Cardiovascular system Flashcards
Systemic circulation
is connected to the heart “in parallel”, which means that there is a choice of which part of the body receive more or less of the available blood volume.
The systemic circulation
There is not enough blood to fully support the
metabolic needs of every tissue. Because of this,
blood supply to inactive tissues is reduced and
blood supply to active tissues is increased
Pulmonary circulation
is connected to
the heart “in series”, which means that the entire blood volume has to pass through it every time it circulates the body
The pulmonary circulation
It needs to accommodate the entire blood
volume every minute, even at rest.
During peak exercise it needs to accommodate
up to 5 times the entire blood volume (~25 litres)
every minute.
Properties of fluids and how flow is regulated
fluids are incompressible
Contractions of the ventricles of the heart generates pressure
which is transferred to the blood causing it to flow along the blood vessels.
DRIVING PRESSURE
(flow is created by the pressure difference between two points
Flow= pressure difference divided by resistance- Darcy’s law
resistance
changed by the radius
halving vessel radius increases resistance by a factor of 16 visa versa
Flow relation to pressure and vessel radius is directly proportional
This means that the higher the driving pressure and the larger the radius of a vessel,
the higher the flow is (and vice versa)
velocity
velocity= flow rate divided by cross-sectional area
The narrower the vessel, the faster the flow
velocity and total cross-sectional area relationship
Flow velocity of blood in the cardiovascular system depends on the
cumulative radius of all vessels at a similar distance from the heart.
In tissues, the large cross-sectional area and very low blood flow velocity
is very important for diffusion of gases and exchange of nutrients/waste
Lecture summary
- The organisation of the cardiovascular system and the implications of
blood flow in the pulmonary and systemic circulations. - Properties of blood flow in the cardiovascular system
Ø Darcy’s Law: What does flow depend on?
Ø Poiseuille’s Law: What changes resistance and how is flow affected? - Relationship between flow velocity and cross sectional area.
Ø Difference between the cross-sectional area of a single vessel and
total cross-sectional area of many vessels
External anatomy of the heart
The heart is surrounded by a fibrous sac called the pericardium
The pericardium protects the heart
by providing lubrication during its
constant movement.
It also helps anchor the heart against
the diaphragm and the spine.
cardiac muscle
9
The cardiac muscle
Cardiac muscle is similar to skeletal muscle
in terms of its structure, but it operates
similarly to smooth muscle (involuntarily).
Electrical and mechanical events of the heart cycle
title
The sinoatrial (SA) node
The SA node has an intrinsic rate of approximately 100 beats per minute and can initiate cardiac contraction in the absence of any external control (hormonal or nervous)
Pacemaker potential
SA node cells initiate an action potential due to an unstable
membrane potential that is continuously drifting towards threshold.
Rate of firing from the cells of the SA node
The rate of firing depends:
A. On the initial value of the membrane potential
B. On the slope of the drift towards threshold.
Sympathetic stimulation to the SA-
node leads to an increase in the slope
of the drift due to an increase of the
permeability of the Na +f current
Parasympathetic stimulation to the
SA-node leads to a decrease in the
slope of the drift due to an overall
decrease of the inward current, and to
hyperpolarisation of the membrane
due to increased K + permeability.
Conduction of stimulation through the cardiac muscle
Stimulation in the atria spreads from fibre to fibre through the gap junctions.
Ventricular stimulation spreads initially via a network of conduits which begins at the
atrioventricular (AV) node and terminates at cardiac muscle fibres via the Purkinje fibres.
electrical events
Stimulus spreads through the muscle fibres and stimulates both atria almost simultaneously.
* The AV-node and the bundle of His are the only pathway of the electric stimulus to travel from the
atria to the ventricles and the signal is delayed at the AV-node in order to allow the atria to empty
before ventricular contraction begins.
* The signal travels down the interventricular septum via the two bundle branches.
* The ventricles begin to contract as the stimulus spreads upwards depolarising muscle fibres via the
Purkinje fibres.
How and when valves open and close in the heart
Tricuspid valve
Separates R atrium and R ventricle
Open when R atrial pressure > R ventricular pressure
Closed when R atrial pressure < R ventricular pressure
Mitral (bicuspid) valve
Separates L atrium and L ventricle
Open when L atrial pressure > L ventricular pressure
Closed when L atrial pressure < L ventricular pressure
Pulmonary valve
Separates R ventricle and pulmonary artery (PA)
Open when R ventricular pressure > PA pressure
Closed when R ventricular pressure < PA pressure
Aortic valve
Separates L ventricle and aorta
Open when L ventricular pressure > Aortic pressure
Closed when L ventricular pressure < Aortic pressure
cardiac output
title
what is cardiac output
Cardiac output is the volume of blood that is pumped out by the heart every minute
what is the abbreviation of cardiac output
The correct abbreviation for cardiac output is Q, although it is often abbreviated as “CO”
units of cardiac output
The volume is measured in millilitres (ml) or litres (l), so the units are ml·min-1 of l·min-1
what does cardiac output depend on
It depends on: Heart Rate (beats per minute), and
Stroke Volume (volume of blood ejected per beat in ml or l)
how is cardiac output calculated
it is calculated from: Cardiac Output = Heart Rate x Stroke Volume
control of heart rate
Parasympathetic nerve endings (vagus
nerve) secrete the neurotransmitter
acetylcholine, which slows heart rate. Parasympathetic stimulation is concentrated to the atria, at the SA and AV nodes.
Sympathetic fibres (cardiac nerves) supply the SA and AV nodes, and increase heart rate by releasing norepinephrine.
Circulating epinephrine also
triggers an increase in heart rate but lags behind neural stimulation
control of heart rate
Parasympathetic stimulation of the SA node
cells cause their membranes to become more
hypepolarised, and the depolarising drift to
become slower. As a result the SA node rhythm
becomes slower and heart rate decreases
Sympathetic stimulation of the SA node cells
cause their membranes to become more
depolarised, and the depolarising drift to
become faster. As a result the SA node rhythm
becomes faster and heart rate increases.
It also:
* Shortens the AV node delay
* Shortens myocyte action potential
* Increases speed of relaxation
control of stroke volume
Stroke volume is regulated by two opposing factors:
* The force by which the muscle cells contract, and
* The arterial pressure against which they have to eject the blood
In turn the force of contraction is regulated by two processes:
* The length-tension properties of cardiac muscle cells, and
* The effects of hormonal influence on the contractility of cardiac muscle
starlings law of the heart
also called frank starling mechanism
RELATIONSHIP BETWEEN END-DIASTOLIC VOLUME AND STROKE VOLUME
length tension relaionship in cardiac muscle
End-diastolic volume can be increased by greater filling of the heart (venous return). This leads to
greater stroke volume because stretching cardiac muscle fibres causes them to contract more forcefully
Contractility of cardiac muscle
Sympathetic nerve activity
(norepinephrine) and circulating
epinephrine increase the force of
contraction, or contractility
Under conditions that increase contractility,
cardiac muscle will contract more forcefully for
a given amount of stretch
cardiac output summary
Cardiac output is the product of Heart Rate and Stroke Volume and it is a
representation of the workload of cardiac muscle.
* Control of Heart Rate
Ø Parasympathetic stimulation puts the “brakes” on the SA node and slows
down the heart.
Ø Sympathetic stimulation and stimulation by circulating epinephrine are the
heart’s “accelerator” and speed up the heart.
* Control of Stroke Volume
Ø Blood returning to the heart stretches cardiac muscle fibres. The more blood
returns to the heart, the more forceful the contraction.
Ø Sympathetic stimulation and stimulation by circulating epinephrine increase
cardiac muscle contractility.
blood pressure
title
Pressure across the cardiovascular system
Blood pressure is the driving force for flow in the cardiovascular system (pressure gradient between
arteries and veins) and it must be maintained at all times.
Blood pressure must be regulated so that it is high enough to create flow, but not too high to put excessive
stress on the cardiovascular system
Blood pressure in the arteries is pulsatile, reflecting the pressure oscillations seen in the heart during the
cardiac cycle, and it is continuous in the veins, reflecting the loss of forward energy mainly due to friction
mean arterial pressure
Mean arterial pressure (MAP) is calculated from the values of systolic and
diastolic blood pressure, but it is not an average of the two.
Diastole lasts almost twice as long as
systole does and MAP is closer to the
diastolic blood pressure value. MAP is
calculated from the following formula:
MAP= diastolic+ (systolic- diastolic/3)
blood pressure, “normal” values
So-called normal blood pressure is 120 mmHg for systolic and 80 mmHg for
diastolic blood pressure, and ~93 mmHg for mean arterial pressure.
These numbers would commonly be presented as: 120/80
120/80 would be accurate enough if we were all young adults of average build. Obviously, blood
pressure shows a lot of variability between groups, but also within groups depending on conditions.
factor affecting blood pressure
arteries are stiffer with age
pressure is higher in the foot and low leg due to gravity
women have a lower blood pressure compared to men
Effects of static and dynamic exercise on blood pressure
In dynamic exercise (walking, running, cycling, etc.),
mean blood pressure remains relatively steady
Conversely, in static exercise (lifting weights) mean
blood pressure rises dramatically
What physiological factors DETERMINE arterial blood pressure?
Mean arterial blood pressure = Cardiac Output x Total Peripheral Resistance
Heart Rate x Stroke Volume
Sum of resistance to blood flow
in all tissues of the body.
MAP = Q x TPR
