Properties of special circulations Flashcards
What are the three characteristics that special circulations have to consider?
- Special requirements met by the circulation
- Special structural or functional features of the circulation
- Specific problems relating to that circulation
What are examples of special circulations with unique requirements?
- Cerebral
- Pulmonary
- Skeletal muscle
- Renal
- Gastrointestinal
Where do the two coronary arteries originate from?
The two coronary arteries originate from the left side of the heart at the beginning (root) of the aorta, just after it exits the left ventricle
What is the role of the cardiac veins?
The cardiac veins carry blood with a poor level of oxygen, from the myocardium to the right atrium.
Where does most of the blood from the coronary veins return through?
The coronary sinus
What are the special requirements of coronary circulation?
- Needs a high basal supply of O2: 2 times that of the supply of O2 in the resting skeletal muscle
- Increase in O2 supply is proportional to an increased demand/cardiac work
What are the special structural features of coronary circulation?
- High capillary density
- Large surface area: needed for O2 transfer
- Together these reduced diffusion distance to myocytes: diffusion
- Time is proportional to distance required: so O2 transport is fast
What does the higher numbers of fibers and capillaries in the cardiac muscles give rise to?
Shorter diffusion distances
Fibre diameter in the skeletal muscles
50 um
Capillaries in skeletal muscle
400 /mm^2
Fibre diameter in the cardiac
18 um
Capillaries in the cardiac muscle
3000 /mm2
Describe normal activity in the coronary circulation:
- High blood flow: 10x the flow per weight of the rest of the body
- Relatively sparse sympathetic innervation
- High nitric oxide concentration released during vasodilation
- High O2 extraction (75%), whereas the average in the rest of the body is 25%
Describe what occurs during increased demand in the coronary circulation
- Coronary blood flow increases in proportion to demands
- Production of vasodilators (adenosine, K+, acidosis) out-compete relatively low sympathetic vasoconstriction
- Circulating adrenaline dilates coronary vessels due to abundance of B2-adrenoreceptors
What are the effects of the curve shifting left (for the curve plotted for Oxyhaemoglobin against PO2)?
- Decreased temperature
- Decreased 2-3 DPG
- Decreased [H+]
- CO
What are the effects of the curve shifting right (for the curve plotted for Oxyhaemoglobin against PO2)?
- Reduced affinity
- Increased temperature
- Increased 2-3 DPG
- Increased [H+]
Describe the blood in the coronary sinus when it is returning to the right atrium from the myocardial tissue:
The blood has a greater carbon dioxide content due to high capillary density, surface area and small diffusion difference
Describe the effect of the Bohr shift
- blood in the coronary sinus has high CO2 content
- High CO2 and low pH means that the haemoglobin has less affinity for oxygen
- This means that more O2 is given up to the myocardial tissues
What is unique about the myocardium?
It is able to extract 75% of oxygen as opposed to typically 25% in other tissues
What is metabolic hyperaemia?
Increase in organ blood flow due to metabolic activity
How does metabolic hyperaemia occur?
Myocardium metabolism generates metabolites to produce vasodilation, which then increases blood flow
What is the need for metabolic hyperaemia (increasing blood flow)?
- O2 extraction is near maximum during normal activity
- To provide more O2 during demand, must increase the blood flow
Example of metabolic hyperaemia
Adenosine is produced by ATP metabolism and is released from cardiac myocytes
What are the effects of metabolic hyperaemia?
Increased pCO2, H+ and K+ levels
Why is it important that the coronary arteries function correctly?
Coronary arteries are functional-end arteries and therefore decreased perfusion produces major problems, such as Ischaemic heart disease
What is the heart susceptible to?
Sudden and slow obstruction
Sudden Ischaemic heart disease
Acute thrombosis, produce myocardial infarction
Slow Ischaemic heart disease
Atheroma (sub-endothelium lipid plaques) chronic narrowing of lumen, produces angina
Role of systole in coronary blood flow
Systole obstructs coronary blood flow
What are human coronary arteries?
Functional end-arteries
What are arterio-arterial anastomoses?
Low numbers of cross-branching collateral vessels
What does the atheromatous plaque do?
Obstructs flow
What does the zone of necrosis do?
INFARCTION: obstruction of the blood supply to an organ or region of tissue, typically by a thrombus or embolus, causing local death of the tissue.
What is an example of thrombosis?
Total occlusion of left anterior descending coronary artery
What does occlusion lead to?
Occlusion leads to obstruction of blood flow to the anterior (front) ventricle: leads to myocardial infarction
Possible physiological symptoms of Thrombosis
- Ischaemic tissue, acidosis, pain stimulation (of C-fibres)
- Impaired contractility
- Sympathetic activation
- Arrythmias
- Cell death (necrosis)
What is Stenosis?
Partial occlusion due to plaque formation
What is an angiography?
X-ray after opaque dye has been injected into the vessels
When does angina become more of a problem?
During increased activity
What happens in the normal heart at rest and during exercise?
AT REST:
- Resistances in series: add together
- In the large coronary arteries resistance is low but in the arterioles resistance is high
DURING EXERCISE:
- Metabolic vasodilation of the arterioles reduces the total resistance
- Therefore there is increased blood flow to meet the increased O2 demands
What happens at rest and during exercise for a person with Angina?
AT REST:
- Stenosis in the large coronary artery increases resistance
- Metabolic hyperaemia occurs at rest, so blood flow meets needs
DURING EXERCISE:
- The arterioles further dilate to reduce resistance, but total resistance is still too high due to dominance of stenosis
- O2 demand cannot be met and so therefore angina develops
What are three mechanical factors that reduce coronary flow?
- Shortening diastole, e.g. high heart rate
- Increased ventricular end-diastolic pressure, e.g. Heart failure (aortic stenosis, stiffening of the ventricle)
- Reduced diastolic arterial pressure, e.g. hypotension, aortic regurgitation
When is coronary blood flow restricted?
During systole: pressure is greater in he ventricles than in the aorta so there is no coronary perfusion
What is an example of cutaneous circulation?
A skin ulcer due to impaired microvascular flow
What are four special properties of cutaneous circulation?
- Defence against the environment
- Lewis triple response to trauma (increased blood flow)
- Temperature regulation
- Skin temperature is dependent
How is temperature regulated in curtaneous circulation?
- Blood flow delivers heat from body core
- Radiation (proportional to skin temperature)
- Conduction to skin: convection from skin (skin temperature)
- Sweating (latent heat of evaporation)
- The skin is an organ
- Skin temperature can range from 0C to 40C briefly without damage (poikilothermic rather than homeothermic)
What does skin temperature depend on?
- Skin blood flow
- Ambient temperature
What are three special structural features of cutaneous circulation?
- Arterio-venous anastosomoses
- Sympathetic vasoconstrictor fibres
- Sudomotor vasodilator fibres
Describe arterio-venous anastosomes (AVAs)
They are direct connections of arterioles and venules, which expose blood to regions of high surface areas.
- Convection, conduction, radiation, evaporation
Role of sympathetic vasoconstrictor fibres in cutaneous circulation
They release noradrenaline acting on a1 receptors
Role of sudomotor vasodilator fibres in cutaneous circulation
- Acetylcholine acting on endothelium to produce nitric oxide
- Driven by temperature regulation nerves in the hypothalamus
How does the cutaneous circulation help heat loss?
Increasing ambient temperature causes vaso- and venodilation
How does the cutaneous circulation help to conserve heat?
- Decreasing ambient temperature causes vaso and venoconstriction
- Severe cold causes ‘paradoxical cold vasodilation’
- Core temperature receptors in hypothalamus control sympathetic activity to skin and hence skin blood flow
What are the effects of cold-induced vasodilation?
CONSERVES HEAT:
- Sympathetic nerves react to local cold by releasing noradrenaline
- Noradrenaline binds to a2 receptors on vascular smooth muscle in the skin
- a2 receptors bind noradrenaline at lower temperatures than a1 receptors
What are the effects of paradoxical cold vasodilation?
PROTECTS AGAINST SKIN DAMAGE:
- Caused by paralysis of sympathetic transmission
- Long-term exposure leads to oscillations of contract/relax
What is the effect of increased curtaneous perfusion with increased core temperature, e.g. exercise
Increased core temperature results in the stimulation of warmth receptors in the anterior hypothalamus
What does the stimulation of warmth receptors in the anterior hypothalamus cause?
SWEATING: Increased sympathetic activity to sweat glands mediated by acetylcholine
VASODILATION: Increased sympathetic sudomotor activity such that acetylcholine acts on the endothelium to produce NO which dilates arterioles in the extremities
Explain ADH-stimulated vasoconstriction of skin blood vessels
- Blood is directed to more important organs/tissues during loss of BP following haemorrhage, sepsis, acute cardiac failure
- Mediated by sympathetic vasoconstrictor fibres, adrenaline, vasopressin, argotensin II. Responsible for pale cold skin of patient in shock
- During haemorrhage warming up the body too quickly may reduce cutaneous vasoconstriction and be potentially dangerous, blood flow to the skin is not vital
in organs/tissues
Examples of functional specialisations of the cutaneous circulation
- Emotional communication, e.g. blushing
- Response to skin injury, such as The Lewis triple response
What is the Lewis triple response of skin to trauma?
- Local redness: site of trauma
- Local swelling: inflammatory oedema (wheal)
- Spreading flare: Vasodilation spreading out from the site of trauma
What is the role of the C-fibre axon reflex?
It mediates the flare to trauma
How are invading pathogens dealt with in the Lewis triple response?
Increased delivery of immune cells and antibodies to the site of damage
Response to trauma
Trauma -> C-fibre (nocieptive afferent) -> main axon -> Dorsal root ganglion
OR
Trauma -> C fibre -> Axon collateral -> Substance P -> Histamine (FLARE) -> Mast cell degranulation -> Dilation
Problems that can occur as a result of prolonged obstruction of flow by compression
Severe tissue necrosis
- Bed sores, on heels, buttocks, weight bearing areas
How can bed sores be avoided?
- Shifting position/turning causing
- Reactive hyperaemia (on removal of compression)
- High skin tolerance to Ischaemia
Problems that can occur as a result of postural hypotension / oedema due to gravity
- Often standing for long periods of time in hot weather will decrease central venous pressure (hypotension) and increased capillary permeability (oedema)
- Feeling fain and rings of fingers can be tighter as a result
What are the effects of increasing curtaneous perfusion with increased core temperature?
SWEATING: Increased sympathetic activity to sweat glands mediated by acetylcholine
VASODILATION: Increase sympathetic sudomotor activity such that acetylcholine act on endothelium to produce NO which dilates
