Of course drawing this picture made it take longer and I had to scan it, but I don't pretend for a second to be a rational person.
Anyway this is a little different to what I planned on doing (apparently all my posts come with disclaimers about how not to expect more posts like them :P) this is how I explained the kidney to my friends, but in that case the diagrams were only an aid, rather than the focus, and most of it was me talking. What this verbose vichyssoise ironically attempts to convey is that reducing verbiage is the aim of this project and so I have attempted to remove as much of the associated chatter as possible. We'll start with the gross anatomy.
Put simply, the kidney is divided into a number of lobes, each with a renal pyramid (the medulla) and its associated cortex. Each of the renal pyramids (the peak of which is called a papilla) drains into a minor calyx (calyx meaning cup) which drains into a major calyx, which drains into the renal pelvis and drains into the ureter.
Seriously, the circulation of the kidney makes sense. It's a nice change. The artery that goes to the kidney is called the renal artery. It divides into segments called segmental arteries. Between each lobe there is an interlobar artery. These split at a 90 degree arc into the arcuate arteries. They then shoot off interlobular arteries. That's the only bit that's slightly confusing. And the veins all have the same names. Isn't it nice when the people naming things are being logical instead of artistic or egotistic?
The functional unit of the kidney is the nephron, that's what it looks like . Note that the afferent arteriole is bigger than the efferent arteriole, it's important for maintaining a pressure gradient.
The high hydrostatic pressure of the blood in the glomerulus pushes the blood out into the glomerular capsule. It has to be filtered through the fenestrations in the leaky blood vessels of the glomerulus, which stops large particles including red blood cells. The basement membrane is a gel that acts like a negatively charged sponge, small particles (like water) come through it fine, but large particles (like sand in our sponge metaphor) are filtered, though a few may make it through, it's negativity also keeps out small negatively charged particles like albumin. The final barrier is the podocytes, whose legs wrap around glomeruli and interdigitations (laced fingers) form slits that keep out large particles.
The regulation of filtration rate is quite important. And there are three main regulatory methods. The first is the myogenic reflex, when the arterioles are stretched by a high blood pressure they respond by constricting, which lessens the blood flow into the glomerulus keeping the filtration rate steady. The second is tubular feedback, the mechanisms of which aren't entirely clear, but when the PCT detects a lower or higher flow the juxtaglomerular cells constrict or relax the arterioles to change the blood flow. The final method is the renin-angiotensin cycle, the kidney secretes renin, which converts angiotensin to angiotensin I, which is converted by ACE into angiotensin II. Which acts on the brain, vessels, adrenal glands and kidneys, stimulating thirst, raising blood pressure secreting aldosterone and increasing NaCl and H2O retention respectively.
Proximal Convoluted Tubular re-absorption is driven by sodium. The cells which line the tubule constantly pump out Na+ keeping the concentration low, causing Na+ to diffuse across the membrane. Glucose is bound to Na+ by transport proteins, causing it to follow along, along with negatively charged molecules such as chlorine . As the osmolarity changes water also diffuses out. Bicarbonate is unable to be absorbed, but none is usually found in the urine, so within the cell water and CO2 combine to form bicarbonate and H+. The H+ moves into the tube and breaks the bicarbonate into water and CO2, which can diffuse out.
This is possibly one of the coolest things ever. No really, I know it's nerdy. But seriously, the thin part of the nephron loop is permeable only to water, so when it dips into the salty medulla the water leaves the tube and the osmolarity increases. When it reaches the thick part which is non-permeable to water the concentration of sodium has increased, and so as it leaves the medulla the sodium bails as well, preserving the sodium gradient.
Now think about this, what would happen if the vessels went straight through the medulla? The salt would go in and the water would go out. Very very bad. Hence the vasa recta, they go enter and leave the medulla next to each other and thus as the gradient increases the salt enters and the water leaves, but as the vasa recta returns more or less on its original path the reverse occurs, meaning the osmolarity and concentration is essentially unchanged.
Finally we reach the Distal Collecting Tubule and Collecting Duct, these remove the last of the water and the salt, and are modulated by aldosterone, anti-natriuretic peptide, antidiuretic hormone and parathyroid hormone depending on whether or not the body needs to conserve or excrete water. The method of the collecting duct is similar to the thin section of the nephron loop, as the medulla becomes saltier more water leaves the duct.
