So, it’s finals week. I’d planned a book review, but instead I needed to rehearse an essay for the physiology exam today. Which I thought I’d share here. Because… oh, no reason. More than most of you ever wanted to know about what does, and doesn’t, go into your pee. If you want fun animations and an even more animated narrator, check this video out.
Kidneys are an important part of maintaining homeostasis. Your excretory system is responsible for maintaining levels, not only of waste, but water in your body. The kidneys pull waste, water, and more from the blood stream and filter it, separating the good from the bad, before the urine travels into the ureters to the bladder for storage until micturation. The kidneys are also responsible for monitoring and maintaining your blood pressure.
The main function of the kidneys happens in the nephron, a tiny, complex structure. How tiny? There are about a million of them in each kidney, and you can survive just fine with only one kidney. The kidney is where blood goes in, and pee comes out. There’s about 180 liters of blood filtered on a daily basis, but we only make about 1-2 liters of urine for that. So there’s a lot going on in our body’s filtration system.
The kidney is made up in layers. First, the outer later is like a shell (squishy, but a shell) then there’s the cortex, which is where the blood encounters the nephron first at the glomerulus. The middle layer is the medulla, and the center is the calyces where the urine collects and enters the ureter. The hypertonic medulla is very salty and allows us to draw back out most of the water from the filtrate.
Blood enters the glomerulus from the afferent arteriole, and leaves it via the efferent arteriole. Inside the Bowman’s capsule, the bundle of tiny capillaries is very leaky, and much of the plasma leaves them, while blood cells and large proteins stay in the blood stream. This is the filtrate. After the filtrate is extracted in the glomerular capsule, it enters the proximal convoluted tubule, where sodium, chloride, amino acids, water, and glucose leave the filtrate and re-enter the cortex of the kidney where they are taken up by the web of capillaries that surround the nephron and re-enter the blood stream. The PCT reabsorbs about 65% of the nutrients that were in the filtrate.
The Loop of Henle then straightens out and plunges into the medullar section of the kidney before rising back up and becoming the Distal Convoluted Tubule. The renal medulla is very salty. The descending limb of the loop of henle is permeable to water, and water leaves the filtrate. The ascending limb is impermeable to water, allowing ions such as sodium, chloride, and potassium to be pulled out and into the medullary tissues (and ultimately into the capillary network). Countercurrent multiplication occurs because the limbs go in opposite directions. The ions that leach from the ascending limb causes water to be reabsorbed passively from the descending limb. The ions are pumped out by active transport. The countercurrent multiplier creates the concentration gradient, the countercurrent exchanger maintains the concentration gradient.
The distal convoluted tubule is responsible for reabsorption of yet more ions, like sodium and chloride. More importantly, the configuration of the DCT as it passes very close to the glomerulus creates the juxtaglomerular apparatus, which controls blood pressure, not just for the kidney, but the whole body. The wastes leave the DCT after the body has re-absorbed as much as possible, and enter the collecting duct. In the CD, urea is reabsorbed into the medullar tissues where it maintains the necessary osmolarity to pull the water from the loop of henle.
