Cardiovascular Histology Flashcards
(20 cards)
1
Q
In Simple terms, what does the cardiovascular system consist of?
A
Consists of: a pump (heart), conducting vessels (arteries & veins), exchange vessels in the tissues (capillaries)
Heart → Artery → Capillary → Vein → Heart
2
Q
What are the three layers of tissue in the heart?
A
The heart is a pump for the CV system and is organised into three layers:
- Epicardium (outer layer): a simple squamous epithelium, subepicardial connective tissue (will contain fat, vessels and nervous tissues), blood vessels, fat, nervous tissue
- Myocardium: muscle cells and capillaries (very blood demanding layer)
- Endocardium (inner layer): an endothelial layer (monolayer of epithelial cells), subendocardial connective tissue, conducting tissue
3
Q
What are the features of cardiac muscle cells?
A
- cardiac muscles cells form the myocardium; it is an example of striated muscle (similar to skeletal muscle)
- contraction depends on myofibril and sarcomere and intrinsic properties cause it to contract spontaneously (not generated by a neuron/action potential; have an intrinsic contraction mechanism)
4
Q
Explain cardiac muscle histology,
A
- Cardiac muscle cells are small and have a central nucleus
- They form branching fibres and are joined by intercalated discs.
- Cardiac muscle cells must co-ordinate contractions
- Gap junctions electrically couple cells and coordinate action potentials in neighbouring cells. (not possible for cells that are further away; requires conducting pathways to transfer those messages)
5
Q
Explain the conducting pathways with respect to histology.
A
- the gap junctions co-ordinate electrical activity on a local level
- to co-ordinate contraction of different chambers a separate conducting system is needed - Purkinje fibres
- The heart beats at the rate of the fastest beating cardiac muscle cells. These are in the SA node (conductor of HR) and the AV node. (The fastest cells train the rest of the heart - AV node tends to follow SA node unless there is associated physiological abnormalities with the SA node)
- The Purkinje fibres conduct these signals to the ventricles
- The innervation of the heart doesn’t control the beating of the heart (this is intrinsic) but instead controls the speed of heart beat (faster or slower)
6
Q
What is the histology of the purkinje fibres?
A
- they are modified cardiac muscle cells; larger
- have limited contractile machinery (not a contractile cell)
- full of glycogen
- form bundles in subendocardium
(control the beating of the chambers relative to each other)
7
Q
What are the three layers of blood vessels?
A
- All blood vessels have a stereotyped structure
- three layers or tunics make the wall
- an internal tunica intima, a middle tunica media and an external tunica adventitia.
- the layers can be modified depending on whether it is a vein, artery or capillary.
8
Q
Explain the histology of the intima.
A
- the intima provides the surface which contacts the blood. It is lined with a simple squamous epithelium which lies on the basal lamina. It is supported by a thin, subendothelial connective tissue layer.
9
Q
Explain the histology of the media
A
- the media, or middle layer consists of smooth muscle arranged concentrically (or helically). It constricts the lumen, changing the lumenal diameter which can alter the resistance and therefore BP.
- the smooth muscle secretes the connective tissue in which is embedded (collagen type II, elastin and ground substance). It can vary from a single later of smooth muscle to up to 40-50 layers.
10
Q
Explain the histology of the adventitia.
A
- the adventitia binds the blood vessel into the surrounding tissue. It is made up of connective tissue (collagen type I and elastin with ground substance), with embedded fibroblasts. It anchors the vessel to the surrounding tissue and has its own blood supply (vasa vasorum) in larger vessels.
11
Q
Explain the role of edothelium.
A
The role of the endothelium is to:
- Actively inhibit clotting by secreting inhibitors (clotting is not wanted in vessels)
- Prime underlying subendothelial connective tissue with Von Willebrand’s factor (which activates clotting) - promotes clotting in the thin layer of connective tissue beneath the endothelium so that leaky vessels do not occur
- Release vasoactive substances like endothelin (vasoconstrictor) and NO (vasodilator) - controlling the tone/contraction it is able to regulate BP of the blood vessel.
12
Q
Explain the structure of arteries.
A
- Arteries themselves vary in their structure depending on where they are in the circulation
- Generally arteries have to withstand high, rapidly changing blood pressure
- They also regulate the BP by constricting or relaxing.
- No exchange occurs in arteries. It serves as simply a pipeline.
13
Q
What is the difference in structure between elastic arteries and muscular arteries?
A
Elastic Arteries:
- Close to the heart - highest BP fluctuations
- BP rises during heart beat, but it doesn’t fall to zero during diastole.
- Layers of elastin in the media store energy and compress the blood in the lumen
- The passive contraction means blood flow is continuous, but pulsatile
Muscular Arteries:
- Muscular arteries distribute blood to the tissue
- Little elastin in the media
- Elastin concentrated in the internal (and external) elastic laminae
- Contractions of the media regulate BP
14
Q
Explain the histology of arterioles and capillaries.
A
- Arterioles are very small arteries less than 0.1mm in diameter
- They have between 1 and 3 layers of smooth muscle
- They contribute most to BP changes (resistance inversely scales to the 4th power of diameter)
- Arterioles give rise to capillaries, the exchange vessels
- Quite often, an intermediate vessel exists
- The meta arteriole is characterised by an incomplete smooth muscle coat
- Single smooth muscle cells act as sphincters controlling capillary flow.
Capillaries:
- less an the diameter of a red blood cell, so when the RBC comes into the capillary, it is distorted as it is in direct contact with the vessel walls, so it is in the ideal situation to exchange gases and metabolites with the surrounding tissues.
- thin walled to facilitate exchange
- most cells in the body are within 50 microns of a capillary
15
Q
Explain the histological relationship between capillaries and endothelial cells.
A
- a single endothelial rolled into a tube and sealed with a tight junction
- basal lamina
- sometimes associated with a pericyte (media) but a media is not usually seen.
- surrounded by only a few collagen fibres in the adventitia.
16
Q
What are fenestrated capillaries?
A
- found in pancreas, intestines and endocrine glands
- diaphragms are not just simple membranes, they contain 8 wedge shaped channels
- sometimes, the diaphragm is completely missing (such as in the kidney)
17
Q
Explain the histological features of veins.
A
- blood from capillaries is collected in veins
- veins carry deoxygenated blood at low pressure and contain the highest volume of blood (~70%) acting as a blood reserve.
- Venous structure has the same layers as arteries however the media is thinner, adventitia is thicker (to help withstand hydrostatic pressure) and veins contain valves to force blood one way.
- This movement is assisted by skeletal muscle contractions (pumps)
18
Q
What are the histological features of venules?
A
- blood from capillaries is collected by venules
- the media of venules is initially pericytes but they are soon replaced by smooth muscle. Generally there is a large lumen.
- exchange across walls (like capillaries) and they are the preferred site of diapedesis of leukocytes
- affected by histamine and other cytokines (present during pathological insult & inflammation)
19
Q
What are some features of medium to large veins?
A
- Subendothelial connective tissue is well developed
- Adventitia enlarge often at expense of media
- Sometimes, longitudinal smooth muscle bundles are formed in the adventitia (stiffening)
20
Q
Explain the histological features of lymphatic vessels.
A
- Extracellular fluid leaks out of capillaries and venules constantly
- It is carried away by system of vessels called lymphatic vessels
- Leaky lymphatic capillaries collect fluid
- These vessels are characterised by absence of red cells (some white cells are present)
- The walls only have a single epithelial layer with many gaps for exchange.
- Usually collapsed in post mortem tissue
- They have valves and quite often, large lymphatic vessels resemble veins.
