What is Labor?
Labor is a process by which contractions of the uterine muscle bring about the birth of the baby. The uterus is a hollow muscle with a hole at one end, and during labor the contractions of the uterus cause the cervix to dialate. As the contractions of the uterus continue, the cervix effaces, lowering the head of the fetus into the birth canal. Once the cervix is fully dialated, the baby descends into the lowest portion of the pelvis. The continuously strengthening contractions cause the fetus to be expelled from the body allowing the birthing process to be complete.
Preparing for Action
Hormones called prostaglandins prepare the body for labor by softening the cervix and making the uterine muscle cells more sensitive to the hormone oxytocin. The onset of birth is an unknown signal from the baby that is detected by the mother. This signal is recieved by the pituitary gland in the brain and this signals direct the release of adrenocorticotropic hormone (ACTH). Once this hormone is released into the bloodstream, it travels directly to the adrenal glans uptop the kidneys. Cortisol, another hormone, is released as a result and this is thought to begin the labor process. The labor process is divided into three stages, the first being from the moment of the first contractions to full dialation of the cervix (~10cm); the second stage beginning at full dialation and ending with the delivery of the fetus; the third stage, the complete birth of the baby to the delivery of the placenta.
Muscle Contractions and Their Components
Muscle contractions occur by a sliding filament mechanism whereby the action of the actin filaments sliding over the myosin filaments creates the contraction of the muscle. They would be impossible if the myosin molecules didnt have a hinge along the shaft that allows the ratchet movement of these tiny myosin heads toward the center. Muscle contractions require a great deal of energy. Some of which is required to break the bond between the myosin heads and the actin active sites, as well as for the removal of calcium from the cytoplasm by the use of a pump within the sarcoplamic reticulum. When the myosin head is tilted foward after the power stroke, a binding site for ATP is exposed. The breakdown of ATP to ADP releases the head from the actin filament and cocks it for the next ratchet power stroke. The actual contaction of the muscle occurs in a three short steps.
1 - Excitation
a) The sarcolemma is depolarized and the action potential propagates
b) The action potential spreads inside along the T-tubules
c) The signal is transmitted from T-tubule to terminal sacs of sarcoplasmic reticulum
d) Calcium is released from sarcoplasmic reticulum into sarcoplasm
2 - Contraction
a) Calcium binds to troponin
b) Cooperative conformational changes take place in troponin-tropomyosin system
c) The inhibition of actin and myosin interaction is released
d) Crossbridges of myosin filaments are attached to actin filaments
e) Tension is exerted, and/or the muscle shortens by the sliding filament mechanism
The Animation of a Muscle Contraction
3 - Relaxation
a) Calcium is pumped into sarcoplasmic reticulum
b) Crossbridges are detached from the thin filaments
c) Troponin-tropomyosin regulated inhibition of actin and myosin interaction is restored
d) Active tension disappears and the rest length is restored
A Breaking Physics Update
The labor process is driven by two forces.
1. The primary force is generated involuntarily
by uterine muscle contractions. This force affects dialation and
is important in the delivery of the fetus from the uterus to the birth canal.
2. The secondary force is produced by voluntary contractions of the abdominal muscles to increase
intra-abdominal pressure and intra-uterine pressure. This force has no effect on dialation, but it is critical
to the delivery of the infant from the uterus and birth canal after the cervix is completely dialated.
The force of a muscle contraction is very powerful, on the order of about forty three pounds per square inch. The design of this force is such that if an unexpected shock is applied to the limb, that the bone is as likely to break as the tendon and muscle to tear. This force can be referred to as the isometric force, because there is no net movement of a limb, no weight lifted and no external work performed. The efficiency of the contraction is zero, but there is a great deal of energy consumed and released as heat to the skin by the flow of blood from the muscles. As the primary and secondary forces are generated, they continue to increase the contractions, thereby progressing the labor process.
Importance of Pressure
The volume of amniotic fluid controls the pressure inside the uterus. As the volume of amniotic fluid decreases, the pressure inside the uterus increases. While the pressure inside the uterus increases, the amount of force exerted on the fetus increases as well. This can be demonstrated by the following formula:
P(pressure) = F(force)/A(area)
(Pressure can be in the unit of a Pascal or Newton/meter². Standard pressure is 1atm (atmosphere) or 1.01 x 10^5 Pa (Pascals). Normal intra-uterine pressure is considered any measurement 20mm over water pressure.)
When the fetus passes through the birth canal, she or he can better adapt to the pressure difference outside the womb. Intra-uterine pressure, which is influenced by the volume of amniotic fluid, is more highly compressed than the pressure on the exterior of the body. Therefore, it is beneficial for the fetus to pass through the birth canal during labor. Fetuses born via Caesarean section can often suffer complications following birth due to the extreme change in pressure experienced by their bodies. The pressure exerted on the fetus as it passes through the birth canal, expels amniotic fluid from the lungs. This prepares the fetus to take its first breath once it is born, while a baby born by Caesarean section may have trouble taking the first breath because of fluid remaining in the lungs.
(Pressure changes during a contraction)
Once all the signals have been sent and the contractions cause the cervix to dilate to ten centimeters, then it is time for the birth of the fetus. Check out the actual birthing process at this link. (Viewer discretion is advised.)
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