What are the units of blood pressure?
mm Hg (mm of mercury) Kilopascals (kPa)
Describe series
2 paths in a row. Flow in both paths is the same.
In series which path has the higher pressure?
The first path has higher pressure than the second since energy is lost as blood experiences friction and resistance
Describe parallel
Branching paths where flow is split between 2 paths.
In parallel which path has the higher pressure?
Both paths have equal pressure provided the resistance is the same in both paths
Define portal system
2 Capillary beds lie in series
Capillary bed reassembles to a blood vessel that then splits again into another capillary bed
What is the function of a portal system
Allows transport of chemicals from one tissue to another without being diluted by mixing with blood at the heart
What is the advantage of a portal system?
High concentration of nutrients from 1st capillary bed is delivered to the 2nd capillary bed
List the 3 ways blood pressure is controlled
Blood volume
Cardiac output
Peripheral resistance
Give the formula for mean arterial pressure
P mean = P diastolic + (P systolic - P diastolic)
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3
What is mean arterial pressure?
Diastolic pressure plus a third of pulse pressure
What is pulse pressure?
The difference between diastolic and systolic pressure
Define stroke volume
Volume of blood pumped out if a ventricle during one beat of the heart
What is the heart rate reciprocal?
RR interval
What is heart rate measured in?
Beats per minute
Define cardiac output
The volume of blood pumped out of a ventricle in one minute
Give the formula for cardiac output
CO = SV X HR
Give the stroke volume of an average sized male
70 ml
What is the human heart rate of an average sized adult?
70 bpm
Give the figure for the RR interval
0.86 seconds
Give a value for cardiac output of an average sized adult
4.9 L/min
What is the RR interval?
The interval between successive R peaks on an ECG in the QRS complex
What percentage of total blood volume does atrial systole add to fill the ventricles?
20-25%
Define end diastolic volume
Volume of blood in the ventricle at the end of diastole
What is end diastolic volume associated with?
Pre load
Define end systolic volume
The volume of blood remaining in a ventricle at the end of diastole
Define ejection fraction
The percentage of ventricular volume pumped out during a heart beat
Give the value for average end diastolic volume
120ml
Give the value for average systolic volume
50ml
Give the average ejection fraction value
58%
What does an ejection fraction of 40% indicate?
The patient has heart failure
What does vasoconstriction cause?
Smaller radius
Higher resistance
Lower flow rate
Give the formula for conductance
conductance (g) = 1
—————-
Resistance
Name 4 causes of vasoconstriction and vasodilatation
Central regulation
Local regulation of pressure
Immune
Haemostasis
Give the name for the opposite of haemostasis
Haemorrhage
Define haemostasis
The stopping of bleeding
Define splanchnic circulation
The circulation of the gastrointestinal tract originating at the coeliac trunk, the inferior mesenteric artery and the superior mesenteric artery. In the splanchnic circulation, venous blood coming from the intestines rebranches into the hepatic portal system (going to the liver) rather than going directly back to the heart.
What happens to blood pressure and heart rate during exercise?
Systolic BP increases
Diastolic BP decreases
Heart rate increases
What occurs to the blood vessels during exercise?
Peripheral vasodilation at the muscles and skin
Vasoconstriction of the splanchnic circulation
What happens to blood pressure and heart rate upon standing?
Systolic BP not changed
Diastolic BP increases
Heart rate increases
What happens generally upon standing?
Blood pressure drops initially since blood pools in the legs. Then compensatory response occurs to increase blood pressure to normal. Peripheral vasoconstriction occurs in both arteries and veins and heart rate increases
List the 3 ways systemic blood pressure is controlled
Local- endothelial and nitric oxide
Neurological- The autonomic nervous system
Humoral- renal/pituitary/adrenal
Define shear force
The force on the endothelium in the direction of the flow of blood
Define laminar flow
Flow without turbulence or eddies, where the flow of all molecules is directly in the direction of the overall flow of all fluid.
How is local vasomotor controlled?
Release of vasodilator compounds such as nitric oxide by epithelial cells
Stimulation of the sympathetic nervous system does what?
Constricts blood vessels leading to an increase in peripheral resistance, blood pressure, heart rate and cardiac output
State the function of baroreceptors
To detect pressure changes and feedback to the brain
Where are baroreceptors located?
In the transverse aortic arch and carotid sinuses of left and right internal carotid arteries
What do chemoreceptors do?
Detect when oxygen levels are low and feedback to the brain
Where are the chemoreceptors located?
Aortic bodies
Carotid bodies
What is the frank-starling mechanism?
The stroke volume of the heart increases in response to an increase in the volume of blood filling the heart
Define venous return
the volume of blood flowing back to the heart through the veins
Define preload
The initial stretching of cardiac myocytes prior to contraction
When does volume overload occur?
When preload becomes too large
Eg in hypertension
List the factors causing an increase in preload
- Increase in atrial contractility
- Increase in ventricular compliance
- Increase in central venous pressure caused by a decrease in venous compliance and an increase in thoracic venous blood pressure
- Decrease in heart rate
- Increase in aortic pressure
Define afterload
The resistance that the chambers of the heart must overcome in order to eject the blood from the heart
What does an increase in afterload cause?
Pressure overload
How can afterload be increased?
Increased by back pressure
Increased if exit valves fail to open completely
Describe how the thoracic pump works
During inspiration the intrathoracic pressure is negative whereas the abdominal pressure is positive. Therefore creates a pressure gradient and pulls blood towards the right atrium
Describe how the muscle pump of venous return works
Rhythmical contractions of limb movements occurs during normal locomotor activity. Blood is squeezed out of veins and valves maintain direction
Describe why pulmonary circulation is special
High capillary density Endocrine control of blood pressure (ACE) Acts as a filter Low vascular resistance Acts as a blood reservoir
When do the arterioles in the lungs constrict?
When o2 is low and H+ and CO2 is low
Poor ventilation causes what?
Poor perfusion
What is the advantage of arterioles in the lungs constricting when o2 is low?
To minimise the amount of blood that is poorly oxygenated
Give some interesting points about coronary circulation
Cardiac muscle has high demand
Most flow occurs during diastole
Obtains blood almost before aorta
What can chronic hypertension lead to?
Aneurysm or stroke MI Kidney failure Heart failure Cardiac hypertrophy
What is orthostatic hypotension?
Low BP on standing causing low venous return
Dizziness or syncope
Caused by drugs, old age or hypovolaemia
What is hypovolaemia?
Where a person doesn’t have enough fluids in system
Give the signs and symptoms of cardiogenic shock
Tachycardia Tachypnoea Low urine output Hypotension Confusion Syncope Acidosis