CVS S1 - Introduction to CVS + Anatomy of CVS + DR Work Flashcards
Where does exchange of substances between the blood and tissues occur?
Capillaries
What is the major mechanism of substance exchange between the blood and tissues?
Diffusion (98%)
Diffusion is affected by what 3 factors?
Area available for exchange
Diffusion resistance
Concentration gradient
How is area for exchange of substances through capillaries determined?
Determined by ‘Capillary density’ - Capillaries per unit volume
Where is capillary density highest?
Are all capillaries always perfused?
In tissues with the highest metabolic activity
Nope
What is meant by diffusion resistance and how is it determined?
The difficulty of movement through the barrier formed by the capillaries
Determined by:
The nature of the barrier and the nature of the molecules that are diffusing
The distance over which the diffusion occurs
i.e.’Path length’
What effect does concentration gradient between the blood and tissues have on diffusion?
Larger concentration gradient means a greater rate of diffusion, the opposite is also true
How is concentration gradient between blood and tissues determined?
Gradient depends on concentration of substances in blood
Which is in turn affected by the rate of flow, unless blood is supplied at an adequate rate the gradient will dissipate
How much blood must the cardiovascular system supply to the body as a whole?
between 5 and 25L of blood per minute
5L at rest
Up to 25L in strenuous conditions
What is the minimum and maximum rate of blood flow (per minute) to the following organs/systems?
Brain Heart Kidneys Gut Muscle Skin Other
Brain, 0.75L - 0.75L
Heart, 0.3L - 1.2L
Kidneys, 1.2L - 1.2L
Gut, 1.4L - 2.4L
Muscle, 1.0L - 16.0L
Skin, 0.2L - 2.5L
Other, 0.2L - 0.2L
What is meant by ‘Perfusion rate’?
Rate of blood flow
Apart from the brain, where else must the cardiovascular system maintain a contant perfusion of?
Heart Muscle
Kidneys
What are the 4 main functional components of the cardiovascular system?
A Pump (The heart)
Distribution vessels (Arteries)
Flow Control
Capacitance
What is meant by ‘Flow control’ in terms of the cardiovacsular system?
How is this control achieved?
Flow control is the control of cardiac output, so that it might be distributed appropriately
Flow is restricted in parts of the body that are easy to perfuse, driving blood to areas that are not so easy to perfuse
This is achieved with resistance vessels, namely arterioles and pre-capillary sphincters
What is meant by ‘Capacitance’ in terms of the cardiovascular system?
Capacitance is the ability to cope with changes in cardiac output
This is a store of blood that can be called on to cope with temporary imbalance in the amount of blood returning to the heart and the required cardiac output.
This store is the veins
How is the blood distributed between:
Arteries and arterioles
Capillaries
Heart and lungs
Veins
What is the total blood volume?
Arteries and arterioles - 0.55L (11%)
Capillaries - 0.25L (5%)
Heart and lungs - 0.85L (17%)
Veins - 3.35L (67%)
Total volume of ~5L
Where is the heart located in the body?
The middle mediastinum
What is the pericardium?
Describe its location and function
A fibroserous sac surrounding the heart and the roots of the great vessels
It is made up of 2 serious membranes, the visceral (lies against the heart) and the parietal layer
The space between is filled with serous fluid, allowing relatively frictionless motion of the heart
What relationship do the phrenic nerves have with the heart?
Phrenic nerves are responsible for innervation of the pericardium
What is pericarditis?
What is the major problem associated with pericarditis?
Inflammation of the pericardium
Commonly due to infection or post infarct
Accumulation of fluid in the pericardium (pericardial effusion) can lead to cardiac tamponade.
Cardiac tamponade is a condition in which there is so much fluid in the pericardium it restricts inflow of blood into the heart.
This can be rapidly fatal
What are the major vessels entering and leaving the heart?
Entering:
Inferior and superior vena cavae to the right atrium
Pulmonary enter the left atrium
Leaving:
Pulmonary artery from the right ventricle
Aorta from the left ventricle
Where do the coronary arteries originate from?
The right and left aortic sinus found branching from the ascending aorta
Describe briefly the course of the Right coronary artery and list the major branches.
Originates from right aortic sinus
Passes anteriorly between the right auricle and the pulmonary trunk
Passes through the coronary sulcus between the right ventricle and atrium until it reaches the base of the heart
Branches:
Right marginal branch
Posterior interventricular artery
Describe briefly the course of the Left coronary artery and list the major branches
Originates from the left aortic sinus
Passes between the pulmonary trunk and the left auricle before entering the coronary sulcus
While still posterior to the pulmonary trunk, it divides.
Divisions are:
Anterior interventricular branch - Descends towards apex
Circumflex branch - Courses towards the left through the coronary sulcus, gives off the left marginal branch
Where is the coronary sinus found and what are its major tributaries?
Found inferior to the left atrium
Tributaries are the great, middle, small and posterior cardiac veins
Where is the great cardiac vein found?
Originates from the apex
Travels up the anterior interventricular sulcus as the ‘anterior interventricular vein’
Upon reaching the coronary sulcus the great cardiac vein turns to the left and continues on in the coronary sulcus until it reaches the coronary sinus
What is the function of arteries?
Carry blood away from the heart to the capillary beds
Outline the progression in types of artery from the heart to the capillary beds
Heart
Large elastic arteries
Medium muscular arteries
Arterioles
Meta-arterioles
Capillaries
What is the basic structure of an artery or vein?
Tunica intima - Next to lumen
Tunica media
Tunica adventitia - Outer layer
Describe in detail the structure of Elastic arteries
Tunica intima:
Endothelial cells have long axis parallel with long axis of artery
Narrow sub endothelium of connective tissue and discontinuous elastic lamina
Tunica Media:
40-70 fenestrated elastic membranes (elastin) with smooth muscle cells and collagen between these lamellae
Thin external elastic lamina may be present
Tunica adventitia:
Layer of fibroelastic connective tissue Vasa vasorum (Lymphatic vessels and nerve fibres)
What is the function of elastic arteries?
Expand slightly during systole
Describe in detail the structure of muscular arteries
Tunica intima:
- Endothelium with thick internal elastic lamina
Tunica media:
- 40 layers of smooth muscle connected by gap junctions for coordinated contraction
- Prominent external elastic lamina with fenestrations
Tunica adventitia:
- Thin layer of fibroelastic connective tissue
- Contains vasa vasorum (not very prominent)
- Also contains lymphatic vessels and (unmyelinated) nerve fibres
What are the vasa vasorum?
Why might an artery contain vasa vasorum?
Smaller blood vessels contained within the tunica adventitia of an artery
Diffusion of nutrients through the endothelium is not sufficient to supply the tissues of the vessel
What is the function of the unmyelinated nerves in the tunica adventitia of a muscular artery?
Nerve endings release noradrenaline (sympathetic nerve fibres)
Diffuses through fenestration in the tunica media’s external elastic lamina and into the superficial tunica media
Creates a depolarisation of the smooth muscle cells which propagates to all muscle cells via gap junctions causing vasocontriction
As muscular arteries diminish in diameter, what happens to their structure?
Number of smooth muscle layers in the tunica media decreases
How do arterioles differ from muscular arteries in terms of size and their tunica media?
Arterioles are less than 0.1mm diameter, arteries are larger than 0.1mm diameter
Arterioles have 1-3 layers of smooth muscle cells in the tunica media
Would you ever find arterioles with thin internal elastic lamina?
Yes, but only those of larger diameter
What are the main features of small arterioles?
No internal or external elastic lamina
Tunica media is composed of a single smooth muscle cell encircling the endothelial cells
Tunica adventitia is scant
By what percentage can arterioles perform sustained dilation or contraction?
Why is this useful?
Give an example of this process
Most arterioles can dilate up to 60-100% resting diameter
Or can constrict up to 40%
This directs blood to where it might be needed
During strenuous physical activity, arterioles in the gut will constrict to allow greater blood flow to the skeletal muscles
What are meta-arterioles and how do they differ from arterioles?
Meta-arterioles supply blood to capillary beds
Their smooth muscle layer is not continuous
Instead, smooth muscle cells are spaced apart and each encircles the endothelium of a capillary arising from the meta-arteriole (pre-capillary sphincter)
What is the function of smooth muscle cells in meta-arterioles?
Function as pre-capillary sphincters
Allow the arterioles and meta-arterioles to serve as flow regulators for the capillary beds
They control whether a capillary bed is perfused or bypassed
Describe the structure of capillaries
Made up of a single layer of endothelium and a basement membrane
7-10um in diameter
How much of the total blood volume is contained within capillaries?
What is the total exchange surface area of all capillary beds in the body?
5%
Estimated 600m2
What features of capillary structure aid in exchange of substances through the capillary wall?
Narrow lumen is almost completely filled by an RBC, minimising the diffusion path length
As velocity is inversely proportional to cross sectional area, velocity is very low in capillaries compared to the rest of the vasculature, this allows more time for exchange
What are the three types of capillary?
Continuous
Fenestrated
Sinusoidal
Describe the structure of continuous capillaries
Endothelium with basement membrane
Pericytes form a branching network around the outer surface
What are pericytes?
What processes are they involved in?
Cells capable of dividing into smooth muscle cells or fibroblasts
Divide during angiogenesis, wound healing or tumour growth
How does the structure of fenestrated capillaries vary from continuous capillaries?
Pores (fenestrations) in the endothelium
Describe the structure of sinusoidal capillaries
Fenestrated endothelium with large holes
Discontinuous basal lamina (basement membrane)
Associated pericytes
Some do not have tight junctions and are called ‘discontinuous sinusoidal capillaries’
Generally larger in diameter than continuous or fenestrated capillaries
What does the unique structure od sinusoidal capillaries allow for?
Where are sinusoidal capillaries normally found and why?
Allows RBCs and WBCs to pass through the capillary wall
Normally found in the liver and spleen, where greater amount of movement of materials is necessary
List the progression of vascular structures that the blood passes through from the capillaries to the heart (in order)
Capillaries
Post-capillary venules
Venules
Veins
Large veins
Heart
How does the structure of veins differ from that of arteries?
Thinner wall with more connective tissue and less elastic fibres or smooth muscle
Lumen wider and irregular (than accompanying arteries)
Paired semilunar valves that prevent retrograde travel of blood
Describe the structure of post-capillary venules
Structure much like capillaries (endothelium and associated pericytes)
Generally more permeable than capillaries
10-30um diameter
No valves as in larger venules/veins
What do the structural features of a post-capillary venule allow for?
How is this relevant to inflammatory response?
Because pressure is lower (larger diameter) than in capillaries fluid tend to drain into them
Except in inflammatory response where fluid and leukocytes emigrate out (these venules are the preferred location for leukocyte emigration)
How will low blood pressure affect veins?
Veins may collapse if blood pressure isn’t maintained
What apart from blood pressure determines blood flow in veins?
Moving blood towards the heart:
- ‘Muscle pump’ action of the lower limb
- Pressure factors in the abdominal and thoracic cavities
Preventing retrograde flow:
- Valves
In what diameter range will muscle fibres begin appearing in venules?
50um - 1mm
How do the superficial veins of the legs differ from other small or medium sized arteries?
Why is this difference necessary?
Most small/medium arteries have a well defined tunica adventitia
In superficial veins of the leg, there is a well defined muscular wall
This may be to resist distension by gravity
Give some examples of large veins
Vena Cavae Pulmonary Portal Renal Internal jugular Iliac Azygous
How do large veins differ from small/medium veins?
Have well developed longitudinal smooth muscle in the tunica adventitia in addition to the circularly arranged smooth muscle in the tunica media
