21. Blood Vessels Flashcards
Structure of Blood Vessels
1) Tunica intima/interna – closest to lumen
- simple squamous epithelium (endothelium) + basement membrane + C.T.
2) Tunica media
- Smooth muscle
3) Tunica externa – outermost
- C.T
Vasoconstriction v. Vasodilation
Vasoconstriction – decrease diameter of the lumen and increase blood pressure
Vasodilation – increase diameter of the lumen and decrease blood pressure
What’s diffrent about the structure of a capiliiary compared to other blood vesseles?
Capiliaries do NOT have these 3 layers
consist only of endothelium and a basement membrane
Blood pressure
the force exerted by the blood against the wall of a blood vessel (units = mmHg)
Systolic pressure v. Diastolic pressure
Systolic pressure: maximum pressure against the arterial wall during ventricular contraction. ~120 mmHg
Diastolic pressure: minimum pressure against the arterial wall during ventricular relaxation. ~80 mmHg
3 things that make up the arterial system
Elastic arteries + muscular arteries + arterioles
Elastic Arteries
- include the aorta (its main branches) and the pulmonary trunk
- presence of elastica interna/externa
- act as pressure reservoir - expand and recoil to drive blood forward; felt as a pulse
- BP is highest in the aorta and large systemic arteries
Muscular Arteries
- medium-sized and branch to various organs
- thickest tunica media with more smooth muscle cells and fewer elastic fibers; can’t recoil
- considered to be distributing arteries because they deliver blood to specific body organs
Arterioles
- microscopic vessels with diameters ranging from 10 to 300 μm
- contains mostly smooth muscle with very few elastic fibers
- smallest arterioles regulate flow into capillary beds
- also known as resistance vessels
- major drop in BP (~40mmHg) occurs between arteries and arterioles
Capillaries
- Microscopic vessels with a diameter of only 8-10 μm
- Wall has a single layer of endothelial cells and a basement membrane – NO muscular layer!
- some capillaries contain pericytes along their walls that act to stabilize capillary wall and help control permeability
- have a large surface area in order to function in the exchange of materials between the blood and interstitial fluid via diffusion and active transport
- BP is about 35 mmHg at arteriole end and 17 mmHg at venule end
3 Structural Types of Capillaries
1) Continuous capillaries
2) Fenestrated capillaries
3) Sinusoids
Continuous capillaries
- Location: skin and muscles
- endothelial cells are continuous and have intercellular clefts (gaps)**
- held together by tight junctions
- allow limited passage of fluids and small solutes
Continuous since there’s no holes, pores or fenestration. (there are slight gaps between the cells, but its very very small since nothing can pass through). The brain is dif and there is zero gaps whatsoever.
• Most common (majority) of capiliares is continuous
Fenestrated capillaries
- Location: villi of s.i., kidneys, endocrine glands
- endothelial cells have fenestrations (pores)
- greater permeability to fluids and small solutes
Sinusoids
Capiliary type
• Location: liver, bone marrow, lymphoid tissue
• endothelial cells have large fenestrations
• large intercellular clefts
• large lumen; incomplete basement membrane
• allow large molecules (blood cells and proteins) to pass between blood and tissues
Type 3 is for really large things. Very wide, very leaky, large pores. Lymphoid needs to move WBC out of spleen and explore. Liver is making a lot of albumin (maintain osmotic pressure), and other things that are larger like clotting factors. Doesn’t even have a complete basement membrane.
Capillary Beds
- Organized into networks known as capillary beds
- terminal arteriole –> metarteriole continuous with a thoroughfare channel–> joins postcapillary venule
• about 10-100 “true” capillaries branch off the metarteriole
• precapillary sphincter alternately relaxes/contracts to regulate flow into capillary bed
> relaxed (open): blood flows into capillaries, exchange occur
> contracted (closed): blood flow through capillaries stops
venules
- receive blood from capillaries
* larger venules have several layers of smooth muscle
Veins
- venules join to form veins
- vein walls are thinner than artery walls and typically a • vein looks collapsed
- no elastic interna/externa
- tunica media has few layers of smooth muscle cells and few elastic fibers
- tunica externa consists of thick bundles of collagen fibers and elastic networks (often thicker than media)
- veins are blood “reservoirs” as can hold ~60% of the blood volume
- pressure gradient in veins is from about 15 mmHg to about 0 mmHg at the right atrium
- must rely on factors other than pressure alone to help return blood back to heart (venous return)
on factors other than pressure alone to help return blood back to heart (venous return)
- gravity: only helps you above the heart level, it will hinder below the heart
- presence of valves in veins (folds of tunica intima)
- contraction of skeletal muscles =milking the veins. (You can’t stand still for more than 20min because your not doing this skel contraction so the blood starts pooling in your legs and you’ll eventually pass out, you start swaying, moving your feet to try to stimulate this contracting.)
- respiratory pump: Inspire (breathing in) you decrease pressure in chest and increase pressure in abdomen, this pushes the blood in the direction of the heart.
- sympathetic vasoconstriction: All vessels themselves also have smooth muscle (even if its just a bit) which can be told to sqeeze, and contract in order to push the blood towards the heart.
Driving force of blood circulation is
- blood pressure
* ↑ Pressure Difference, ↑ Blood Flow
Blood Flow (formula)
- is the volume of blood flowing through a vessel = cardiac output (CO)
- F = CO
- ↑ Pressure Difference, ↑ Blood Flow
cardiac Output
- Refers to the volume of blood pumped per minute by each ventricle of the heart
- CO = stroke volume (SV) x Heart rate (HR)
- eg. average adult cardiac output: CO = 70 ml/beat x 75 beats/min
End Diastolic Volume (EDV)
total volume of blood in ventricle at end of ventricular relaxation phase (diastole)
End Systolic Volume (ESV)
total volume of blood in ventricle after the ventricle has contracted and ejected about 60% of the blood from its chambers (40% of the blood usually remains in the ventricle)
Stroke Volume
- volume of blood pumped by each ventricle during each heart beat
- Stroke volume = EDV - ESV
End Diastolic Volume (EDV)
End Systolic Volume (ESV)
