Histology of the Cardiovascular System Flashcards
What is the main role of the circulatory system?
Pumps and directs blood cells and substances carried in the blood to all tissues of the body
Main components of the circulatory system.
- heart, arteries, capillaries, veins
- blood and lymphatic vascular systems
Characteristics of Arteries
- a series of vessels efferent from the heart
- become smaller as they branch into various organs
- carry blood to the tissues
Characteristics of Veins
- carry blood back to the heart to be pumped again
- organized in the pulmonary circulation and the systemic circulation
Characteristics of Capillaries
- smallest vessels
- sites of O2, CO2, nutrient, and waste product exchange between blood and tissues
- form a complex network of thin, anastomosing tubules called the microvasculature (in almost every organ)
How are Veins formed?
Results from the convergence of venules into a system of larger channels
Continue enlarging as they approach the heart
What are the two types of circulation?
(1) Pulmonary Circulation
(2) Systemic Circulation
Pathway of Pulmonary Circulation
The right side of the heart pumps blood through pulmonary vessels, through the lungs for oxygenation
Comes back to the left side of the heart
Pathway of Systemic Circulation
Pumps blood from the left side of the heart through vessels
Supplies either the head and arms or the lower body
Comes back to the right side of the heart
What are the 3 layers of heart chambers?
(1) Internal endocardium
(2) Middle myocardium
(3) External epicardium
What are the histological characteristics of the endocardium?
- endothelium
- a thin layer of connective tissue with smooth muscle cells
- subendocardial layer
What are the histological features of the subendocardial layer?
Lies below the endothelium and myoelastic layer
Contains the conducting (Purkinje) fibers in the ventricles
Myofibrils are displaced to the periphery due to glycogen filling much of the cytoplasm
What are Purkinje Fibers?
Cardiac muscle cells joined by intercalated disks
Role: specialized for impulse conduction
In the atria, Purkinje fibers are closer to the endothelium and intermingle with the contractile fibers within the myocardium
Give the histological features of epicardium (aka. visceral pericardium)
- forms the external tunic of the heart
- site of coronary vessels
- contains considerable adipose tissue
- consists of loose connective tissue containing autonomic nerves and variable amount of fat
- covered by simple mesothelium that also lines the pericardial space
What cells are responsible for the secretion of the pericardial fluid?
Mesothelial cells secrete a lubricant fluid
What is the significance of pericardial fluid?
This fluid prevents friction as the beating heart contacts the parietal pericardium on the other side of the pericardial cavity
What is the role of the cardiac skeleton?
Connective tissue:
- anchors the valves
- surrounds the two atrioventricular canals, maintaining their proper shape
Histology of the cardiac skeleton
dense irregular CT, primarily in the endocardium
Where do Chordae Tendineae attach?
Chordae tendineae attach to the atrioventricular valve
What is the histology of Chordae tendineae?
These structures are largely dense connective tissue covered with a thin layer of endothelium
What are the 3 tunics of arteries and veins?
(1) Intima
(2) Media
(3) Adventitia (or externa)
(these tunics correspond roughly to the heart’s endocardium, myocardium, and epicardium
How do arteries and veins differ?
ARTERIES: thicker media, relatively narrow lumen
VEINS: larger lumen, adventitia is the thickest layer, intima of veins is often folded to form valves
CAPILLARIES: only an endothelium, with no subendothelial layer or other tunics
What is the role of the internal elastic lamina
Separate the intima from the media
What is elastic lamina comprised of?
- internal elastic lamina = a sheet of elastic
- Simple squamous endothelial cells line the intima that has subendothelial loose connective tissue
- media contains many elastic lamellae and elastic fibers alternating with layers of smooth muscle
Compare the histological features of a LARGE artery and vein.
- media is much thicker in large arteries than veins, with relatively more elastin
- elastic fibers are also present in the outer tunica adventitia, which is relatively thicker in large veins
- the adventitia of large arteries is also supplied more sparsely with small sympathetic nerves for control of vasoconstriction
- media also has muscle fibers and elastic lamellae
Where is the vasa vasorum?
Vasa vasorum are seen in the adventitia of the aorta
What is the vasa vasorum’s role?
The microvessels bring O2 and nutrients to local cells that are too far form the lumen to be nourished by blood there
What is the vasa vasorum comprised of?
It is the microvasculature of the adventitia of larger arteries (aorta): capillaries, arterioles, and venules
What are the histological features of elastic artery?
- a large elastic artery shows a thick media with many well-developed elastic lamellae
- the intima of the empty aorta is typically folded
- the dense irregular conenctive tissue of the adventitia is thinner than the media
What is the advantage of elastic lamellae in elastic arteries?
- the largest arteries contain considerable elastic material and expand with blood when the heart contracts
- strong pressure of blood pulsating into such arteries during systole expands the arterial wall
- this expansion reduces the pressure and allows strong blood flow to continue during diastole
Where is the glomus body located?
The glomus bodies are two small (0.5-5mm diameter) ganglion-like structures found near the common carotid arteries
What is the glomus body comprised of?
- large capillaries intermingled with clusters of large glomus cells
- glomus cells filled with vesicles of various neurotransmitters
- supportive satellite cells with elongated nuclei that ensheath each glomus cell
What is the role of the glomus body?
- specialized regions in the walls of certain elastic arteries contain tissues acting as chemoreceptors
- chemoreceptors - provide information to the brain regarding blood chemistry
What do glomus cells sense and how is the message relayed to the brain?
Glomus cells form synaptic connections with sensory fibers
Significant changes in the blood CO2, O2, or H+ concentrations are detected by these chemoreceptive cells
Release a neurotransmitter that activated the sensory nerve to relay this information to the brain
What are the histological features of the muscular artery?
Arteries have relatively less elastin and more smooth muscle in their walls
Slightly folded intima with only sparse connective tissue between the endothelial cells and internal elastic lamina
Multiple layers of smooth muscle in the media that are thicker than the elastic lamellae and fibers
Vasa vasorum are seen in the adventitia
(most arteries are of mucular type)
What are the components of microvasculature?
Arterioles
Capillaries
Venules
What is the significance of microvasculature?
In almost every organ, molecular exchange takes place between blood and the interstitial fluid of the surrounding tissues
What are the histological features of capillaries?
Lack media
Adventitia tunics with diameters of only 4-10um
How are capillaries identified?
Can be recognized by nuclei adjacent to small lumens or by highly eosinophillic red blood cells in the lumen
What are the histological features of lymphatic vessels?
Thin-walled
Irregularly shaped
What is the role of lymphatic vessels?
Help in draining interstitial fluid
Give the histological features of arterioles.
Classified as microvessels
Can be various sizes
Intima - consists of only endothelium comprised of cells with rounded nuclei
media - consists of only 1-2 layers of smooth muscle
Have thin inconspicuous adventitia
Adventitia merges with neighboring connective tissue
Give the histological features of a large mesenteric arteriole
Endothelial cells
1-2 layers of smooth muscle cells
What are the histological features of capillaries?
Consist only of an endothelium across which molecular exchange occurs between blood and tissue fluid
Endothelial cells have more flattened nuclei in capillaries
Normally associated with perivascular contractile cells: pericytes
What are pericytes
Perivascular contractile cells
Have a variety of functions, but ultimately a symbiotic relationship with endothelial cells
What are the histological characteristics of pericytes?
Bigger nuclei than endothelial cells
How are pericytes and endothelial cells of capillaries differentiated?
The more flattened nuclei belong to endothelial cells
What are the 3 types of capillaries and their common location?
CONTINUOUS CAPILLARY: common in almost all organ systems
FENESTRATED CAPILLARY: intestinal walls, choroid plexus, restricted to endocrine organs
SINUSOID CAPILLARY: bone marrow, liver, spleen
What are the histological features of a continuous capillary?
Does NOT have any perforations
- a prominent endothelial cell nucleus
- abundant tight or occluding junctions in the junctional complexes at folds between the endothelial cells
- numerous transcytotic vesicles
What are the advantages of a continuous capillary?
Exert the tightest control over what molecules enter and leave the capillary lumen
All the material that crosses continuous capillary endothelium must pass through the cells, usually by diffusion or transcytosis
What is the advantage of a fenestrated capillary?
Specialized for uptake of molecules such as hormones in endocrine glands or for outflow of molecules, such as in kidney’s filtration system
Allows a freer exchange of molecules than continuous capillaries
What are the histological features of a fenestrated capillary?
- Typical fenestrae closed by diaphragms
- a continuous basal lamina surrounding the endothelial cell
- Golgi apparatus, nucleus, and centrioles
What are the main features of sinusoidal capillaries?
- have a much greater diameter than most capillaries
- surrounded by tissue containing adipocytes and masses of hematopoietic cells (in bone marrow)
- the endothelial cells are very thin
- cell nuclei are more difficult to find than in smaller capillaries
- have large fenestrations through cells
- large discontinuities between the cells and through the basal lamina
What are the specialized roles of Sinusoidal capillaries?
- maximal molecular exchange between blood and surrounding tissue
- easy movement of blood cells across the endothelium
What are the differences between an arteriole and a venule?
- venules are larger and more organized
- venules lie between capillaries and veins
- postcapillary venules have large lumens compared to arterioles and intima of simple endo-cells with occasional pericytes
- larger collecting venules have much greater diameters than arterioles
- arterioles have thicker walls than venules
What are the main features of a muscular venule?
- a better defined tunica media
- 3 layers of smooth muscle in some areas
- a very thin intima of endothelial cells
- a more distinct adventitia
What are the main features of a small vein?
- relatively large lumen compared to the small muscular artery (artery has a thick media and adventitia)
- its wall is very thin (contains only 2-3 layers of smooth muscle)
- valves
How are valves formed?
Valves are thin folds of intima projecting well into the lumen of a vein
Act to prevent backflow of blood
How is interstitial fluid formed?
Produced when plasma forced from the microvasculature by hydrostatic pressure
All of this fluid does not return to blood by the action of osmotic pressure
What is lymph composed of?
Rich in proteins and other materials
What are the main histological features of lymphatic capillaries?
Wall of endothelial cells - quite variable in diameter (10-50um)
Filled with lymph
Openings between endo-cells (that are held in place by anchoring filaments containing elastin - where interstitial fluid enters)
Lymphatic endothelial cells are typically larger than those of blood capillaries
What are the differences between a venule and a lymphatic vessel?
Lymphatic vessels: have very thin walls - formed by the merger of lymphatic capillaries
Venules have thick walls compared to lymphatic vessels
Lymphatic vessels do not contain red blood cells (in contrast to venules)
There is a valve that is resonsible for the unidirecitonal flow of lymph
What are the changes happening in an atherosclerotic artery that can contribute to ischemic heart disease?
- Atheromatous deposits in the coronary arteries (of adults)
- Asymptomatic, but can progress
- Cause acute myocardial infarction and its complications: dysrhythmia, heart failure
What are the 2 main consequences of coronary atherosclerosis?
ANGINA - chest pain caused by cardiac ischemia
MYOCARDIAL INFARCTION
What is angina?
A type of temporary chest pain, pressure, or discomfort
Pain: chest, arm, and neck (due to exertion, cold, and excitement)
Caused when the oxygen supply to the myocardium is insufficient
What are the changes in a coronary artery that can lead to myocardial infarction?
Coronary arteries become blocked by thrombus - lipid deposits once they start forming also attract platelets, then a clot can form
Cardiac myocytes rely on aerobic metabolism, and they die if ischemia occurs from thrombus
What are the 2 common consequences of myocardial infarction?
Common cause of death:
(1) Mechanical failure of the ventricle
(2) Dysrhythmia - electrical system going haywire and pumping lots
