Kidney

The kidney is responsible for many different processes in the body – such as getting rid of waste/water, regulating blood pressure and acid-base balance.

The kidney is made up of several different structures, we are going to look at;

1. The glomeruli
2. The tubules
3. Juxtaglomerular apparatus

The glomerulus filters the blood, and initially removes fluid/substances that the body wants to get rid of.

Many diseases affect the glomerulus, and so it is important to understand the normal structure in order to understand and identify how it can go wrong.

Here is a picture with the glomeruli pointed out.

 

Blood flows through the capillaries lined by endothelial cells. Fluid (containing certain smaller molecules) passes through the endothelium (either directly through the cytoplasm of the endothelial cells or through fenestrations).

 

 

Glomerular tufts are largely made up of a network of capillaries. The endothelial cells lining these capillaries are thin so the nuclei bulge slightly into the capillary lumen.

 

After passing through the endothelial cells fluid then reaches and passes through the basement membrane.

 

On the outside of the basement membrane, there are epithelial cells that have little bits sticking out, these are called podocytes. The podocytes mean that only part of the epithelial cell membrane is in contact with the basement membrane.

 

Once through the epithelial cells, the fluid has reached Bowman’s space (also referred to as the urinary space). From Bowman’s space, the urine then moves to the convoluted tubule.

 

The glomerulus also has another type of cell, the mesangial cells, which sit between the capillaries in an area referred to as the mesangial matrix or mesangium. They have long processes which extend between the capillary endothelial cells and the basement membrane. The mesangial cells are related to macrophages and have phagocytic properties, enabling them to mop up various bits of debris.

 

 

Normally there is only a small amount of mesangium – however, in certain diseases, this can become more prominent.

Following on from the glomerulus is the renal tubule. Tubules make up most of the kidney and consist of; the proximal convoluted tubule, the loop of Henle (descending and ascending), distal convoluted tubule, and the collecting duct.

 

Each tubule consists of a lumen and an epithelium. The tissue between the tubules is called interstitial tissue.

 

The proximal convoluted tubule consists of an empty space surrounded by cuboidal or low columnar cells with abundant eosinophilic cytoplasm and a microvillous (or brush) border which protrudes into the lumen of the tubule. These features are essential as the main role of the proximal tubule is reabsorption. The proximal tubule is responsible for the reabsorption of many substances including; water, sodium, potassium, chloride, bicarbonate, urea, amino acids and glucose. The proximal tubule also plays a role in secretion, mainly of ammonium and medications.

 

The thin loop of Henle is made up of simple squamous epithelium. The cells are small and flatten, and so thin that the nuclei stick out into the lumen of the tubule. The thin loop of Henle connects the thick (ascending and descending) segments of the loop of Henle. The thick loops of Henle (both ascending and descending) are lined by simple cuboidal epithelium. The main function of the loop of Henle is to concentrate the urine by reabsorbing water and sodium. The loop of Henle also plays a role in reabsorption, mainly of calcium and magnesium.

 

The distal convoluted tubule is similar to the proximal convoluted tubule. However, it contains cells that are smaller, more closely packed and are less abundant in cytoplasm than the proximal convoluted tubule. Meaning, there are fewer mitochondria and eosinophils, and the cells do not protrude as far into the lumen. The distal convoluted tubule also does not have a brush border. The distal convoluted tubule is responsible for the absorption of ions (including sodium, chloride and calcium) and secretion of potassium.

 

The collecting duct has cuboidal cells and mainly acts to reabsorb water. The collecting duct also plays a small role in the regulation of electrolytes such as chloride and potassium.

The juxtaglomerular apparatus consists of different types of cells; including the macula densa cells, and the juxtaglomerular cells.

The macula densa is a point in the distal convoluted tubule where it is met by the thick ascending limb of the loop of Henle. It is an area of closely packed, specialised cells. The macula densa cells are sensitive to the sodium chloride concentration and increased renin release from the juxtaglomerular apparatus in response to a drop in sodium chloride.

 

The juxtaglomerular cells (also known as granular cells) also produce renin which is released in certain circumstances – such as when blood pressure is low, activating the RAAS.

 

In summary, the kidney is made up of several different structures which are all specialized for their function.

The glomerulus consists of:

1. Endothelial cells
2. Basement membrane
3. Epithelial cells
4.  Bowman’s space
5. Mesangial cells

 

The renal tubule consists of:

1. Proximal convoluted tubule
2. Loop of Henle (descending and ascending)
3. Distal convoluted tubule
4. Collecting duct.

 

The juxtaglomerular apparatus includes:

1. Macula densa cells
2. Juxtaglomerular cells