Here’s a fancy topic of the newer spec… I’ve never done the kidney so I spent my post-A levels life in total kidney ignorance and utter lack of knowledge of my urination habits and their complexity, oh what a fool I have been. You on the other hand are going to have the damn honour of actually having a clue about how the brain and the kidneys freshen our blood up all the time. Oh ye enlightened children, rise.
Osmoregulation refers to the control of water potential of the blood. The blood is complicated, it has all these ions and proteins and stuff. Cells use various things up all the time and some more often than others at different times, night, day, sweat, tears, etc.
There are systems in place that keep the blood at the right composition and pressure. The hypothalamus and posterior pituitary in the brain release a hormone into the blood that reaches the kidney and enables its cells to take up more water, to prevent it being wasted in urine as the case may be. This is detected by osmoreceptors.
The hormone is known as vasopressin or antidiuretic hormone (ADH), has a very short half life of 16-24 minutes as you can imagine since it regulates fast-changing things like water retention and blood pressure. It acts in the negative feedback loop that maintains optimal plasma concentration.
More specifically, the hypothalamus synthesises it while the posterior pituitary which is actually an extension of the hypothalamus, stores it for release into the blood.
ADH stimulates water retention by the kidney by:
1. Increasing water permeability in a part of the kidney cell which results in retaining more water and excreting more concentrated urine
2. Increasing urea permeability by another part of the kidney cell which results in its concentration in the urine
3. Increasing sodium absorption across the section of the kidney cell which circulates the solution, resulting in reabsorption of water
This uses the principles of osmosis where water moves from a higher water potential (less concentrated solution) to a lower water potential (more concentrated solution). Here urea is the solute and water is the solvent. Guess the solution! …pee.
Ok let’s look at the actual kidney cell and all this mystery of the different “parts” that do different things.
The cell in the kidney that executes all this action is the nephron. It looks a bit weird and has all these tubes hanging off it. A kidney has about a million of these bad boys.
The path that the fluid takes via the nephron and to becoming urine is threefold: filtration, reabsorption and secretion. This means that there is a middle section that allows for reabsorption into the bloodstream before releasing the contents into urine.
Oooh almost got poked by that loop of Henle… keep to yourself loop.
Let’s start at the top with the glomerulus. It’s a scrunched up bunch of capillaries that allow the high pressure needed to filter the blood forwards. The fluid passes from the capillaries into the capsule that surrounds them, Bowman’s capsule.
Oooh creepy. This is where the subsequent glomelural filtrate is formed. Still similar to blood plasma, but minus all those proteins and large molecules. Capillaries consist of a squamous endothelium (flat, single-celled layer). The wall of Bowman’s capsule contains podocytes (spider-like cells that prevent proteins from passing though).
Next, water and glucose are reabsorbed by the proximal convoluted tubule (in yellow)… At least it’s honest about being convoluted. Unlike the distal convoluted tubule, it doesn’t have many mitochondria. The distal convoluted tubule needs many mitochondria to generate ATP for active transport of ions such as sodium ions back from the filtrate. As the useful minerals get absorbed back into circulation, waste materials such as urea accumulate in the fluid (urea is produced in the liver from excess amino acids by joining two ammonia molecules with one carbon dioxide molecule in what is termed the ornithine cycle).
The distal and proximal convoluted tubules are made of cuboidal epithelium (single-celled layer of cuboidal cells with large, centrally-located nuclei). The proximal tubule cells contain a high concentration of mitochondria, villi and basal invaginations.
For the water to be reabsorbed, a countercurrent exchange system needs to exist. This is provided by the loop of Henle which has a downwards part and an upwards part. The upwards part is impermeable to water, enabling the countercurrent exchange and the pumping out of the sodium ions.
Its job is to maintain a gradient of sodium ions to enable the water potential gradient underpinning water movement and reabsorption. This is carried away in the distal tubule and collecting ducts. The water and waste is then excreted via urine.
Microscope slides of the kidney and nephron reveal these structures. Under scanning electron microscopy (right), the glomeruli look particularly striking.