Blood vessels (9) Flashcards
What are blood vessels?
> 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
What is the structure of arteries and veins?
3 layers :
1) Tunica intima (interna)
2) Tunica media
3) Tunica externa (adventitia)
What is the lumen?
Central blood containing space surrounded by the tunics, the hole in the middle
What is the structure of capillaries?
> Composed of thin tunica intima surrounded by basement membrane
Sometimes, only single endothelium cell forms the entire circumfrence of capillary wall
What is the structure of the tunica intima (interna)?
> 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
What is the structure of the tunica media?
> 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
What is the structure of the tunica externa (adventitia)?
> 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
Are all vessels the same size?
> Arteries, veins and capillaries vary in length, diameter, wall thickness and tissue makeup
What is blood flow?
> 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
What is blood pressure?
> 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
What is resistance?
> 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
What is blood viscosity?
> 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
How does blood vessel length and diameter effect resistance?
> 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
How does the heart causes changes in blood pressure?
> 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
What is pulse pressure?
> 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