Flashcards in Histology of the CVS Deck (134):
What path does the blood take around the body?
Blood is pumped from the heart to the large elastic arteries, then to the medium muscular distributing arteries. It then goes to the arterioles, then metarterioles, the capillaries. From the capillaries, a small about of blood returns to the heart via the lymphatic system, but most goes to the post capillary venules, to the medium veins, to the large veins, then back to the heart.
At what rate does blood return to the heart via the lymphatic system?
Why does most blood return to the heart by going to post capillary venules?
Because of the pressure
When is blood flow fastest?
When total cross sectional area is least, and so in the aorta
What happens as the arteries branch?
The total cross sectional area of the vascular bed increases
Where is blood flow slowest?
Capillaries, as there are many divisons
Why is blood flow being slowest at the capillaries advantageous?
Because blood needs to hand around for gas exchange
What are arteries?
Blood vessels that carry blood away from the heart to the capillary beds
What is the major artery arising from the right ventricle?
The pulmonary trunk
What happens to the pulmonary trunk?
It bifurcates into the left and right pulmonary arteries
What do the pulmonary arteries do?
Supply the lungs with deoxygenated blood
What is the major artery arising from the left ventricle?
What does the aorta do?
It courses in a posteriorly oblique arch to descent into the thoracic cavity
What arises from the arch of the aorta?
Three major arterial trunks, the brachiocephalic artery, the common carotid artery, and the left subclavian artery
How does the aorta terminate?
In the abdominal cavity it bifurcates into left and right common iliac arteries in the pelvis, near the belly button
What happens in systole?
Left ventricular contraction causes blood pressure in the aorta to rise to approx. 120mm Hg- this is systolic pressure.
What happens to the aorta under systolic pressure?
The walls of the elastic aorta stretch
What happens in diastole?
The aortic semi-lunar valve closes, and the walls of the aorta recoil. Aortic pressure drops to 70-8mmHg- this is diastolic pressure.
Why do the aortic walls relax in diastole?
To maintain pressure on the blood, moving it forwards into the small vessels
What is the result of diastolic pressure?
It is stil high, so blood is moving the whole time, meaning that it doesn’t start and stop
What do elastic arteries acting do?
Conduct blood away from the heart
Act as pressure reservoirs
What do elastic arteries act as during diastole?
Axillary pumps, giving back the elastic energy stored during systole
What types are arteries classified into?
Elastic conducting arteries
Muscular distributing arteries
What layers do the walls of arteries and veins have?
Tunica intima (next to lumen)
Tunica adventitia (outside)
What does the tunica intima consist of?
Endothelium and subendothelial layer
What do some arteries have in addition to the three layers?
Internal elastic lamina between the tunica intima and media, and an external elastic lamina between tunica media and tunica adventitia
How are the arteries and veins connected?
How do the walls of the elastic arteries appear in the fresh state?
May be yellow
Why do the walls of elastic arteries look yellow in the fresh state?
Because of abundant elastin
Why do the walls of elastic arteries look white in cadavers?
Because of the fixation
What does the tunica intima consist of in elastic arteries?
Endothelial cells with long aces, orientated parallel to the long axis of the artery.
Narrow sub-endothelium of connective tissue, with discontinuous elastic lamina
What is the main feature of the tunica media in elastic arteries?
40-70 fenestrated elastic membranes
What else does the tunica media of elastic arteries contain?
Smooth muscle cells and collagen between lamina
What is the purpose of smooth muscle cells in the elastic arteries?
Produce elastin, collagen, and matrix
How do the features of the tunica media of elastic arteries stain?
Fenestrated elastic lamallae stain black
Collagen and extracellular matrix stain turquoise
Smooth muscle stains red
What does the tunica adventitia consist of?
A thin layer of connective tissue, containing vasa vasorum, lymphatic vessels, and nerve fibres
Why are the vasa vasorum required?
Because the wall is so thick that they need their own blood supply
How does an aortic dissection arise?
If blood somehow breaks the endothelium and works its way into the tunica media, it can start getting between the elastic lamallae. The high pressure means that sections are forced apart.
What is the problem with aortic dissections?
Can be catastrophic
The collected blood is susceptible to clotting. If this continues, may rupture the aorta
When does aortal dissection happen a lot?
Marfan’s syndrome, due to the defected elastin
Where in the aorta can a dissection occur?
Anywhere along the length of the aorta
How are aortal dissections treated?
An endovascular stent is placed, which means there is no flow to the aortic dissection, and the blood can flow through the stent graft
How does the tunica intima appear in small elastic arteries?
Indistinct endothelial cells
What does the tunica media of an elastic artery have?
Elastin lamellae, smooth muscle cells and extracellular matrix
What is the tunica adventita of small elastic arteries made of?
What is the importance of the elastic walls of small elastic arteries?
They can store energy in systole, helping move blood along
What does the tunica media of muscular arteries consist of?
Endothelium, a sub endothelial layer, and a thick internal elastic lamina
What is the main feature of the tunica media of muscular arteries?
40 layers of smooth muscles, connected by gap junctions
Why are the muscle cells of the tunica media connected by gap junctions in muscular arteries?
For coordinated contraction
How are the smooth muscle layers in muscular arteries arranged?
What does the tunica adventitia consist of in muscular arteries?
A thin layer of fibroelastic connective tissue, containing vaso vasorum, lymphatic vessels and unmyelinated nerve endings
How do the vaso vasorum in muscular arteries differ in muscular arteries from those in elastic arteries?
They are not very prominent
What stimulates vasoconstriction?
Sympathetic nerve fibres
How does vasoconstriction of muscular arteries occur?
The neurotransmitter noradrenaline is released at nerve endings, and diffuses through fenestrations in the external elastic lamina into external tunica media, to depolarise some of the superficial smooth muscle cells. Depolarisation is propagated to all cells of the tunica media via gap junctions
What happens as branch and diminish in diameter?
The number of smooth muscle layers in the tunica media diminishes
What happens when smooth muscle is contracted?
The endothelial layer and nuclei stick out
What is an end artery?
The terminal artery supplying all or most of the blood to a body part, without significant collateral circulation, meaning there is no significant contribution to tissue from another artery
What do end arteries undergo?
Progressive branching, without the development of channels connecting with other arteries
What happens if an end artery is occluded?
There is insufficient blood supply to dependant tissues, so nutrient and oxygen supply is seriously compromised
Give 3 examples of end arteries
What are the best examples of an absolute end artery?
The central artery to the retina
The labyrinthine artery of the internal ear
What is bridging?
The compression of a segment of a coronary artery during systole, resulting in a narrowing that reverses during diastole
What are arterioles?
Arteries with a diameter of less than 0.1mm
What do arterioles have in their tunica media?
1 to 3 layers of smooth muscles
Do arterioles have a thin elastic lamina?
Only larger ones
What is the tunica media composed of in small arterioles?
A single smooth muscle cell that completely encircles the endothelial cells
Do arterioles have an external elastic lamina?
What is the tunica adventitia of arterioles made up of?
A few fibrous cells with a bit of connective tissue
What are metarterioles?
Arteries that supply blood to capillary beds
How do metarterioles differ from arterioles?
The smooth muscle layer is not continuous
What do individual muscle cells in metarterioles do?
They are spaced apart, and each encircle encircles the endothelium of a capillary arising from the metarteriole, called the precapillary sphincter
What is the function of the precapillary sphincter?
On contraction, it controls the blood flow into the capillary, reducing it
What does the precapillary sphincter allow?
Arterioles and metarterioles to serve as flow regulators for capillary beds
What happens when precapillary sphincters are open?
There is plentiful blood flow through the capillary bed
What happens when precapillary sphincters are closed?
Blood flow through the capillary bed is greatly reduced
How can the capillary bed be bypassed?
By a central channel, consisting of a metarteriole and a thoroughfare channel.
This happens when the sphincters are closed
What are most arterioles able to do?
Dilate to 60-100% of their resting diameter
Maintain up to 40% constriction for a long time
What does regulation by arterioles do?
Directs blood flow to where it may be most needed
What happens during strenuous physical exertion?
Blood flow to skeletal muscles is increased by dilation of arterioles, and blood flow to the intestine is decreased by constriction of arterioles
What do lymphatic capillaries do?
Drain away excess extracellular fluid
Where do lymphatic capillaries return extracellular fluid to the blood?
At the junctions of the internal jugular and subclavian veins
How much of the total blood volume do capillaries hold?
What do capillaries present?
The largest surface area for gas and nutrient exchange
How is the diffusion path to adjacent tissues minimised?
Passing red blood cells fill virtually the entire capillary lumen
How does blood velocity in capillaries differ from elsewhere?
It is lowest during passage through the capillaries
What does the low blood velocity through the capillaries allow?
Time for gas exchange and nutrient exchange with surrounding tissues
How big are capillaries?
30µm in diameter
Usually less than 1mm long
Essentially, what is the capillary?
A tube thats just large enough to allow passage of blood cells one at a time
What are capillaries made of?
A single layer of endothelium and its basement membrane
What are the types of capillaries?
Sinusoidal, or discontinuous (sinusoids)
What are the most common type of capillaries?
Where are continuous capillaries located?
Nervous, muscle and connective tissues, exocrine glands, and the lungs
What do continuous capillaries consist of?
Continuous endothelial layer, with the cells joined by tight or occluding junctions
Where are fenestrated capillaries found?
In parts of the gut, endocrine glands and the renal glomerulus
What exists in fenestrated capillaries?
‘Little windows’, or interruptions, across thing parts of the endothelium, which are bridged by a thin diaphragm (except in the renal glomerulus)
What are the possible routes of transport across the endothelial wall of a fenestrated capillary?
Diffusion through intercellular cleft
Diffusion through fenestration
Through pinocytic vesicles
How do sinusoids differ from other capillaries?
They have a larger diameter and a slower blood flow
What is the diameter of sinusoids?
Where are sinusoids found?
In the liver, spleen and bone marrow
What do sinusoids have?
Gaps in the wall
What is the purpose of the gaps in the walls of sinusoids?
Allows whole cells to move between blood and tissue
What do pericytes form?
A branching network on the outer surface of the endothelium
What are pericytes capable of?
Dividing into muscle cells or fibroblasts, during angiogenesis, tumour growth and wound healing
How are postcapillary venules similar to capillaries?
The wall is similar, in that it has an endothelial lining with associated pericytes
What is the diameter of postcapillary venules?
What is more permeable, capillaries or post-capillary venules?
What tends to drain into post-capillary venules?
Why does fluid tend to drain into post-capillary venules?
Because their pressure is lower than that of capillaries or the surrounding tissues
When does fluid not drain into the postcapillary venules?
When an inflammatory response is operating, in which case fluid and leukocytes emigrate
What are the post-capillary venules the preferred location for?
Emigration of leukocytes from the blood
At what point do smooth muscle fibres begin to be associated with the endothelium of venules?
As the diameter of merging venules begin to increase to more than 50µm
What is happening as smooth muscle fibres begin to appear in venules?
A tunica media is beginning to appear
What is the diameter of venules?
Up to 1mm
What is the endothelium of venules associated with?
What can venules have?
What do venules valves consist of?
Thin, intimal extensions
What do venules valves do?
By pressing together, they restrict retrograde transport of blood, stopping blood flow in the opposite direction
What is the general rule regarding veins?
They have a larger diameter than any accompanying artery, and a thinner wall that has more connective tissue and fewer elastic and muscle fibres
What do small and medium sized veins have?
A well developed adventitia
What does the tunica media of veins consist of?
2 to 3 to layers of smooth muscle
What is the diameter of large veins?
Can be over 1cm
How do large veins differ from smaller ones?
The tunica intima is thicker
Do large veins have a prominent tunica media?
What is well developed in large veins?
What large veins differ from most?
The superficial veins of the legs, which have a well-defined muscular wall
Why do the superficial veins of the legs have a well defined muscular wall?
Possibly to resist distension caused by gravity
How do valves in the vein act?
Together with muscle contraction, to propel blood towards the heart
What are venae comitantes?
Deep paired veins that, in certain anatomical positions, accompany one of the smaller arteries on each side of the artery.
The three vessels are wrapped together in one sheath
What does the pulsing of the artery in vanae comitantes promote?
Venous return within the adjacent, parallel, paired veins
Give 3 examples of venae comitantes
Brachial, ulnar and tibial
Give 7 examples of large veins
What do large veins have in the tunica adventitia?
Well-developed, longitudinally orientated smooth muscle
What do large veins have in the tunica media?
Circularly arranged smooth muscle