Chapter 1 Physiological Readiness I n order to potty train your child as successfully and as easily as possible, it is really important that you understand the physiological functions involved. Not only can this help you select an appropriate time to commence training, it can also help you to understand and troubleshoot any problems you may face both now and in the future. The excretory system is responsible for the elimination of waste from our bodies-primarily, urine (pee) and stools (poo). Let''s take a look at how each of these works in turn. Urinary excretion Urine is excreted in order to eliminate the waste products of cell metabolism. It also regulates blood pressure, volume and pH, and the levels of ions such as potassium, sodium, and chloride in the body. Urine is formed in the kidneys, a pair of small bean-shaped organs located in the upper abdomen, toward the back. Organic waste is removed by the nephron, the basic functional unit of the kidney, which filters certain objects out of the body that are not needed, while reabsorbing those that are.

This process uses around 25 percent of the body''s cardiac output, or blood flow. The major waste product filtered out by the kidneys is known as urea, a toxic substance formed mainly from the ammonia made by the liver during the breakdown of amino acids. After the process of secretion and reabsorption that occurs in the nephrons, waste substances travel to the collecting tubules, where they will later be excreted in urine. Urine is mostly composed of excess water, as well as urea, excess ions, and other waste products. The amount of urine produced by the body, per kilogram of weight, reduces as we age. Newborns produce 3 ml/kg per hour, while older babies produce around 2 ml/kg and toddlers produce 1.5 ml/kg per hour. Older children produce 1 ml/kg per hour, and adults produce approximately 0.

5 ml/kg per hour. The urine next passes from the kidneys through to the ureters (two tubes of smooth muscle fiber, around 30 centimeters long when fully grown) and, finally, the bladder. The bladder is a hollow, muscular, and distensible (elastic) organ that sits at the base of the pelvis. Urine leaves the bladder via the urethra, a muscular tube. Bladder capacity increases with age, until at adulthood it can hold approximately 455 ml of urine. The following table shows the capacity of the bladder at the different ages predominantly covered in this book. Bladder capacity by age Age of child Bladder capacity Six months 85 ml Twelve months 115 ml Eighteen months 142 ml Two years 200 ml Three years 213 ml Four years 227 ml Adult 455 ml The process and control of urination The neck-or section close to the opening-of the bladder is held closed by two strong bands of muscle. These are known as the internal and external sphincters.

The internal sphincter is made from smooth muscle that contracts involuntarily. The external sphincter is formed of skeletal muscle and is voluntarily controlled; that is, the individual can open and close it on demand. When the bladder is full, it signals to the parasympathetic nervous system, which, in turn, contracts a layer of the bladder composed of smooth, involuntary muscle fibers known as the detrusor muscle, causing the internal sphincter to open, ready to excrete urine. Actual urination itself is a combined response of both the parasympathetic nervous system (the part that regulates the body''s unconscious actions, such as digestion) and the central nervous system, or CNS (consisting of the brain and spinal cord). This teamwork causes the internal and external sphincters to open. It is the voluntarily, CNS-controlled external sphincter that is particularly important in potty training. It is not just the external sphincter, however, that is related to potty training but the detrusor muscle. Babies commonly urinate frequently, in small amounts.

Around a third will show something known as "interrupted voiding." This is characterized by incomplete urination of varying amounts and frequency. This is due to unsustained muscular contractions of the bladder, which are, in turn, believed to be a result of poor coordination between the detrusor muscle and the sphincters of the bladder. This lack of coordination may indicate that control of the bladder in early infancy is related to connections in the developing brain rather than the detrusor muscle simply stretching and causing the internal sphincter to open, as happens later in life. This suggests that urination in babies is not completely conscious or voluntary but rather related to their brain development, like the conscious and voluntary control of their limbs and movements in the early months. This pattern disappears completely by toddlerhood when the nervous system matures. Their new neurological maturity enables the toddler''s and preschooler''s brain to receive and send messages to the bladder, preventing urination from occurring automatically before they have found a potty or a toilet. Control of urinary output at night Nocturnal urinary output is controlled by the body''s circadian rhythm, or body clock.

At night, the lowering and, finally, absence of light is detected by the optic nerves, in the eyes, which transmit a signal to the hypothalamus and pineal gland in the brain. This signal causes several changes within the body, including: the secretion of melatonin-a hormone that causes sleepiness the lowering of body temperature the increased secretion of antidiuretic hormone (ADH), also known as vasopressin. Secretion of ADH causes the body to reabsorb more water and thus reduce production of urine at night. ADH is also a vasoconstrictor, which means that it constricts blood vessels within the body, leading to higher blood pressure. This is necessary for the increased reabsorption of water at night. There is some evidence to show that in instances of persistent enuresis (bed-wetting), ADH levels are lower than in children who are dry at night; however, they are not so different that this could be considered the sole cause of bed-wetting. When thinking about babies and toddlers, it is important to understand the development of the circadian rhythm. Research shows that it develops as children grow-that is, the circadian rhythm of a baby is not comparable to that of an adult.

It is estimated that circadian rhythm becomes established to the level of an adult by around three years of age. This maturation coincides with control of the levels of ADH production at night. Research has found that these are similar in three-year-olds and teenagers. We can understand, therefore, that there is a strong correlation between the age of night dryness and the maturation of the child''s circadian rhythm and ADH secretion. Stool excretion The intestines absorb nutrients and move any remaining waste through the body, first through the small intestine and then the large, so that it may be excreted. The small intestine comprises the duodenum, jejunum, and ileum. In adults, it measures about twenty feet in length. Its primary functions are to absorb nutrients from food and to break it down, ready for excretion.

Small fingerlike projections, known as villi, cover the inside of the small intestine and aid in the absorption of nutrients by increasing its surface area. The small intestine adds further enzymes to those that were introduced to the food in the stomach, so continuing the process of digestion, absorption, and breaking down, as food is moved along, toward the large intestine, via a series of muscle contractions known as peristalsis. The large intestine is about five feet in length and comprises the cecum, colon, rectum, and anal canal. The colon is a long muscular tube connecting the cecum (the first part of the large intestine) to the rectum (the last part). It is subdivided into the ascending, transverse, descending, and sigmoid colon-the ascending being connected to the cecum and the sigmoid being connected to the rectum. As food passes through the colon, salt and water are extracted, turning soft and liquid waste into solid matter consisting of food debris and bacteria. This remains in the sigmoid colon until it is emptied into the rectum, approximately twice every day, where it is stored temporarily until stretch receptors in the rectal wall sense fullness and send a message to the brain to excrete the waste (stool or poo) via the anal canal. Like urination, the process of defecation depends on the opening of two sphincters, one internal and one external, once again a mix of involuntary and voluntary control.

The internal sphincter is opened when the intrarectal pressure is increased due to the presence of waste, which pushes the internal walls of the anal canal open slightly. But as with urination, it is the external sphincter that is under our conscious control, and it is this that children must learn to master when potty training. The external sphincter is opened by the muscles of the pelvic floor, which cause the anal canal to open fully and the rectum to temporarily shorten in order to expel stools. This conscious control-or lack of it-can lead to soiling, withholding, and constipation in children. Constipation occurs when stools remain in the rectum for too long and the solid waste is moved back into the colon, where further salt and water extraction occurs. This may then lead to a buildup of hard, compacted waste. (Soiling, withholding, and constipation are discussed at length in chapter 6.) Understanding the physiological basis of bowel and bladder control.