The Urinary System

Now we've moved onto the next system- the urinary system! Once again I don't have that many notes on this system, and whatever notes I have aren't that great, so please a) forgive me and b) correct me if you find any blatant errors.

1. Describe the anatomy of a nephron in detail.

A nephron is basically the functioning unit of the kidney, and as far as I can tell each nephron is basically just a duct surrounded by capillaries. However, it gets a bit more complicated than that (read: a LOT more complicated), but I'll try and describe it as simply as possible.

First, I will describe the shape of the urine-carrying renal tube. At its beginning, located in the renal cortex (the outer region of the kidney), the tube widens out to form a capsule known as Bowman's capsule. The tube then quickly becomes convoluted- the proximal convoluted tubule- as it proceeds downwards. Eventually it forms a long U-shaped loop called the Loop of Henle, which dips down into the renal medulla (the middle portion of the kidney) before going back into the renal cortex again. At this point, it becomes the distal convoluted tubule, which then joins a larger urine-carrying duct.

Next I will describe the blood supply. Fresh blood travels to the nephron in an afferent arteriole. This leads into a convoluted ball of capillaries called the glomerulus, which tucks neatly into the cup-like Bowman's capsule of the renal tube. Blood leaves the glomerulus in an efferent arteriole which is thinner than the afferent arteriole, causing blood in the glomerulus to be at relatively high pressure, which is great for filtration purposes. This blood then branches off into capillaries that surround the other sections of the renal tube, allowing for further transfer of substances between the blood and the urine.

2. Describe the internal blood flow of the kidneys. How is it different from a venous portal system?

I have already described the blood flow of nephrons, the main functioning units of the kidney, in my answer above. I would, however, like to draw attention to one particular point. Did you notice that the blood entering the glomerulus went from an afferent arteriole to capillaries to an efferent arteriole (i.e. arteries to arteries) as opposed to the vein-to-vein systems of the hepatic and hypophyseal portal systems? I would assume that this would result in a higher pressure blood flow due to the arterioles' ability to contract, but I'm not 100% sure so don't quote me on that.

Now I'll mention the more "general" (for lack of a better term) blood flow to the kidneys, which is fortunately much less complex than talking about nephrons. Each kidney has a renal artery and a renal vein. These branch out within the kidney and form circular-like rings so that if one part gets cut off, the rest of the kidney can still survive. This is an example of an anastomosis.

3. Describe the mechanism for water reabsorption and name the locations for reabsorption in the nephron.

Most water reabsorption (around 90%) occurs via osmosis, or the movement of water from an area of higher concentration (i.e. with fewer dissolved solutes) to an area of lower concentration. The remaining 10% of water absorbed is absorbed with the help of hormones such as ADH (antidiuretic hormone) and aldosterone.

4. What is the effect of severe and prolonged hypotension (low blood pressure) on renal filtration? Renal blood flow?

Since filtration requires pressure to push solutes across into the renal tube, severe hypotension would likely decrease the rate of filtration. In fact if the pressure in the capillaries was lower than the pressure in the capsule, perhaps the solutes would filter back across, but I haven't done any physics for around four years now so don't quote me on that. Renal blood flow would likewise decrease, especially since the body is likely to prioritise more critical organs such as the brain and heart over the kidneys. In fact, in very severe cases, blood flow to the kidneys can stop entirely, which can kill the tubules or even the entire kidney.