Your Body posts are a new series on the blog, exploring the mind-blowing ways that your body keeps you alive. The first post in this series was requested by the delicious Bella, who is currently in hospital with possible kidney problems. Science nerds bring the gift of knowledge. Get better soon, lovely Belmo!
Your kidneys are busy little organs. Busy, busy, busy. They’re responsible for such a range of things in your body, but their main function, and the way that they achieve all their stunning regulatory work, is to filter your blood, reabsorb any of the good bits, and get rid of the junk in your urine. I remember learning about the way in which the kidney filters your blood during my A Levels: it took me hours and hours of frustrated diagrams (and frustrated teachers) to get my head around it but oh, when I did, how I loved it. It’s just so CLEVER, I had to run out and tell everyone how exciting their kidneys were. And yes, I was a very cool and popular teenager, why do you ask?
So, the filtration unit of the kidney, the bit where all the exciting action takes place, is the nephron. The nephron is mostly based in the cortex of the kidney and consists of four sections: Bowman’s capsule, the proximal convoluted tubule (PCT), the Loop of Henle and the distal convoluted tubule (DCT).
Bowman’s Capsule: Spring cleaning the blood
In the Bowman’s capsule there is a network of teeny blood vessels. When blood is forced from a wider normal-sized bloody vessel into these tiny skinny tubes, the pressure is increased. And, under pressure, the blood gives up the goods, shoving water, cells, proteins and ions through the permeable membrane of these specialised weeny blood vessels into the (PCT). Blood would be a rubbish spy, it’d never hold up under torture.
The Proximal Convoluted Tubule: Getting back the good stuff
Because the blood was so hasty in getting rid of everything, the PCT has to do a bit of work at this point, reabsorbing some of the good things the blood was carrying around. I imagine the PCT spends much of its time grumbling at the Bowman’s capsule like an irritable old man… “Why did you throw glucose out, hey? You NEVER know when you might need glucose! And look, you’ve got hardly any water or salt left either. Do you never think before you act?! Just guess who’s got to put in the work to pick up the pieces. SIGH” , to which the blood merely responds… “It was the pressure… I panicked”. After the PCT reabsorbs some water, salt and glucose, the remaining filtrate consists mainly of water, urea, and sodium and chloride ions.
The Loop of Henle: Making the grade(ient)
The loop of Henle is a large loop, it drops down from the PCT in a descending arm that reaches into the medulla region of the kidney, and then comes back up in an ascending arm to rejoin the rest of the nephron in the kidney cortex. As the filtrate from the blood moves down the loop, water decides it has frankly had quite enough of the company of urea and over-excited salt ions and gets the hell out of there, through the membrane of the loop. Now the filtrate is mostly just salt and urea, until, on the way back up the ascending tube, salt is actively transported out of the loop, to hang out in the medulla, leaving urea surrounded by the remaining water, feeling rather lonely and sorry for itself as it finishes the somewhat disappointing fairground ride that is the loop of Henle.
The distal convoluted tubule and collecting ducts: Go go H2O
After leaving the loop of Henle, the filtrate passes through the DCT and into collecting ducts. These ducts pass through the medulla, which is where the salt that was removed during the loop of Henle is currently hanging out, having a party. Now water, whenever it sees an area that has a high concentration of salt immediately wants to rush in and dilute the fun, returning the system to a balanced equilibrium. Water is a serious party pooper. Luckily, the membrane of the collecting ducts restricts how much water is allowed to rush towards the salt, and this amount depends on the concentration of salt in the blood.
The hypothalamus in your brain detects the concentration of salt in your blood: a high concentration of salt means that there is less fluid and water need to be conserved. In this case, the hypothalamus sends out an alarm call to the pituitary gland, which reacts by releasing a dedicated response team of antidiuretic hormone (ADH) molecules. These bad boys rocket to the kidneys, and tell them to let more water out of the collecting duct and back into the body. This leaves you with very concentrated urine. On the other hand, if there’s very low salt concentration in your blood, no ADH is released and no water is allowed out to crash the salt party, it all stays in the collecting duct to be passed out of the kidney, to the bladder and out of your system.
And THAT is how your kidneys filter your blood, getting rid of all the nasties and keeping the useful bits and pieces. Such an elegant system, so clever and beautiful. Take a moment now and congratulate your kidneys for being so freaking awesome.