Blood Vessels Flashcards
What is the tunica intima?
The innermost layer of a blood vessel –> single layer of endothelial cells
What is the tunica externa?
The outermost layer of a blood vessel
What is a capillary?
Any of the small blood vessels that connect arteries to veins
What is the tunica media?
The middle layer of a blood vessel
What is the anastomosis?
The junction between blood vessels
How many tissue layers are arteries and veins composed of?
3 tissue layers
What is the tunica externa made of?
Connective tissue
What is the tunica externa surrounded by?
External elastic lamina which functions to anchor vessels with surrounding tissues
Why is the tunica externa often thicker in veins?
Prevent collapse of blood vessel and provide protection from damage since veins may be superficially loaded
Where is the tunica media?
Surrounds the tunica intima
What is the tunica media comprised of?
Smooth muscle cells and elastic and connective tissues arranged circularly around vessel
Why is the tunica media in arteries especially rich in smooth muscle cells?
Helps control caliber of vessel to maintain blood pressure
What is the external elastic membrane/laminar?
Thick elastic band separating tunica externa and tunica media
What is the internal elastic membrane/laminar?
Elastic band separating tunica media and tunica intima (endothelium)
Predominantly elastic in aorta, predominantly muscular in smooth muscle cells
Do veins have an external elastic laminar?
No, only an internal one
What is the tunica intima?
The inner layer and thinnest layer
What is the tunica intima formed from?
A single continuous layer of endothelial cells and supported by a subendothelial layer of connective tissue and supportive cells
In smaller arterioles and venules, what is the sub endothelial layer supporting the tunica intima like?
Single layer cells but can be much thicker in large vessels such as the aorta
What do capillaries consist of?
A single layer of endothelium and associated connective tissue
Describe route from arteries all the way back to veins
- Arteries carry blood away from heart
- Branch into smaller arterioles throughout body
- Eventually form capillary network (gas exchange)
- Capillaries merge into venules
- Then into larger veins which carry blood back to heart
Are valves present in arteries or veins?
Present in valves not arteries
Why are valves not present in arteries?
Blood is pumped under pressure from heart so backflow cannot occur
Why do veins have valves?
Passing through the capillary network results in a decrease in blood pressure meaning backflow of blood is possible. To counteract this, veins contain numerous one-direction valves to prevent backflow.
What are the lumina like in venules compared to arterioles?
Larger lumina and thinner walls
Describe the structure of venules
Clear tunica intima layer without any elastic fibres (composed of squamous endothelial cells that act as membrane)
A tunica media with one or two layers of muscle fibres (middle layer is poorly developed so venules have thinner walls than arterioles)
Tunia externa fuses with surrounding tissue
Describe the structure of veins
Tunica intima
Tunica media
Tunica externa/adventitia (broadest layer)
What is the return of blood to the heart assisted by?
The action of the skeletal-muscle pump. As muscles move, they squeeze veins running through them.
Most veins carry deoxygenated blood back to the heart. What are the 2 exceptions to this?
Pulmonary and umbilical veins both carry oxygenated blood
What are the 3 main types of arteries?
- Elastic arteries
- Muscular arteries
- Arterioles
What are the elastic arteries?
Receive blood directly from heart –> the aorta and pulmonary artery
Why do elastic arteries need to be elastic?
Relatively thin compared to diameter:
- When heart contracts and ejects blood into these arteries, the walls need to stretch to accomoate pressure
What is the arteriole hydrostatic pressure that results from ventricular contraction
Systolic blood pressure
Between heart contractions, what happens to elastic walls of arteries?
Elastic walls recoin to maintain blood pressure (continues to move blood even when ventricles are relaxed)
What is the arterial hydrostatic pressure between contractions called?
The diastolic blood pressure
What is tunica media like in elastic arteries?
Broad and elastic with sheets of elastin and only few smooth muscle fibres
What is one of the first signs of atherosclerosis?
Tunica intima has fibroblasts and ‘myointimal cells’ that accumulate lipid with ageing, and the intima layer thickens, one of the first signs of atherosclerosis.
Picture of difference in arteries and veins
What are the muscular arteries?
Distribute blood to various parts of the body. These include arteries such as the femoral and coronary arteries.
What do the walls of muscular arteries have lots of and why?
Have lots of smooth muscle, which means that they are able to contract or relax (dilate) to change the amount of blood delivered, as needed
Describe the difference in structure between muscular and elastic arteries
- Sheets of elastin reduced in muscular
- Internal elastic layer between tunica intima and tunica media
- External elastic layer between tunica media and tunica externa (less well defined)
- Well defined layer of smooth muscle in tunica media (in muscular)
What are arterioles?
Small arteries that deliver blood to capillaries
How do arterioles control blood flow through capillary beds?
Contract and dilate size of lumen –> the tunica media layer contains concentric rings of smooth muscle to do this
What is the tunica intima like in arterioles?
Very thin and mostly consists of a single layer of squamous epithelium
What does elastic recoil of vascular wall help?
To maintain pressure gradient that drives blood through the arterial system
Picture of the different structures of the 3 types of arteries
What happens to structure of arteries farther from the heart?
The % of elastic fibres in an artery’s tunica intima decreases and the amount of smooth muscle in its tunica media increases (muscular artery)
What does the thick tunica media in muscular arteries allow? What does their decreased quantity of elastic fibres limit?
Vasoconstriction
Limits their ability to expand
What is general appearance of arteries?
Thick walls with small lumens
Generally appear rounded
What is general appearance of veins?
Thin walls with large lumens
Generally appear flattened
What is endothelium?
A tissue which forms a single layer (lines blood vessels). The tunica intima is a thin layer composed of a simple squamous epithelium (endothelium) and a small amount of connective tissue.
What is structure of capillaries?
Only have a tunica intima layer
What is function of endothelium?
Blood pressure regulation
Regeneration and repair
What can damage to clot lead to?
Clot formation
What is tunica intima like in arteries?
Endothelium usually appears wavy due to constriction of smooth muscle
Internal elastic membrane present in larger vessels
Describe tunica intima in veins
Endothelium appears smooth
Internal elastic membrane absent
Describe tunica media in arteries
Normally thickest layer in arteries.
Smooth muscle cells and elastic fibres predominate (proportions of these vary with distance away from heart –> more elastic towards heart, more muscle away from heart)
External elastic membrane present in larger vessels
Describe tunica media in veins
Normally thinner than tunica externa
Smooth muscle cells and collagenous fibres predominate
Nervi vasorum and vasa vasorum present
External elastic membrane absent
What are nervi vasorum?
Small nerve fibers found in arteries and veins that trigger contraction of the smooth muscle in their walls
What are vasa vasorum?
A network of small blood vessels that supply the walls of large blood vessels, such as elastic arteries (e.g., the aorta) and large veins (e.g., the venae cavae)
Describe the tunica externa in arteries?
Normally thinner than the tunica media in all but the largest arteries
Collagenous and elastic fibres
Nervi vasorum and vasa vasorum present
Describe the tunica externa in veins?
Normally the thickest layer in veins
Collagenous and smooth fibres predominate
Some smooth muscle fibres
Nervi vasorum and vasa vasorum present
What is a capillary?
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter, and having a wall one endothelial cell thick (tunica intima)
How do RBC pass through capillaries?
Capillaries only large enough for single flow of RBC. RBCs are deformed to squeeze through
Why are capillary walls leaky?
To allow substances to pass through (O2, CO2, waste etc)
What are the 3 major types of capillaries?
- Continuous
- Fenestrated
- Sinusoid
What is the most common type of capillary?
Continuous –> found in most vascularised tissues
How do muscular arteries affect BP?
Contraction and relaxation of smooth muscle layer that decreases and increases size of blood vessel diameter
Vasoconstriction –> decreases blood flow as smooth muscle contracts and makes lumen narrower and increases BP
Vasodilation –> increases blood flow as smooth muscle relaxes, lumen gets wider and BP drops
What are continous capillaries not associated with the brain rich in and why?
Rich in transport vesicles, contributing to endocytosis/exocytosis
What do continuous capillaries in the brain form?
Part of the blood brain barrier
Are there intercellular clefts in continuous capillaries in brain?
No, only tight junctions plus a thick basement membrane and astocyte extensions –> prevent movement of nearly all substances
What are fenestrated capillaries?
One that has pores (or fenestrations) in addition to tight junctions in the endothelial lining
What is purpose of pores in fenestrated capillaries?
These make the capillary permeable to larger molecules (permeability varies depending on location)
Where are fenestrated capillaries most common?
In the small intestine/kidneys
Why are fenestrated capillaries most common in small intestine/kidneys?
The primary site of nutrient absorption
Filter the blood
Where else are fenestrated capillaries found?
Endocrine organs- pituitary releasing thyroxine or GH
Hypothalamus, pituitary, pineal, and thyroid glands.
What is the least common type of capillary?
Sinusoid capillary
What are sinusoid capillaries?
Flattened with extensive intercellular gaps and incomplete basement membranes, in addition to intercellular clefts and fenestration
How many layers do veins have? Do they have elastic laminar?
3 layers (tunica intima, media, externa). Elastic laminar absent (or very thin). Thin muscular wall
What 3 mechanisms aid blood back to heart?
- Skeletal muscle contraction system which helps compress the veins
- Pressure of artery next to vein helps blood move in right direction
- Breathing in helps lower intrathoracic P and draw blood back into the heart
What are sinusoid capillaries for?
Large openings allow for the passage of the largest molecules (like plasma and proteins)
For example, when bone marrow forms new blood cells, the cells must enter the blood supply and can only do so through the large openings of a sinusoid capillary
Where are sinusoid capillaries found?
Liver, spleen, lymph nodes
Why is blood flow through sinusoid capillaries very slow?
Allowing more time for exchange of gases, nutrients, and wastes
What is frictionless flow?
Blood vessel doesn’t exert resistnace at all on blood –> doesn’t happen in blood vessels
What is laminar flow?
Faster flow in centre and all blood flowing in layers without mixing and touching each other –> most efficient way of blood flow in vessels
What is turbulent flow?
Mixing of all blood, different directions, noisy blood flow (can cause thrombosis)
What happens to blood flow when taking blood pressure?
Compres and cause turbulent flow –> noisy
What is Virchow’s Triad?
Describes the 3 broad categories that contribute to thrombosis:
- Endothelial injury
- Abnormal blood flow
- Hypercoagulability
How is blood flow measured?
Volume of blood / unit of time
How does blood flow and pressure relate?
Higher blood pressure –> lower flow
What is the pressure that the blood exerts on vessels themselves?
Hydrostatic pressure
What is systemic arterial blood pressure?
Refers to the pressure exerted on blood vessels in systemic circulation, and is often measured using arterial pressure (usual blood pressure)
Why is pressure in arteries higher than in veins?
- Receiving blood from heart after contraction
- Due to their contractile capacity
How is blood pressure measured?
Blood pressure cuff wrapped around person’s upper arm –> measures pressure in brachial artery
Usually expressed in terms of systolic over diastolic pressure (140/90 mmHg)
What is systolic blood pressure?
The peak arterial (blood) pressure when heart is contracting
What is diastolic blood pressure?
The minimum arterial pressure between contractions, during relaxation and dilatation of the ventricles of the heart when the ventricles fill with blood
What is pulse pressure?
Difference between systolic and diastolic blood pressure
What is mean arterial pressure?
Average pressure in a patient’s arteries during one cardiac cycle
What is hypertension?
Blood pressure in arteries is persistently elevated
What is hypotension?
Blood pressure in the arteries is persistently low
What 5 factors affect blood flow?
- Cardiac output
- Compliance
- Volume of blood
- Viscosity of blood
- Blood vessel length and diameter
How does cardiac output affect blood flow?
The amount of blood the heart pumps through the circulatory system (determined by stroke volume and heart rate)
Increase in HR or SV will increase blood flow (adrenaline, noradrenaline, calcium levels)
What is stroke volume?
The volume of blood pumped from the left ventricle per beat
How does compliance affect blood flow?
Greater compliance of artery can expand more and accommodate blood flow
Vascular disease causes stiffening or calcification so resistance increases and compliance decreases causing turbulent blood flow
Are veins or arteries more compliant?
Veins are more compliant than arteries and expand to accommodate changing volume
Why are veins called capacitance vessels?
Over 50 % of the blood volume is in veins
What is Poiseuille’s law?
Describes non turbulent laminar flow
Flow is inversely proportional to viscosity –> thicker the more resistance
Flow is directly proportional to pressure of liquid and 4th power of radius
Length of vessel directly proportional to resistance (longer it is, lower the flow)
If you constrict/dilate arteriole by half of its radius, how much does resistance increase/decrease by?
16 times
- Vessel diameter
- Total cross-sectional area of vessels
- Average blood pressure
- Velocity of blood flow
Graphs
How are arterioles main component to resistance?
Given their small lumen, they dramatically slow the flow of blood from arteries and are site of greatest resistance in the entire vascular network
Vasodilation and vasoconstriction of arterioles play more significant roles in regulating blood pressure than do the vasodilation and vasoconstriction of other vessels.
Blood flow decreases dramatically as the blood moves from arteries to arterioles to capillaries. This slow flow rate allows more time for exchange processes to occur
Why are arterioles the greatest components of resistance despite their smallest size?
There are vastly more capillaries in the body than there are other types of blood vessels
What does left coronary artery branch into / supply?
Left branches into left anterior descending and left circumflex arteries
Supplies the LA, LV and septum
What does the right coronary artery supply?
RA, both ventricles, conducting system
How much does resting blood have to increase by when doing stressful exercise?
Tenfold
When do coronary arteries fill?
When the heart is relaxed (disatole), the backflow of blood fills these valve pockets and allows blood to enter coronary arteries
What are precapillary sphincters?
Segments of smooth muscle that help direct bloodflow into capillaries. Because there is a limited amount of blood in the body, these sphinters are important in directing blood to the tissues that need it the most, and reducing the flow to inactive tissues.
How do vasodilators/vasoconstrictors affect precapillary sphincters?
Vasodilators –> relax precapillary sphincters
Vasoconstrictors –> constrict precapillary sphincters
What are examples of vasodilators?
Decreased O2, increased CO2, metabolic acids (e.g. lactate), inflammation, increased body temp
What are examples of vasoconstrictors?
Prostaglandins, products released by activated platelets, leukocytes, endothelins
How does pulmonary circulation regulate blood flow?
Pulmonary circulation undergoes hypoxic vasoconstriction. While tissues and organs tend to increase blood flow by vasodilating in response to low oxygen supply, the blood vessels in the lungs actually vasoconstrict to decrease blood flow in response to low oxygen
What is the diameter of capillaries?
Between 5-10 um in diameter with an endothelial vessel wall of only one cell thick
What are capillaries surrounded by?
Basal lamina of connective tissue
What is vasculogenesis? What is angiogenesis?
Vasculogenesis –> during embryological developmennt, new blood vessels are made from scratch
Angiogenesis –> formation of new capillaries from pre-existing blood vessels
