1
David Emerine
Fetal blood flow/Cardiac
Supplemental Instruction Nov 17 & 20
Fetal Respiration
In addition to being an endocrine organ, the placenta is also a respiratory organ.
During fetal development, oxygen is obtained from the placenta.
The umbilical vein carries the oxygenated blood from the placenta to the
fetal venous circulation (it connects to inferior vena cava).
The oxygenated blood enters the right atrium heart (right atrium).
Fetal circulation is distinctly different than adult circulation.
Fetal and maternal blood do not mix.
The circulation is shunted so that most blood bypasses the lungs.
The blood coming from the placenta is already oxygenated when it enters the
right atrium.
There is no air in the fetal lungs so alveoli and associated capillaries are
collapsed.
Additionally, there is very little surfactant produced by they Type II
alveolar cells during most of embryonic development.
The collapsed alveoli creates a large amounts of resistance to blood flow to the
lungs and greatly reduces the amount of blood travelling through the lungs and
returning to left atrium.
Two structures help shunt blood away from the lungs.
1)Foramen ovale is an opening with a flap-like valve between right and
left atria. As atria contract, blood is shunted from right to left
atrium. Valve opens easily because there is little pressure in the
left atrium (very little blood coming from lungs).
However, some blood still enters right ventricle and sufficient blood must go to the lungs to support their growth and development.
Foramen Ovale
Forms a one-way
valve
2) Ductusarteriosus connects pulmonary artery to aorta.
Blood entering the right ventricle is pumped into pulmonary artery
and then most is shunted into the aorta through the ductus
arteriosus and thus bypasses the pulmonary circulation
DuctusArteriosus
Shunts blood from
Pulmonary to
Aorta
At birth, breathing inflates the alveoli which greatly reduces resistance to blood
flow in the lungs
This increases blood flow through the lungs and back to the left atrium
Increase blood pressure in the left atrium closes the flap over the foramen ovale,
So blood flow stops between the two atria and the flap eventually grows
shut.
In many people, the formenovale does not completely shut after birth
(“patentformenovale”, “hole in the heart” atrial septal defect)..
Additionally, the ductusarteriosus between the pulmonary artery and the aorta
constricts as a reflex associated with birth, so blood no longer travel from
pulmonary artery to the aorta.
Before
Birth
Circulation
After
Birth
Cardiac Muscle
Cardiac Cycle ----*the following is extra information on the heart*
The period from the beginning of one heart beat to the beginning of the next.
a) Diastole
It is the relaxation phase of the cardiac cycle.
The heart is relaxed and filling with blood.
The blood pressure in arteries during this period referred to as "diastolic".
Blood pressure is maintained by the elasticity of the major arteries.
Normally diastolic pressure is about 80 mm of Hg.
b) Systole
Contraction phase of the cardiac cycle,
"Systolic" blood pressure (normally reaches a maximum of 120 mm of Hg)
Electrical Activity of Heart Associated with the Cardiac Cycle
The heart has two basic types of electrically responsive tissues in heart
1) Contractile fibers that normally do not initiate their own action potentials.
2) Autorhythmic tissue that is responsible for the initiation and conductance of action
potentials that control heart's contraction.
Membranes spontaneously depolarize because Na+ channels leak.
There is an extensive network of this type of tissue throughout heart.
Different tissues in the heart have different rates of depolarization.
Examples: * the following quantitative information is not tested in this class*
SA node: 70 to 80 depolarizations per minute
AV node: 40 to 60
AV bundle: 15 to 40
Purkinje fibers: 15 to 40
Spread of Depolarization Through the Heart
It is carefully orchestrated to ensure efficient pumping of blood
a) Sinoatrial node (SA node)
It is an elliptical strip of tissue located in upper wall of right atrium.
It is responsible for "pacemaker" activity because it has the fastest rate of
all of the pacemaker tissues.
Its endogenous rate is about 70 to 80 depolarizations per minute.
SA node tissue is continuous with contractile tissue of the right
atrium.
So right atrium depolarizes immediately after SA depolarization.
Depolarization is rapidly conducted to left atrium via an interatrial
pathway so that both atria contract simultaneously.
But the depolarization does not immediately spread to ventricles.
Nonconductive connective tissue between atria and ventricles
blocks the direct transfer of action potential from atria to
ventricles.
Why is it advantageous not to have the depolarization spread
immediately to the ventricles?
The delay allows time for the efficient filling of the ventricles.
b) Atrioventricular node (AV node)
It is located at the base of right atrium and it connects with AV bundle.
It depolarizes and then conveys that depolarization to AV bundle.
AV node slows conductance of action potential
into ventricles (by about 0.1 second delay)
This is referred to as the “AV nodal delay”.
What causes the delay?
There is decreased number of gap junctions and small diameter
fibers in a portion of the AV node tissue.
c) AV bundle (i.e. Bundle of His)
It originates from the AV node.
It extends down and through the interventricular septum
It has two main branches.
AV bundle conveys depolarization down to the bottom of
ventricles to the Purkinje fibers.
d) Purkinje fibers
They are fibers that extend throughout the ventricular “myocardium”
(muscular portion of the heart wall)
They are fast conducting fibers.
They rapidly spread the depolarization from AV bundle through
ventricular myocardium.
Ventricular contraction
In general contraction occurs from the bottom up.
This is an efficient way to pump blood out of the heart.
Does depolarization continue to spread after the ventricles depolarize?
No, depolarization gets to the top of the ventricles and stops due to the
prolonged refractory period in cardiac muscle.
Control of Heart by Autonomic Nervous system
Your basal heart rate is determined by the SA node,
However, the autonomic nervous system can increase and decrease your basal heart rate
Parasympathetic Nervous System
Stimulates heart via the vagus nerve (Cranial Nerve X)
***this will not be tested in this class Acetylcholine is released,
and it opens K+ channels in heart tissue***
This results in hyperpolarization that causes:
a) Decreased excitability of SA node which decreases heart rate
b) Decreased conduction speed of AV node.
Sympathetic stimulation of heart:
***this will not be tested in this class Norepinephrine is released from sympathetic neurons (or epinephrine from
adrenal glands)
This opens Na+ and Ca++ channels in heart tissue which causes***:
a)Increase excitability of SA and AV node, resulting in increased heart
rate and conduction speed of depolarization
b) Increased contraction strength of the heart.