Session 1 - Histology of the CVS Flashcards
What factor has the largest effect on diffusion rate?
-Concentration gradient
What three main things effect the rate of diffusion?
- Area available for exchange
- Diffusion resistance
- Concentration gradient
What determines the area available for exchange in diffusion?
-Capillary density (capillaries/unit volume)
What is the relationship between area for exchange and diffusion rate?
-Proportional -> increase area increase diffusion
What is advantageous about the branching of arterioles into capillaries?
-One arteriole branches into several capillaries -> increasing area for exchange
What is meant by diffusion resistance?
-The difficulty of movement through a diffusion barrier
What determines diffusion resistance?
- Barrier nature
- Nature of diffusing molecule
- Diffusion pathlength
What is meant by the nature of the barrier when determining diffusion resistance?
-Whether the barrier is continuous or fenestrated
What is meant by the nature of the molecules when determining diffusion resistance?
- Size of molecule
- Chemical properties eg lipophilic/hydrophilic
Will lipophilic or hyrdophilic molecules have a smaller diffusion resistance?
-Lipophilic as they do not need pores to diffuse (eg O2/CO2)
What is meant by pathlength when determining diffusion resistance?
-The distance/thickness of the barrier
What is the relationship between pathlength and the rate of diffusion
-Rate is proportional to (pathlength)2
What is the main factor in determining the concentration gradient between the capillaries and the tissues?
-The rate of blood flow
Why will a decreased blood flow decrease the concentration gradient between the capillaries and the tissues?
-Lower concentration of substances in the blood as it is being used by the tissues and not being replaced
How does an adequate flow rate help maintain concentration gradients between capillaries and tissues?
-Limits the amount the substrate drops by in the capillaries as the flow keeps replacing the substrate
What is the cardiac output, in an average male, at rest?
-5.0L/min
What is the maximum cardiac output in an average person?
-25L/min
Describe the amount of blood flow to the brain?
-Remains constant at 0.75L/min
Can bloodflow to the brain alter?
-No it cannot tolerate any changes
What is the minimum and maximum blood flow to the heart?
-0.3->1.2L/min
Describe the amount of bloodflow to the kidneys?
-Needs to be constant at 1.2L/min
What is the minimum and maximum bloodflow to the gut?
-1.4->2.4L/min
What is the minimum and maximum blood flow to skeletal muscle?
-1->16L/min
What is the minimum and maximum bloodflow to the skin?
-0.2->2.5L/min
Why does the bloodflow to the kidney need to be kept at a constant high pressure?
-Needed to ensure efficient filtration
Why can the bloodflow vary to the gut?
-Increased flow needed after a meal
Why can the bloodflow alter so much in skeletal muscle?
-Needed during exercise to match supply to demand of O2
Why can the bloodflow to the skin vary so much?
-Thermoregulation
Describe the distribution of blood in the circulation at rest
- Arteries/arterioles->11%
- Capillaries -> 5%
- Heart and lungs -> 17%
- Veins -> 67%
What are the two types of arteries?
-Elastic and Muscular
Name some large elastic arteries
- Aorta and its trunks
- Pulmoary trunk which bifurcates into L and R pulmonary arteries
- L and R common iliac arteries
Describe the bloodflow in large elastic arteries near the heart
Hint: one word
-Pulsatile
Which arteries are the largest?
-Elastic
Which arteries have the widest lumen?
-Elastic
What determines bloodflow to organs?
-Resistance vessels (arterioles) and pre-capillary sphincters
Why does flow distribution need to be controlled?
-To restrict flow to the parts which are easily perfused and drive flow to those parts which are harder to perfuse
What is capacitance?
-Store of blood in the veins
Why is capacitance important?
-Provides the ability to cope with necessary changes in CO
Describe the lumen of veins
-Wide and irregular
What are the three layers of an artery wall?
- Tunica intima
- Tunica media
- Tunica adventitia
Describe the tunica intima of an elastic artery
- Thin
- Endothelium running parallel to long axis
- Subendothelial connective tissue
- Discontinous internal elastic lamina
Describe the tunica media of an elastic artery
- Thick
- Many smooth muscle fibres
- Collagen
- Many fenestrated elastic fibres
- Thin external elastic lamina
Describe the tunica adventitia of elastic arteries
-Connective tissue containing vaso vasorum and nerve fibres
What are vaso vasorum?
-Vessels of vessels
How does the tunica intima of muscular arteries differ from elastic arteries?
-Thicker internal elastic lamina
How does the tunica media of muscular arteries differ from that of elastic arteries?
- Prominent external elastic lamina with fewer elastic fibres within the media itself
- Smooth muscle connected by gap junctions to allow coordinated contraction
Why is the elastic lamina of tunica media fenestrated?
-To allow noradrenaline released from the nerve fibres in the adventitia to reach the smooth muscle
How does the tunica adventitia of muscular arteries differ from that of elastic?
-Less vasovasorum
What happens to the number of smooth muscle layers as diameter of arterioles decreases?
-Decreases
-What is the maximum diameter of an arteriole?
-0.1mm
How many layers of smooth muscle do arterioles have?
-1->3
Is there an internal elastic lamina present in all arterioles?
-No, only large ones
What happens to the tunica media in small arterioles?
-Consists of only 1 smooth muscle layer which encircles the endothelium
Is there an external elastic lamina in arterioles?
-No
What do arterioles branch into?
-Metarterioles
What do metarterioles branch into?
-Capillaries
What are the three types of capillary?
- Continuous
- Fenestrated
- Sinusoids
What are the supporting cells of capillaries?
-Pericytes
What is an end artery?
-An artery which supplies most or all of the blood to a body part with little collateral circulation
Why do end arteries propose a significant problem if occluded?
-They are the only blood supply to that body part and infarction will occur at that site
Provide an example of an end artery
- Coronary arteries
- Splenic artery
- Renal artery
Name 2 absolute end arteries
- Central artery to retina of eye
- Labyrynthine artery of the internal ear
How do metarterioles differ from arterioles?
-The smooth muscle layer is not continuous but confined to areas where there is branching
What is the name of smooth muscle cells of branching sites of metarterioles?
-Precapillary sphincters
What is the function of precapillary sphincters?
-Control blood flow into capillary beds
What is the function of lymphatic capillaries?
-Drain away excess extracellular fluid
Where does the excess extracellular fluid drained by lymphatic vessels go?
-Returned to the blood at the junctions of the internal jugular and subclavian veins
What ensures a short diffusion path in capillaries?
- One cell thick walls
- Narrow lumen-> RBCs fill almost entire lumen, minimising diffusion path to adjacent tissues
Where in the circulation is blood velocity at its lowest?
-Capillaries
Why is it beneficial for blood velocity to be low in capillaries?
-Allow adequate time for gas and nutrient exchange
What cell types are pericytes able to produce, and under what conditions would they be produced?
- Muscle cells for re-growth
- Fibroblasts during angiogenesis, tumour growth and wound healing
How are the endothelial cells of capillaries joined together?
-Through tight junctions
What are pericytes?
-Contractile cells that wrap around endothelial cells of capillaries and venules
Where are pericytes located within vessels?
-Within the basement membrane
How do pericytes communicate with endothelial cells?
-Direct contact and paracrine signalling
What is a fenestrated capillary?
-Capillary with pores present in the endothelium, which is bridged by a diaphragm
Where are fenestrated capillaries located?
- Gut
- Endocrine glands
- Renal glomerulus
How do sinusoids differ from fenestrated capillaries?
- Larger openings
- Slower bloodflow
- Discontinous basal lamina
What do the adaptions of sinusoid vessel walls allow which cannot happen in other capillaries?
-Passage of red and white blood cells
Where are sinusoids located?
-Bone marrow, lymph nodes and adrenal glands
What are discontinuous sinusoidal capillaries?
-They are sinusoids which do not have tight junctions between endothelia
Where are discontinuous sinusoids found?
-Liver and spleen (greater movement necessary)
Why does fluid drain into the post-capillary venules?
-Pressure is lower than that of capillaries and surrounding tissue
What is the maximum diameter of a venule?
-1mm
How do vein walls differ from artery?
- Thinner
- More connective tissue
- Fewer elastic of smooth muscle fibres
Describe the three layers of small veins
- Thin tunica intima
- Thin tunica media
- Well developed adeventitia
Which veins are the exception and have a well-defined tunica media?
-Superficial veins of the leg
What are venae comitantes?
-Deep paired veins, that in certain anatomical locations, accompany an artery, wrapped together in a sheath
What is the advantage of venae comitantes?
-Pulsatile artery promotes venous return within adjacent paired veins
Give an example of a venae comitantes
- Brachial
- Ulnar
- Tibial
