelial layer of the tunica intima?

Thin connective tissue

What layer of the tunica intima is distinct in arteries but may be less prominent or even absent in veins?

Internal elastic lamina

What are the functions of the tunica intima?

-Maintains blood-tissue interface

-Regulates vascular tone, permeability, and clotting responses

What makes up the tunica media?

-Concentric layers of smooth muscle cells

-Interspersed elastic fibers, reticular fibers, and proteoglycans

What are the functions of the tunica media?

-Regulates vascular diameter

-Maintains blood pressure and flow distribution

What makes up the tunica adventitia?

Loose connective tissue

What are the special structures of the tunica adventitia?

Vasa vasorum and nervi vasorum

What is the vasa vasorum?

Small vessels supplying blood to the outer wall of large arteries and veins

What is the nervi vasorum?

Autonomic nerves in charge of regulating smooth muscle tone

What are the primary functions of the tunica adventitia?

-Anchors vessel to surrounding tissues

-Provides nutrient and neural support to the vessel wall

What are some examples of elastic arteries?

Aorta, pulmonary arteries, major branches (i.e. carotid artery)

What is the tunica media made up of in elastic arteries?

Numerous concentric elastic lamellae (alternating with smooth muscle)

What does the tunica intima look like in elastic arteries?

It has a prominent endothelium and internal elastic lamina

What does the tunica adventitia look like in elastic arteries?

It is relatively thin with vasa vasorum and nerves

What is the function of elastic arteries?

Maintains a continuous blood flow by storing energy during a systole and releasing it during diastole

What are some examples of muscular arteries?

Most named arteries (e.g., radial, femoral, coronary, mesenteric)

What does the tunica intima look like in muscular arteries?

It has a prominent internal elastic lamina

What does the tunica media look like in muscular arteries?

It contains a thick layer of smooth muscle cells

What separates the tunica media from the tunica adventitia in muscular arteries?

External elastic lamina

What does the tunica adventitia look like in muscular arteries?

It is composed of collagen, elastic fibers, vasa vasorum

What is the function of muscular arteries?

Maintains pressure of vessels

What are arterioles?

The smallest arteries leading into capillary beds

What does the tunica intima look like in arterioles?

Endothelium with thin subendothelial layer; IEL may be present in larger arterioles

What does the tunica media look like in arterioles?

1-3 layers of smooth muscle cells

What layer of arterioles is the key site of resistance control?

The tunica media

What does the tunica adventitia look like in arterioles?

It is minimal and blends with the surrounding connective tissue

What are the functions of arterioles?

-They act as primary resistance vessels which regulates systemic vascular resistance

-They control blood pressure and capillary perfusion via vasoconstriction/dilation

Describe the layers of capillaries

They only have endothelium

What cell type may be present in capillaries and what is their function?

Pericytes- support, repair, regulation

How wide are capillaries?

Just wide enough for RBCs to pass in single file

What are the types of capillaries?

Continuous, fenestrated, sinusoidal (discontinuous)

What joins cells in continuous capilalries?

Tight junctions

What do fenestrated capillaries have that allows for higher exchange rates?

Pores

Describe the endothelium of sinusoidal capillaries

Wide gaps, incomplete basement membrane

What are the functions of capillaries?

-Site of gas, nutrient, and waste exchange

-Regulate fluid balance and permeability depending on the type

What makes up postcapillary venules?

Endothelium and thin connective tissue. There is little to no smooth muscle

What is the function of postcapillary venules?

It is a major site of leukocyte migration (diapedesis) and inflammation

What is diapedesis?

Leukocyte migration

What is the structure of collecting venules and small veins?

There are 1-2 more layers of smooth muscle in the tunica media

What are the functions of venules and small veins?

-Drain blood from capillary beds

-Regulate exchange and immune cell trafficking

-Serve as capacitance vessels, holding much of the body's blood volume

What are some examples of medium and large veins?

Femoral, jugular, portal, vena cava

What does the tunica intima look like in medium and large veins?

Endothelium with thin subendothelial connective tissue

What does the tunica intima in medium and large veins have to prevent backflow?

Valves

What does the tunica media look like in medium and large veins?

It is much thinner than arteries and has many fewer muscle cells relative to the lumen

What is the thickest layer of medium and large veins?

Tunica adventitia

What does the tunica adventitia of medium and large veins contain?

Vasa vasorum and nerves

What are the functions of medium and large veins?

-Returns blood to the heart (valves and skeletal muscle pumps aid venous return)

-Act as capacitance vessels

How thick are the walls of lymphatic vessels?

Very thin

What does the endothelium look like in lymphatic vessels?

It has an incomplete basal lamina

What does the tunica media look like in lymphatic vessels?

There is thin-to-no tunica media

What does the tunica adventitia look like in lymphatic vessels?

It blends with the surrounding connective tissue

What are the functions of lymphatic vessels?

-They collect interstitial fluid (lymph) & return it to venous circulation

-They transport immune cells (lymphocytes, antigen-presenting cells)

-They maintain tissue/fluid balance