Blood vessels (9)

Question 1

What are blood vessels?

Answer 1

> Carry blood in a closed system of vessels that begins and ends at the heart
3 types- arteries, veins and capillaries
Arteries= carry blood away from heart
Veins= carry blood to heart
Capillaries= contact tissue cells and directly serve cellular needs
When heart contracts, forces blood into arteries which branch into smaller arterioles; these feed into capillary beds; drains from capillaries into venules which branch into larger veins which circle back to heart

Question 2

What is the structure of arteries and veins?

Answer 2

3 layers :

1) Tunica intima (interna)
2) Tunica media
3) Tunica externa (adventitia)

Question 3

What is the lumen?

Answer 3

Central blood containing space surrounded by the tunics, the hole in the middle

Question 4

What is the structure of capillaries?

Answer 4

> Composed of thin tunica intima surrounded by basement membrane
Sometimes, only single endothelium cell forms the entire circumfrence of capillary wall

Question 5

What is the structure of the tunica intima (interna)?

Answer 5

> Internal layer that is in direct contact with the blood in the lumen
Has an endothelium layer that lines the lumen of all vessels
Forms a smooth, shiny surface which minimises friction as blood moves through the lumen
In vessels with a diameter >1mm, there is a connective tissue membrane which supports the endothelium

Question 6

What is the structure of the tunica media?

Answer 6

> Middle layer
Mostly smooth muscle cells, arranged in circles and sheets of elastin
Activity of smooth muscle is regulated by autonomic nervous system
Can cause vasodilation and vasoconstriction (controls diameter of blood vessel)
Regulates blood flow and blood pressure
Thickest layer and is thicker in arteries than veins

Question 7

What is the structure of the tunica externa (adventitia)?

Answer 7

> Outermost layer
Composed mainly of collagen fibres which protect and reinforce vessel and anchor it to surrounding structures
Has lots of nerve fibres, lymphatic vessels and in larger veins there is a network of elastic fibres

Question 8

Are all vessels the same size?

Answer 8

> Arteries, veins and capillaries vary in length, diameter, wall thickness and tissue makeup

Question 9

What is blood flow?

Answer 9

> The volume of blood flowing through a vessel, organ or the entire circulation in a given period of time
Measured in ml/min
The same as cardiac output when considering the entire circulation
In resting conditions, it is relatively constant
At any given moment, blood flow through individual body organs may vary massively according to immediate needs

Question 10

What is blood pressure?

Answer 10

> The force exerted on the wall of a blood vessel by the blood which it contains
Measured in mm Hg
Measured as systemic arterial blood pressure in the largest arteries near the heart
The differences in blood pressure within the vascular system provide the driving force which keeps blood moving from areas of high pressure to areas of lower pressure

Question 11

What is resistance?

Answer 11

> The opposition to flow, a measure of the amount of friction blood encounters as it passes through the vessels
Most friction is encountered in the peripheral (systemic) circulation, well away from the heart (generally use the term TPR- total peripheral resistance
3 sources of resistance- blood viscosity, vessel length and vessel diameter

Question 12

What is blood viscosity?

Answer 12

> Thickness/stickiness of fluid
Greater viscosity= more resistance as molecules slide past each other less easily and it is more difficult to keep the fluid moving
Blood is more viscous than water as it contains formed elements and plasma proteins
Viscosity is fairly constant in bloodstream- polycythaemia (excessive no. of red blood cells) can increase viscosity and resistance; anaemia (low no. of red blood cells) decreases viscosity and resistance

Question 13

How does blood vessel length and diameter effect resistance?

Answer 13

> Longer vessel= more resistance
As a child grows, blood vessels get longer and so peripheral resistance and blood pressure increases as they age
Smaller diameter= more resistance
Diameter changes frequently which causes peripheral resistance to change frequently also

Question 14

How does the heart causes changes in blood pressure?

Answer 14

> BP is pulsatile- rises and falls in regular fashion in the elastic arteries near the heart
As left ventricle contracts and umps blood into aorta, elastic aorta stretches and BP reaches its peak- this is systolic pressure and averages as 120mm Hg in healthy adult
During diastole, aortic valve closes to prevent backflow of blood into heart- pressure falls to lowest and this is diastolic pressure (resting), around 70-80 mm Hg in healthy adult

Question 15

What is pulse pressure?

Answer 15

> The difference between systolic and diastolic pressure
Felt as throbbing pulsation in an artery during systole and blood is forced into elastic arteries, causing them to expand
Because pressure in aorta fluctuates with each heartbeat, it is more important to consider MAP (mean arterial pressure)- this is the pressure that propels blood into the tissues
MAP= diastolic pressure + (pulse
pressure/3)
MAP and pulse pressure both decline as you move further away from the heart

Question 16

What is capillary blood pressure?

Answer 16

> By the time blood has reached capillaries, BP has dropped to approximately 35 mm Hg
Low BPs are desirable because high BPs would rupture the capillaries as they are fragile, most are extremely permeable and so would force the fluids out of the bloodstream and into the interstitial space

Question 17

What is venous blood pressure?

Answer 17

> Venous BP is steady and changes little during a cardiac cycle
The differences in BP between a vein and an artery become very clear if they are cut- arteries spurt red blood rhythmically and veins flow dark red blood sluggishly and evenly
BP is down to 0 mm Hg by the time it reaches the venae cavae

Question 18

What happens during venous return?

Answer 18

> Venous pressure is too low to ensure adequate venous return so there are 3 adaptations that veins have to ensure venous return:

1) Muscular pump
2) Respiratory pump
3) Sympathetic venoconstriction

Question 19

What is the muscular pump?

Answer 19

> Consists of skeletal muscle activity
As the skeletal muscles that surround the deep veins contract and relax, they squeeze blood towards the heart, and once blood passes a valve, it cannot flow back (skeletal muscle inactivity, such as standing for a long time, can cause fainting as it reduces venous return)

Question 20

What is the respiratory pump?

Answer 20

> Moves venous blood back up towards the heart as pressure changes in the ventral cavity during breathing
As we inhale, abdominal pressure increases, squeezing local veins and forcing blood towards heart
At same time, pressure in chest decreases, allowing thoracic veins to expand, speeding up the blood going towards the heart

Question 21

What is sympathetic venoconstriction?

Answer 21

> Reduces the volume of blood in the veins
As the layer of smooth muscle around the veins constricts under sympathetic control, venous volume is reduced and blood is pushed towards heart

Question 22

How is blood pressure regulated?

Answer 22

> Maintenance requires cooperation of the heart, blood vessels and kidneys and the supervision of the brain
Main factors that influence BP are cardiac output, peripheral resistance and blood volume
BP= CO X TPR

Question 23

What are baroreceptor reflexes?

Answer 23

> Works on negative feedback mechanism
If BP is too high, baroreceptors in carotid sinuses and aortic arch are stimulated; then impulses from baroreceptors stimulate cardioinhibitory centre and inhibit the casomotor centre; then a decrease in sympathetic impulses to the heart cause a decrease in HR, decrease in contractility and decrease in CO; decrease in rate of vasomotor impulses which causes vasodilation causing a decrease in TPR; the decrease in CO and TPR return BP to a normal range
If BP too low, baroreceptors in carotid sinuses and aortic arch inhibited; impulses from baroreceptors activate cardioacceleratory centre and stimulate vasomotor centre; then an increase in sympathetic impulses to the heart cause an increase in HR, increase in contractility and increase in CO; vasomotor fibres stimulate vasoconstriction causing an increase in TPR; the increase in CO and TPR cause BP to return to normal range

Question 24

What is a pulse?

Answer 24

> You can feel a pulse in any artery that lies close to the body surface by compressing the artery against firm tissue (such as bone or cartilage)
It is a pressure wave that is transmitted through the arterial tree as the arteries alternately expand and recoil
Radial pulse is easiest to access and so most commonly used
Pulse points are also pressure points because they can be compressed to stop blood flow into distal tissues during haemorrhage

Question 25

How is blood pressure measured?

Answer 25

1) Systemic BP is measured indirectly via auscultatory method
2) Sphygmomanometer cuff placed on arm above elbow, in line with brachial artery
> Pressure in cuff is increased until it is greater than systolic pressure in brachial artery
> Pressure slowly released until first sound is heard through stethoscope (systolic pressure)
> The pressure when the sound disappears is diastolic pressure

Question 26

How does blood pressure vary throughout the day?

Answer 26

> It is in a diurnal rhythm (24 hour cycle)
Peaks early in the morning due to sudden rise in corticosteroids released from adrenal cortex
Different factors can also affect BP

Question 27

What are the risk factors that influence blood pressure?

Answer 27

Factors that cant be controlled include; age, genetics, race and gender.

Factors that can be controlled include; obesity, diabetes, smoking, High LDL (low density lipoprotein) and high HDL (high density lipoprotein)

Question 28

What is hypotension?

Answer 28

> Low BP
Systolic lower than 100 mm Hg
Addisons disease- underproduction of hormones from adrenal cortex
Adrenal crisis- potentiall fatal condition linked to underproduction of glucocorticoids

Question 29

What is hypertension?

Answer 29

> High BP
Systolic more than 140 mm Hg
Transient elevations normal and can be caused by fever, physical exertion and emotional upset
Chronic elevation is major cause of heart failure, vascular disease, renal failure and stroke

Question 30

What is normal BP?

Answer 30

120/60- 140/90