factors affecting blood blow Flashcards
what does blood contain?
- erthrocytes (RBS)
- leukocytes (WBC)
- platelets
- plasma proteins
what is the name for the proportion of RBS to total volume of blood?
haematocrit
what does increased haematocrit indicate?
dehydration- risk of deep vein thrombosis
what does decreased haematocrit indicate?
anaemia
what happens to blood at the lungs?
- oxygenated
- CO2 removed
what does blow flow vary according to?
the metabolic demands of the organs
when does skeletal muscle have a greater demand for blood?
during exercise
when does the gastrointestinal system have a greater demand for blood?
following ingestion of food
why are there differences in blood flow within individual organs?
due to alterations in vascular resistance
what are the different categories for blood flow regulation?
- local (intrinsic)
- neural/hormonal (extrinsic)
describe local control of blood flow
Matches blood flow to metabolic requirement of tissue system
Direct action of metabolites on arteriolar resistance
describe neural/hormonal control of blood flow
Action of sympathetic nervous system on vascular smooth muscle
Action of vasoactive substances (histamine, bradykinin, prostaglandins)
what are specific needs of the tissues for blood flow (local control BF)?
- Delivery of oxygen to tissues and nutrients (glucose, amino acids, fatty acids)
- Removal of carbon dioxide and hydrogen ions
- Maintenance of ion concentrations in tissues
- Transport of hormones
what are the three mechanisms through which intrinsic control (local) of blood flow can occur?
- autoregulation
- active hyperemia
- reactive hyperemia
what is autoregulation (LCBF)?
- maintaining constant blood flow while arterial pressure changes
eg decrease in arterial pressure gives vasodilation to decrease resistance and maintain constant blood flow
what is active hyperemia (LCBF)?
- Blood flow to tissues is proportional to its metabolic activity.
- Increased blood flow when metabolic activity increases (exercising skeletal muscle increased O2 consumption / ATP demand)
- Increased arteriolar dilation.
what is reactive hyperemia (LCBF)?
- Increase in blood flow in response to a prior period of decreased blood flow.
e.g., following period of arterial occlusion, an O2 dept accumulates – longer the arterial occlusion, greater O2 debt, greater the increase in blood flow (above pre-occlusion levels) until the O2 dept reversed.
what are factors which determine resistance to blood flow?
- vessel diameter
- vessel length
- viscosity of blood
how does vessel diameter change?
with vasoconstriction and vasodilation- contraction and relaxation of vascular smooth muscle
what does poiseuille’s equation rely on?
- flow is through a uniform straight pipe
- flow is non-pulsatile
- flow is smooth (laminar)
what does poiseuille’s equation describe?
how flow is related to perfusion pressure, radius, length and viscosity
what do veins have that’s larger than arteries?
cross-section
why are venous valves orientated towards the heart?
to maintain flow in one direction
what aids the venous system?
the action of working skeletal muscles intermittently contracting to compress the veins
how is venous return aided during exercise?
as intrathoracic pressure becomes more negative
which activation via noradrenaline constricts veins and increases venous return?
sympathetic
what is preload?
venous return to the right ventricle
what happens if preload increases?
the heart will have to work harder to pump blood out- problem in angina and heart failure
what does coronary circulation do?
- Facilitates perfusion of myocardium- blood to heart
- Maintains high basal rate of O2 supply to cardiac muscle
what does skeletal muscle circulation do?
Adapted to meet metabolic demand of skeletal muscle during exercise
what does cerebral circulation do?
regulation to maintain cerebral perfusion (brain)
what supplies blood to the heart?
the left and right coronary arteries- which originate at the root of the aorta (behind the cusps of the aortic valve)
which coronary artery supplies which side of the heart?
right supplies right, left supplies left
what does the left coronary artery divide into?
- The left anterior descending artery branches off the left coronary artery and supplies blood to the front of the left side of the heart.
- The circumflex artery branches off the left coronary artery and encircles the heart muscle. This artery supplies blood to the outer side and back of the heart.
what is the function of the segments which branch off from the coronary arteries?
Coronary arteries branch into segments dividing into capillary networks that facilitate O2 diffusion into cardiac myocytes that have a high ATP requirement.
what happens to blood once it has passed through coronary capillaries?
it collects in venules that drain to form the epicardial veins which transport blood to the coronary sinus which empties blood into the right atrium
what are thesbian veins?
thesbian veins drain deoxygenated blood from capillary network in ventricular wall directly into the cardiac chambers
what is coronary blood flow in a human at rest?
70-80 ml/min/100g
when does perfusion of the myocardium from coronary arteries occur?
during early diastole
what does exercise training do?
increases the number of arterioles and capillaries
what is blood flow regulated by at rest?
its sympathetic innervation
what is blood flow to skeletal muscle regulated by during exercise?
local control
what are examples of local vasodilators in skeletal muscle?
lactate, adenosine and potassium ions (K+)
what does alpha-1-adronoreceptor activation cause?
vasoconstriction
what does beta-2-adrenoreceptor activation cause?
vasodilation
what does adrenaline cause?
vasodilation
what % of vascular resistance does cerebral arteries account for?
50%
what is the circle of willis?
the joining area of several arteries at the bottom (inferior) side of the brain- can help to preserve cerebral perfusion if carotid artery obstruction occurs
describe cerebral autoregulation
- Cerebral resistance vessels dilate to maintain perfusion when arterial Blood Pressure falls.
- Below arterial blood pressure of ~60mmHg, cerebral blood flow steeply declines.
- Severe hypotension can lead to mental confusion and syncope
what are cerebral resistance vessels highly sensitive to?
local hypoxia and arterial CO2
what does hypERcapnia cause?
cerebral vasodilation, mediated by endothelial NO
(also caused by local hypoxia)
what does hypOcapnia cause?
cerebral vasoconstriction. Hyperventilation can reduce cerebral perfusion (dizziness)
(also caused by systemic hypoxia)
