The kidneys are very important organs. The functional units of the kidneys are the nephrons and we have millions of these in each kidney. The nephron, the kidneys, are important in forming urine and there are four main steps in urine formation. These are filtration, reabsorption, secretion and excretion.
In this video we will focus mainly on reabsorption and secretion. It's important to understand some terminology though. Reabsorption refers to the movement of water and solutes from the nephron back into circulation. Secretion is the movement of solutes and stuff from circulation back into the nephron, back inside the nephron tubule. So let's just recap some anatomy first.
Here you have the nephron. The afferent arteriole brings in blood, brings in plasma to the head of the nephron. where the vessels here will form what's called the glomerulus.
The vessels will then exit the head of the nephron via the efferent arteriole. The efferent arteriole will then form the vasorector, which are basically capillaries which will follow the tube of the nephron. The vasorector allow for reabsorption of things back into circulation, while at the same time allow for secretion of things from the circulation back. inside the nephron tubule.
Now the tubule of the nephron has a few sections. After the head of the nephron, called the Bowman's capsule, you have the proximal convoluted tubules, the loop of Henle, the distal convoluted tubules, and then the collecting duct. These different segments or sections of the nephron are responsible for the reabsorption of different electrolytes and substances, as well as water, but also these different segments of the nephron may absorb the same things but in varying amounts. So let's focus on the reabsorption of things first, beginning with the proximal convoluted tubules. Here you get reabsorption of sodium, chloride, potassium, glucose, amino acids, which are the protein building units, urea, and bicarbonate, as well as water, of course.
In the descending limb, of the loop of Henle, water reabsorption takes place. In the ascending limb of the loop of Henle, so the going up part, sodium, chloride, and potassium reabsorption occurs. The loop of Henle here has a descending limb and it is ascending limb as we talked about. The distal convoluted tubules are responsible for the reabsorption of sodium, chloride, potassium.
calcium, magnesium, and bicarbonate. The collecting duct here is responsible for the reabsorption of sodium, chloride, urea, and water. Now different segments of the nephrons also allow for the secretion of things back inside the tubule from the circulation, specifically from the vasorector.
Now the proximal convoluted tubule allow for the secretion of creatinine, certain drugs, as well as hydrogen ions which are If you remember, these guys are the acidic ones. And the distal convoluted tubule allow for the secretion of hydrogen ions as well, as well as potassium. In the distal convoluted tubule, there's actually an important transporter, which is actually responsible for the exchange of sodium and potassium.
And this is how the potassium enters the tubule of the nephron. And this transporter is also a site where diuretics have an effect. Important things to take note here is that sodium and water play a key role in regulating our blood pressure.
Bicarbonate and hydrogen ions play a key role in acid-base balance, so maintaining the pH of our body. So let us now focus on each segment or each section of the nephron and look into it in a bit more detail. Firstly, focusing on the reabsorption. of things that occur in this area. So let's look at what happens in the proximal convoluted tubule and the ascending limb of the loop of Henle.
The lumen here refers to the tubule of the nephron, so inside the nephron tube. And the cells here refer to the cells which line up the tube of the nephron. And here is the vasorector, which is the circulation. In the proximal convoluted tubule, sodium get reabsorbed together with glucose. or amino acids.
The glucose or amino acid will then get reabsorbed back into circulation. There is another transporter which uses an exchanger, a sodium for hydrogen exchange. The sodium is reabsorbed inside the cell and then is exchanged with potassium via the sodium-potassium ATPase pump. The sodium-potassium ATPase pump is very important to remember and it is situated usually on the basal aspect of the cell, so closer towards circulation.
The proximal covalent tubule is important in regulating acid-base balance. It is a site where hydrogen can be secreted, as you can see here, but also it's a site where bicarbonate can be reabsorbed. What happens is a chemical reaction takes place. Hydrogen ions inside the lumen of the nephron reacts with bicarbonate ions to form...
carbonic acid, H2CO3. Through the enzyme carbonic anhydrase, or CA, carbonic acid gets converted to water, H2O, and carbon dioxide, CO2. Carbon dioxide is a gas which can diffuse back into the cell. Water is also present inside the cell, and so carbon dioxide can diffuse back into circulation because it's a gas.
Further, within the cell, the same reaction can take place with the same enzyme, carbonic anhydrase, which makes carbonic acid again. Carbonic acid can then become hydrogen ions and bicarbonate, and the cycle can continue. And that is why when you have an increase in carbon dioxide levels, you are more acidic, because there's a shift for more hydrogen ion production, and hence more acidity.
The hydrogen here can also come from circulation. Bicarbonate can be reabsorbed into circulation to increase the pH of blood to make it more alkali if it has to. And it does this with a symptoma, a type of transporter which brings back into circulation bicarbonate and sodium. And so bicarbonate reabsorption in the proximal convoluted tubule is sodium dependent.
It's important to note that other electrolytes are also reabsorbed in the proximal convoluted tubule, which I have not actually drawn. And these include calcium ions, for example, and majority of calcium is reabsorbed here. The ascending loop of Henle, not the descending loop of Henle, is a site where electrolytes are also reabsorbed.
There is a trisymptoma, a transporter which brings in one potassium, two chloride, and one sodium back inside the cell. The potassium and chloride then go through another symptom, which carries it back into circulation, while the sodium gets exchanged for potassium through, again, the sodium-potassium ATPase. And so you can see this sort of repetition with the sodium-potassium ATPase on the basal surface of the cell.
Important to note here on the apical surface, on the top of the cells of the loop of Henle, there are passive channels which allow sodium to enter the cell. from the lumen and also potassium to enter the lumen from the cell. Let us now focus on what happens in more detail at the distal convoluted tubule as well as the collecting duct. I wrote here distal convoluted tubule and collecting duct because these parts of the nephrons share some similar functions as well.
Again you have the lumen of the nephron tubule and the cells which line up the tubule and here is the vasorector. which is a circulation essentially. In the distal convoluted tubule, sodium and chloride get reabsorbed via a symptoma.
The chloride has its own channel from here back into circulation, while sodium actually obviously gets exchanged for potassium via, again, the sodium-potassium ATPase on the basal surface of the cell. The potassium can passively go through into the movement of the nephron from the cell. Magnesium and calcium reabsorption is thought to occur via paracellular roots, meaning in between the cells, in between the tight junctions, and is thought to occur via diffusion.
Now towards the collecting duct, there are important processes which take place. And it is here in particular where the last bit of reabsorption and secretion takes place before the final urine product is produced. From the lumen of the nephron, sodium is reabsorbed.
in exchange for hydrogen ions. And the reabsorption of sodium into circulation from inside the cell relies on hydrogen exchange. So now you have sodium in circulation and you have hydrogen in the lumen.
Whenever you have hydrogen in the lumen, you know acid-base balance stuff occurs depending on the pH of the blood. The acid-base regulation allows hydrogens to interact with bicarbonate ions to become carbonic acid. Then again to carbon dioxide and water. The carbon dioxide can diffuse easily inside the cell in and out because it's a gas.
Within the cell, however, carbon dioxide can react with water again, and the reverse reaction can take place. Water and carbon dioxide become carbonic acid, and then again to bicarbonate and hydrogen ions. Bicarbonate reabsorption into circulation uses an exchanger, a bicarbonate chloride exchanger. And so the big difference to the reabsorption of bicarbonate here is that it relies on chloride rather than sodium, and so it is not sodium dependent like the proximal convoluted tubule. Finally, within the distal convoluted tubule and the collecting duct, there are many channels which basically exchange sodium and potassium and not necessarily ATP dependent.
Rather, it is controlled by an important hormone called aldosterone. Aldosterone is a hormone produced and secreted by the adrenal glands, specifically the adrenal cortex. in response to low blood pressure.
An increase in circulating aldosterone means more sodium will be reabsorbed, which means more water will be reabsorbed to increase blood pressure. However, this means more potassium will also be secreted. There is a decrease in potassium reabsorption, which will cause hypokalemia, low potassium in the blood. Finally, let us focus on what happens only in the collecting ducts. The apical surface of the cells here lining the collecting ducts have these special channels called aquaporins, which allow for reabsorption of water.
This is the final concentrating area for urine, which occurs in the collecting duct. When the body reabsorbs water, it will increase the osmolality of urine. So it will...
increase the solute concentration in urine. The number of aquaporins in the collecting duct is controlled also by a hormone, and this hormone is called antidiuretic hormone, also known as vasopressin. This hormone targets the collecting ducts and essentially tells the cells here to make more aquaporins, which means with more aquaporins, it means more water will be retained in circulation. After all the reabsorption and secretion that takes place along the tubule of the nephron, urine is produced.
So what is in urine then? Well, it's mainly water, but it also contains nitrogenous waste, which is toxic if it stays in the body. It also contains lots of metabolites, and also can contain red blood cells and white blood cells, usually in extremely low numbers.
However, red blood cells and white blood cell numbers in urine can obviously be high, but this is... of course, when there is an infection or some form of pathology that is occurring along the urinary tract.