In this lesson, you'll learn about the supportive structures of the nephron, including the juxtaglomerular apparatus, juxtaglomerular cells, macula densa, and mesangial cells. You'll also explore enzymes and hormones like renin and erythropoietin and learn why they are so important to the body.
The Kidney and Red Blood Cells
You've probably heard that your kidneys are involved in the filtration of blood in order to produce urine. You're also likely aware that they are responsible for the regulation of water balance throughout your body. However, not many people know that your body would have trouble producing red blood cells without the kidneys. You might be thinking, 'Red blood cells? They are made in the bone marrow, not the kidneys!' Stick around to find out how specific cells in your kidneys are actually involved in all of these processes and why the kidneys are important in the prevention of anemia.
In between the renal corpuscle (the structure involved in the filtration of your blood) and the distal convoluted tubule are three main types of cells collectively referred to as the juxtaglomerular apparatus. Again, the juxtaglomerular apparatus is a collection of cells responsible for the hormonal regulation of the body's blood pressure, the kidney's blood flow, and the glomerular filtration rate.
It's easy to remember where the JG apparatus, short for 'juxtaglomerular apparatus,' is located. 'Juxta' means 'next to' and 'glomerular' refers to the 'glomerulus' - hence we get 'juxtaglomerular,' or an apparatus next to the glomerulus. You can think of the JG apparatus as a control center that allows a certain amount of blood to flow through the glomerulus and hence influence the glomerular filtration rate.
The location of the juxtaglomerular cells in the kidney
This control center has three main cells that are kind of like its employees. One of the employees is called a juxtaglomerular cell, which is sometimes also known as a 'granular cell.' The juxtaglomerular cells are cells of the juxtaglomerular apparatus that secrete renin, an enzyme that helps to increase your body's blood pressure once it is released.
These cells are mainly part of the afferent arterioles entering the glomerulus. If they sense decreased blood pressure at the afferent arteriole, are stimulated by the sympathetic nervous system, or are signaled about low sodium chloride concentrations by the macula densa cells of the JG apparatus, they will secrete renin to help increase the body's blood pressure.
Macula Densa Cells
The macula densa cells I just mentioned are another employee of the control center, the JG apparatus. The macula densa cells are cells that are also part of the distal convoluted tubule. This positioning is really important, for the macula densa cells are cells that monitor changes in sodium concentration in the kidney's distal convoluted tubules in order to regulate glomerular filtration rate.
When your blood is filtered by the glomerulus, small molecules like water, sodium, and chloride flow through the nephron of your kidney. The nephron has many structures, including the glomerulus and distal convoluted tubule involved in the JG apparatus.
Since water essentially follows sodium in your body like a tail follows its dog, the macula densa cells, by implication, can tell how much water is in the distal convoluted tubules by sensing how much sodium is in the distal convoluted tubules. I hope that wasn't too convoluted of an explanation.
The macula densa cells monitor sodium levels in the distal convoluted tubules
If there is too much sodium in the filtrate, the macula densa cells start to believe the glomerular filtration rate is too fast, leading to too much water being excreted by the body. This is bad, as it can lead to things like dehydration!
In this case, the macula densa cells cause the increase of a substance called adenosine, which constricts the afferent arteriole. Since the afferent arteriole constricts, little blood can enter the glomerulus. This then decreases the pressure inside of the glomerulus and therefore decreases the glomerular filtration rate back to normal in order to try and preserve fluid in your body.
Conversely, if not enough sodium is sensed by the macula densa cells, they signal the juxtaglomerular cells to release renin. Renin will then cause another substance to constrict the efferent arteriole. This means blood can enter the glomerulus through the afferent arteriole while a lot less is allowed to leave through the efferent arteriole, causing a backup of blood in the glomerulus. This backup of blood, like a traffic jam of sorts, causes the pressure inside the glomerulus to increase. The pressure forces a lot more blood to be filtered per unit of time and therefore increases the glomerular filtration rate back into a normal range. The process I just described is often called 'tubuloglomerular feedback.'
The final employee of the control center, the JG apparatus, is called the extraglomerular mesangial cell; this cell is sometimes referred to as the 'Lacis cell.' The extraglomerular mesangial cells are juxtaglomerular apparatus cells that may secrete a hormone called erythropoietin, which is critical for the production of red blood cells.
Renal failure can cause anemia because of decreased levels of erythropoietin or EPO
Basically, we're not 100% sure if this is the cell that produces erythropoietin, or EPO for short. However, one thing to keep in mind for sure is that somewhere in the kidneys this hormone is produced and secreted. Therefore, chronic renal failure has been associated with anemia, or decreased amounts of red blood cells, in part due to the decreased ability of the body to produce EPO and therefore stimulate the bone marrow to make red blood cells.
We covered a lot of information, so let's summarize the main points. Your kidneys have a hormonal control center called the juxtaglomerular apparatus, which is a collection of cells located near the glomerulus that are responsible for the hormonal regulation of the body's blood pressure, the kidney's blood flow, and the glomerular filtration rate.
The JG apparatus has three main cells. One is appropriately called the juxtaglomerular cell. The juxtaglomerular cells are cells of the juxtaglomerular apparatus that secrete renin, an enzyme that helps to increase your body's blood pressure once it is released.
Another cell that is part of the JG apparatus is called the macula densa cell. The macula densa cells are cells that are also part of the distal convoluted tubule. This positioning is really important, for the macula densa cells are cells that monitor changes in sodium chloride concentration in the kidney's distal convoluted tubules in order to regulate glomerular filtration rate.
The final cell of the JG apparatus is called the extraglomerular mesangial cell. The extraglomerular mesangial cells are juxtaglomerular apparatus cells that may secrete a hormone called erythropoietin, which is critical for the production of red blood cells.
Following this video, you should be able to explain the function of the juxtaglomerular apparatus and its three main cells: juxtaglomerular, macula densa and extraglomerular mesangial cells.