Seems to be a lot of misinformation in this thread, so I will try to clear up as much as I can.
Both spinal and epidural anesthesia work on the same principle, which is bathing nerves or nerve roots at or in the spinal canal with a local anesthetic medication to provide varying levels of analgesia or anesthesia for a number of different procedures. Neither depends on "vascular uptake" to be effective, though uptake into the vascular system is how the anesthetics are cleared, and why they wear off after a given amount of time.
A spinal anesthetic is a "one shot" anesthetic. A very small needle is inserted through the dura until there is a return of CSF. A small amout of local anesthetic, usually (but not always) with some narcotic, is injected, and the spinal needle is removed. Spinal anesthesia provides a very dense motor and sensory block. Depending on the volume, baricity, and type of agent injected, it can provide complete lack of feeling from about the nipple line down. (It can even go higher, but that is considered a bad thing, because respiratory muscles begin to be involved.) However, it is a one shot thing. Depending on the agents chosen for injection, duration can range from about 30 - 45 minutes, up to 90-120 minutes, and sometimes even longer. If it wears off before delivery, your only options are to deal with it, get another spinal injection, or use IV medications.
In epidural anesthesia, a special needle is used to identify the epidural space just outside of the dura. A very thin catheter is then threaded into this space, and again, local anesthetic, usually with narcotics, is administered. Again, we are attempting to block sensation at the nerve roots where they enter the spinal canal. Much higher volumes are used in epidural anesthesia. Additionally, the catheter is left in the back, so a continuous infusion of the local anesthetic medication can be administered to the catheter. Also, we can administer additional bolus doses of medication as labor progresses.
Generally, there is a greater degree of control with epidural over spinal anesthesia. I can vary the rate of infusion, and change medications to give greater or lesser levels of blockade, and by choice of medication administered, retain a greater degree of motor control, so mom can more effectively push. However, I can use epidural anesthetics to achieve a block as dense as spinal, so that surgery, to include c-section can be done on the patient with little or no discomfort. (It's not unusual to feel pressure, as when the surgeon pushes on the belly to help push the baby out, but it should NOT be painful.)
So, why do we sometimes choose spinal over epidural? Well, there are a lot of reasons. For example, I choose to give spinal anesthetics to patients having c-sections, because spinal is better for the baby than general, and mom gets to be awake when the baby is born. Additionally, time of onset is much faster with a spinal anesthetic. Generally, once I have an epidural in, I tell moms that it is going to take two to three contractions before they really start feeling any relief. With spinal anesthesia, the patient feels relief in about 10-30 seconds.
Generally, if the patient is going to have a procedure of known duration, and pain management post operatively is not going to be anesthesia's concern, we will use a spinal anesthetic. If we cannot predict the duration, or the duration is expected to be longer than can be achieved with spinal anesthesia, then epidural anesthesia is the way to go.
Having answered your question, allow me to step on my soapbox for a moment. As a CRNA, one of my pet peeves is a prejudice some nurses, physicians, and (most commonly) nurse midwives feel towards epidurals for labor and delivery. They believe (and convey to their patients) that they are somehow less of a woman if they need an epidural for labor. It is an antiquated idea, and one I believe harms women. Yes, I know, women had babies for thousands of years without epidurals. But then, we extracted teeth for thousands of years without anesthesia as well. Does it make a dental patient somehow less of a person for demanding local anesthesia before the dentist drills on their tooth?
I've actually been told by a nurse midwife that, with proper breathing and concentration (as she taught her patients) she could achieve the same level of pain control that I could achieve with an epidural. Nonsense. I don't care how much concentration or breathing you work on, you will never be able to make an incision without pain by this method. However, I can do so with an epidural.
I'm not suggesting that every woman must or should have an epidural. In fact, with I discuss them with my patients, one of the first things I say is that "I'm not here to sell you anything." I simply provide information, answer questions, and allow the mom to make her own decisions, and I respect her ability to do that. If she chooses not to have an epidural, great. That is HER choice. By the same token, it is unfair and unethical for others to allow their prejudice to color how they talk to patients. Present patients with options, and allow the patient to make a choice, unsullied by your preconceived notions.
Kevin McHugh, CRNA