Flashcards in CVS 1 Deck (65):
Describe the structure of capillaries
Single layer of endothelial cells surrounded by a basal lamina.
How can small hydrophilic molecules, like glucose, pass out of capillaries into interstitial tissue space?
Through small aqueous pores in between the endothelial cells lining the capillary. Except in the brain where there is tighter junctions between endothelial cells (blood-brain barrier).
State factors that affect the rate of diffusion
1. Concentration gradient
2. Diffusion distance
3. Diffusion resistance - usually low
4. Surface area available for exchange
What affects the diffusion resistance?
1. Nature of molecule - lipophilic, hydrophilic, size
2. Nature of barrier - pore size, number of pores for hydrophilic molecules
3. Path length - depends on capillary density, path length is shorter in more active tissues
How does the rate of blood flow through the capillary bed affect the concentration gradient?
If it is too slow, the concentration gradient will dissipate. Therefore the blood flow must be matched to the rate of use by tissues.
What is the perfusion rate?
the rate of blood flow through the capillaries per unit mass of tissue, expressed in milliliters per minute per 100 g
Which organs receive a steady blood flow?
Brain and kidneys
What is the cardiac output at rest of a 70kg man?
During exercise the cardiac ouput can increase from 5l/min to ...
What are the 4 components of the cardiovascular system?
1. Pump - heart
2. Distribution system - vessels and blood
3. Exchange mechanism - capillaries
4. Flow control - arterioles and pre-capillary sphincters
Why is resistance an essential component of the cardiovascular system?
In order to regulate blood flow so that it doesn't only perfuse those areas which are easiest (e.g. not against gravity). By reducing the ease of blood flow in some areas you direct it to areas that are more difficult to perfuse.
What component of the cardiovascular system provides a temporary store of blood, which can be returned to the heart at a different rate?
The veins. They have thin, distensible walls and therefore easily distend and collapse acting as temporary reservoirs.
The distensibility of blood vessels located within the body; it is inversely proportional to elasticity. Therefore, the greater the amount of elastic tissue in a blood vessel, the greater the elasticity, and the smaller the compliance.
What determines the direction of blood flow around the circulatory system?
Flow travels from higher pressure to lower pressure.
Where is the largest proportion of blood found in the cardiovascular system?
Where is blood flow the fastest?
Where the total cross-section is the least
In which vessels is blood flow the slowest?
What are the major elastic arteries of the body?
Right and left pulmonary arteries
Left carotid artery
Left subclavian artery
Left and Right common iliac arteries
Where are the semi-lunar valves found?
Aortic and pulmonary valves
Where is a bi-cuspid valve found?
Mitral valve - left atrio-ventricular valve
Where is a tri-cuspid valve found?
Right atrio-ventricular valve
What essential function do elastic arteries have during diastole?
They act as pressure reservoirs - they store elastic energy during systole and release it (recoil) during diastole maintaining blood pressure.
Why are pulses only found in arteries and to as lesser extent arterioles?
Because these are elastic vessels which are stretching and recoiling during systole and diastole respectively.
What are the three major types of arteries?
1. Elastic conducting arteries (widest)
2. Muscular distributing arteries (intermediate diameter - most of the named arteries)
3. Arterioles - narrowest
What three layers make up the walls of arteries and veins?
Tunica adventitia - usually connective tissue
What are vasa vasorum? Where are they found?
They are 'vessels of the vessels' - small blood vessels that supply the walls of large vessels such as elastic arteries, muscular arteries and large veins.
What type of cell produces the elastin, collagen and extracellular matrix found in the tunica media?
smooth muscle cells
What is aortic dissection?
The separation of elastic llamelae by high pressure blood forcing its way in e.g. aortic dissection.
Why are individual's with Marfan's syndrome at high risk of aortic dissection?
Marfan's syndrome is a genetic syndrome. A defect in the fibrillin-1 gene (a glycoproteins which forms elastic fibres in connective tissue), weakens elastic fibres, such as those found in the tunica media of elastic arteries. This makes them more vulnerable to damage by the high pressure blood flow in these vessels.
Where do aortic dissections occur? How are they treated?
They can occur anywhere along the aorta. They are treated by insertion of a stent.
What is the main feature of elastic arteries?
40-70 fenestrated elastic membranes
What is the main feature of muscular arteries?
40 layers of smooth muscle
Describe the mechanism of vasoconstriction of muscular arteries
1. Sympathetic nerve fibres located in the tunica adventitia release NA
2. NA diffuses through fenestrations in the external elastic lamina into the external tunica media
3. NA stimulates the depolarisation of some smooth muscle cells in the external tunica media
4. Depolarisation is spread to all cells in the tunica media via the gap junctions
How can you distinguish whether a vessel is vasoconstricted histologically?
Protruding endothelial cells into the lumen.
What is an end artery?
A terminal artery supply most or all of the blood to a body part without significant collateral circulation.
Give some examples of end arteries:
What are the best examples of absolute end arteries?
Central artery to the retina and the labyrynthine artery of the internal ear.
What is meant by 'bridging' of a coronary artery?
Compression of a segment of the coronary artery during systole. This is reversed during diastole.
When are arteries considered to be arterioles?
When they have a diameter of less than 0.1mm.
Describe the structure of arterioles
1. 1-3 layer of smooth muscle in tunica media
2. IEL present in larger arterioles only
3. EEL is absent
4. Tunica adventitia is scant
What are metarterioles?
Arteries that supply blood to capillary beds.
How do metarterioles differ form arteroles?
Their smooth muscle component is discontinous. The individual smooth muscle cells are spaced apart and fully encircle the endothelium of the vessel forming pre-capillary shincters.
What is the function of precapillary sphincters?
They allow the arterioles and metarterioles to regulate the flow through the capillary bed.
Where do lymph vessel ultimately return excess extracellular fluid to?
They return the fluid to the blood at the junction between the internal jugular and subclavian veins (via the thoracic duct).
How are arterioles capable of altering the resistance to blood flow so much?
They are capable of 60-100% of their resting diameter and can maintain up to a 40% constriction for a long time.
Describe the structure of a capillary
Single endothelium layer and their basement membrane..
What are the three types of capillary?
3. Sinusoidal or discontinuous
Describe the structure of continuous capilaries
These are the most common type of capillary. They have a continuous layer of endothelium, with tight or occluding junctions between cells.
Describe the structure of fenestrated capillaries
These have 'windows' or interruptions across parts of the endothelium, bridged by a thin diaphragm (apart from in the renal glomerulus. There are not gaps in the basement membrane.
Describe the structure of sinusoidal or discontinuous capillaries
These have larger diameter and slower blood flow. They have gaps in the walls which allow whole cells to diffuse out. They have an incomplete basement membrane as well as endothelium, unlike fenestrated capillaries.
Where are continuous capillaries found?
These are the most common type of capillary. They are found in nervous, muscle and connective tissue, exocrine glands and the lungs.
Where are fenestrated capillaries found?
In parts of the gut, endocrine glands and renal glomerulus.
Where are sinuosoidal or discontinuous capillaries found?
In the liver, spleen and bone marrow.
Pericytes are cells that form a branching network on the outer surface of the endothelium of capillaries. What is their role?
They are capable of differentiating into muscle or fibroblast cells during angiogenesis, tumour growth and wound healing.
What are the four possible modes of transport across the endothelium of a fenestrated capillary?
1. Direct diffusion
2. Diffusion through the intercellular cleft
3. Diffusion through the fenestration
4. Transmembrane transport - endo- and exo-cytosis
Describe the structure of post-capillary venules
Like capillaries - endothelium and associated pericytes
Describe the functions of post-capillary venules
Even more permeable than capillaries and due to their lower pressure fluid tend to drain into them (unless an inflammatory response is occurring, when fluid and leukocytes emigrate out).
Describe the structure of venules
The endothelium is associated with pericytes or thin smooth muscle cells to form a very thin wall.
What forms the valves of venules and what is their function?
They are formed from thin intimal extensions of the vessel wall and prevent retrograde movement of blood.
Describe the structure of small and medium sized veins and large veins
S/M: thin TI and TM and well developed adventita
L: thicker TI than above and well developed adventitia
How are the superficial veins of the leg different in structure from other veins?
Unlike other veins they have a well-developed muscular tunica media, possibly to resist distension caused by gravity.
How is venous blood helped in its return back to the heart?
Valves act to prevent retrograde movement of blood and muscle contractions propel the blood towards the heart.
What are venae comitantes? Function?
They are deep paired veins that, in some anatomical locations, run either side of small arteries. The three vessels are wrapped together in one sheath and the arterial pulses compress the veins against the vascular sheath and help push venous blood along. Examples are the brachial, ulnar and tibial venae comitantes.
List some examples of large veins
vena cavae, pulmonary, portal, renal, internal jugular, iliac and azygous.