This week’s big question comes from my son who is in second grade (kids always have the best science questions). He was wondering, “How does your pee come out?” So, let’s take a little journey through the urinary system.
We can begin in the kidney where blood is filtered and urine is formed. In humans, the kidneys are a pair of bean shaped organs located in the abdomen near the bottom of the rib cage. Inside of each of your kidneys you have millions of microscopic tubes called nephrons. At the beginning of each nephron is a tiny tuft of tangled blood vessels called the glomerulus (pronounced glow-mare-you-lus). Blood is literally pushed through several membranes and then into the nephrons. During this filtration process, stuff like red blood cells and white blood cells, and proteins are too big to pass through the filter. However, a great deal of fluid (blood plasma) and almost all of the dissolved stuff such as salts and sugars also pass into the tiny tube of your nephron. You can think of the nephrons as pee processing pipes. Any water, salts, sugars, or other molecules that we need are reabsorbed into the blood vessels surrounding the nephron as the blood filtrate flows down the winding tubules. Any extra fluids or waste products are not reabsorbed or are secreted from cells lining the tubules, they keep flowing down the nephron and end up as urine.
Urine leaves the kidneys through a series of tubes. First it is dumped from the millions of nephron tubules into the collecting duct. The collecting ducts empty into a space called the calyx (pronounced ka-licks). The calyces (plural for calyx) join together like tributaries and merge into a wide river in the renal pelvis and this is where the river of pee leaves the kidneys. Urine is transported down from the kidneys to your bladder by muscular tubes called the ureters (yur-re-ters). Gravity helps pee drain down into your bladder, but it can still move down the ureter if you are standing on your head or lying down and sleeping. The walls of the ureter actually contract in coordinated waves called peristalsis, propelling urine to the bladder.
Your bladder is a stretchy storage sac for pee. How stretchy? Well, a bladder that is fairly full can hold about 500 ml (1 pint), but it can still stretch to nearly twice that volume (think about the size of a 1 liter water bottle). Near the bottom of your bladder is a tube to the outside of your body called the urethra (yur-ee-thra). In males the urethra passes through the penis and in women it opens to the outside just above the vagina. You have two circular valves or sphincters that normally keep the urethra pinched off when urine is not passing through. The internal urethral sphincter is closer to the bladder and is controlled by reflexes in your spine and other parts of your nervous system that you cannot consciously control. The external urethra sphincter is under conscious control, so you can “tell” it to open and close as needed.
When your bladder fills, it stretches and nerves fire in response to this stretch and activate a spinal reflex. Usually, you feel the need to pee before urine volume is greater than 400 ml (a little less than 1 pint). The spinal reflex causes a layer of smooth muscle in your bladder, called the detrusor, to contract and the internal urethral sphincter to open. Babies pee whenever their bladders are somewhat filled and this reflex is activated because they haven’t developed conscious control over urination yet. However, most people after infancy have conscious control over the external urethral sphincter and can choose to keep it closed until it is convenient to urinate. So, to recap the whole peeing process, urine is made in the kidneys, travels down the ureter to the bladder, the bladder fills and stretches, the smooth muscle of the bladder contracts, the urethral sphincters open, and urine travels down the ureter outside your body, preferably into a toilet or perhaps the base of a tree (if you are outside).
Marieb, E. N., & Hoehn, K. (2015). Human Anatomy & Physiology (10th edition). Boston: Pearson.