Changes to the Body

00:00 What are the neurological changes that occur? These mainly have to do with the brain size and the amount of myelination that occurs on axons. Let's look at a couple of the different years and go through some of the responses. So at 40 weeks we're about at 400 g. It's pretty rapid in terms of a gain to about 1000 g within the first year. It then tapers off some that the rate arise and when you get into adolescence maybe you're somewhere around 1250 to 1400. Myelination is going to be occurring throughout childhood. It depends on the various structure that we're dealing with how quickly it takes to myelinate but this is one reason why coordination develops and some other brain functions work a little bit better as one is maturing. Other neurological changes that happen are being able to regulate one's own body temperature. In fact, there is a circadian rhythm associated with temperature regulation and early on within the first few months you have a lower nightly temperature but when 3-6 months that magnitude starts to change and finally it takes all the way to about year 2 to maybe even 5 before the adult normal circadian rhythm variation is able to be observed. Infants also have an impaired ability to shiver. They don't have good muscle coordination as of yet so therefore they can't utilize shivering in thermogenesis. What they do use, however, is something called brown adipose tissue. So there are brown adipose tissue sites especially around some of the large organs that go through a futile cycle in which in response to norepinephrine there is increased heat production. The final thing that's different for thermoregulation as a child develops is their surface area to mass ratio. The more surface area they have, the greater ability they have to lose heat. The less surface area they have, the more they retain heat. So you have to factor that into the person's growth patterns.

02:14 So when they have long limbs, they are able to radiate heat better than when they have short limbs.

02:21 There are also sleep-related changes. A neonate sleeps quite a bit 16-17 hours per day.

02:28 In fact, the sleep cycles are fairly short, only 45-60 minutes. They spend a lot of time in REM sleep. If we contrast that to the infant, they start to sleep for longer periods at a time maybe 8-10 hours and by year 1 they're dropping the amount of REM sleep that they need to about 40%. A child will sleep about 10-12 hours a day so still a little bit longer than an adult, but now they are starting to gain a normal sleep pattern in which they're going to go to the various stages of sleep every 90-110 minutes. There are some blood-related changes associated with development. A neonate has a very high hematocrit. In fact, they look like they have polycythemia. They have a hematocrit of about 50%. Hemoglobin levels are also high per deciliter. In early and late childhood hemoglobin drops so in this case it is 30 to maybe upwards to 38 at late childhood and hemoglobin levels are still fairly low if we compare that to the normal adult male or woman. Here, hematocrits are somewhere in between about 40-54% for male and 37-47% for females. The cardiovascular system changes quite a bit as one develops. Neonates, when they first start out, have a very large right ventricle. Because this was hypertrophied while the fetus was in the womb and had to overcome very high pulmonary vascular resistances but as soon as birth happens now you are having a low resistance and so the right ventricle does not have to work as hard to go through the pulmonary vasculature. The infant's side difference starts to equate especially because systemic vascular resistance becomes so much higher but this also involves a large mass change in the heart maybe about doubling by year 1. By late childhood there's probably about a 6-fold increase in the amount of heart mass or heart tissue. If we look at some of the standard heart rates and blood pressure responses, if you look at a preemie you have a fairly high heart rate 120-170 and a pretty low systolic blood pressure. You can see this will start to drop in terms of the amount of heart rate as you have an infant to 1 year old and this keeps dropping as one goes from 4, to 6, to 8, to 10 until you reach adult levels around 12 years of age. The minimum systolic blood pressure continues to climb while heart rate is dropping. Let's go through an example of the change that could happen in cardiac output if we compared an infant to an adult. So in an infant you have a very small stroke volume. That's the volume of blood ejected per beat versus a heart rate which is how many times the heart beats per minute so you have an overall cardiac output of around 1.3 L. If we contrast that to an adult who has a stroke volume of around 70 mL/beat and a heart rate about 72, you get a cardiac output around 5 L. So there are quite a bit of cardiac output difference between infants and adults.

06:16 The other thing that's very different about this response is the ability to change stroke volume.

06:23 During the more maximal stimulation, stroke volume may only go up about 3 mL/beat.

06:29 Therefore, even if you had a higher heart rate you're cardiac output may only increase to 2 L. If we contrast that to an adult who has the ability to really change stroke volume by 50 mL/beat, you get large changes all the way up to 19.2 L/min. Highlighting the differences here are stroke volume in nature, the ability to increase stroke volume and the absolute stroke volume you start with. There are also changes that occur with the respiratory system.

07:06 There are higher respiratory rates, which is your breathing frequency in childhood. There are lower lung volumes and capacities. There are also lower abilities to diffuse gases across the lung but these seem to be fairly proportional to size and why that is important is then it means you don't have a diffusional lung capacity test or problem but rather it is related only to the size of the lungs. By about year 3 is about the first time you're able to really complete a lung function test. Prior to this, it's very difficult to get someone to do the type of breathing procedure necessary to be able to undergo a respiratory test such as a pulmonary function test.

07:57 So what's a normal ventilation rate? If you look at a preemie or looking at somewhere between 40-60 and this will gradually drop as you age until you get to about a normal rate at about age 12 around 12-20 breaths per minute. Other respiratory changes that happen are there are smaller airways in younger children. This means that it's easier to obstruct their airways and that obstruction could be from a foreign object such as something that was aspirated or could be clogged up from some of their own secretions. Another potential problem is that neonates have non-fully calcified ribs and this makes the compliance of the lungs a little bit different so they have to enact different breathing patterns because of this high compliance and once calcification of the ribs happen, they will be able to return to more of a normal respiratory rate. There are also renal changes that occur from neonates to infants to children.

09:17 The biggest thing about a neonate just after birth you have a very low glomerular filtration rate and your ability to concentrate urine is really pretty poor. Infants get a little bit better in being able to have more glomerular filtration rate as compared to their size but they still can't concentrate their urine like an adult. In fact, it takes a while for that ability to concentrate urine comes into play. Children also have a reduced ability to excrete potassium to reabsorb bicarbonate and then to totally buffer hydrogen ions. To buffer hydrogen ions that's working with non-volatile acids. So let's take this idea of inability to fully concentrate the urine and let's show a couple examples of that. So in a normal adult, you have a nephron loop. The nephron loop is an index of how much concentration abilities you have so in adult you might go from an isoosmotic portion of the nephron about 300 milliosmoles all the way down to the hairpin loop which is 1200 milliosmoles, so we are able to concentrate urine by about 900 milliosmoles. If we compare the neonate, they're only able to go from 300 to maybe 450 less of an ability to concentrate their urine. What does that mean? The fluids that they take in they're more likely to urinate out without holding on to that fluid. They don't reabsorb their sodium and water to the same level so it's easier for them to become dehydrated.

11:12 Infants are a little bit better, they can drop their osmolality to maybe somewhere around 900 but you still don't have the full adult ability to fully concentrate the urine so again still infants have that ability to become dehydrated as well because they cannot hold on to their water in times of which they are water challenged. This means that we really have to be careful of water turnover rates in neonates and infants. They have a lower ability to concentrate urine. The other item that we need to think about for homeostasis for acid-base balance is that since usually children have a high metabolic rate they're using ketone bodies to a greater extent and their kidney has a less of an ability to deal with non-volatile acids, they are prime for things like metabolic acidosis. Because they don't have that ability to buffer those hydrogen ions yet, you have to be very careful with acid-base balances in young children.