KIDNEY STRUCTURE

Each kidney consists of about one million functional units, called nephrons. The number of nephrons is individual determined and depending on many factors.  For the kidney to function properly, it is necessary to keep at least 30% of the nephrons in operation to work well. One nephron is composed of the kidney corpuscle and the tubule. The kidney corpuscle consists of a complex network of capillary loops called a glomerulus and an encompassing Bowman's capsule.

What is a Nephron??

Each kidney has one million of tiny filtering machines, called nephrons. These nephrons are like the microscopic heroes, tirelessly working 24/7 to ensure your blood is free from waste and excess fluids. You can learn more about its function here.

The glomerulus as a filtration unit

The kidney corpuscle (glomerulus+ Bowman’s capsule) has two poles: a vascular pole and a tubular pole. The arterioles from the kidney circulation enter and leave the glomerulus at the vascular pole. The glomerular filtrate leaves the Bowman's capsule into the proximal tubule at the tubular pole.

Preliminary filtration of urine takes place in the glomeruli of the kidney – in its capillary network (blood vessel). The wall of this capillary network has a three-layer structure and consists of the following layers:

  1. Endothelial Cells that pad the glomeruli capillaries from the inside.
  2. Glomerular Basement Membrane (GBM), a trilaminar structure composed principally of type IV collagen with laminin, heparan sulfate, and enactin as well as smaller components of other proteins and proteoglycans.
  3. Epithelium, a layer of cells called podocytes. They have long extensions called pedicels, after which they are named (podo-, -cyte). They cover the capillaries from the outside. The pedicels rest on the basement membrane. Between them there are diaphragm filtration gaps containing various proteins.

The glomerular filtration assembly:

The glomerular filtration assembly is composed of three main cellular barriers that are critical for the ultrafiltration process, the fenestrated endothelium, glomerular basement membrane and highly specialised podocytes.

GBM (Glomerular Basement Membrane) is not only a stabilizing element of the structure of capillaries and glomeruli, but also the only continuous, homogeneous layer of the filter barrier. Its thickness changes with age, in children it is from about 110 nm, in adults 310-350 nm; in males significantly thicker than in women.

► The general task of the GBM is to selectively pass only certain components of the blood toxins and metabolic products, while retaining the desirable components in blood circulation.

Possible and common structural changes in kindeys

Changes in the structure of the kidneys can be caused by many factors. In addition to age-related changes, various deformities or changes in the structural constitution of the kidneys can occur at birth or in the course of a person's life as a result of diseases. Here are some common examples.

Kidney Cyst

A Kidney cyst is a round fluid collection formed in or on the kidney. There are several types of cysts, and they can subvert the kidney structure and cause loss of kidney function. Cysts are usually small, and they can derive from any segment of the nephron (microscopic functional unit of the kidney), including the glomeruli.

Kidney Hyperechogenity

means an increased response (echo) during the ultrasound examination of the organ, which is generally displayed as increased brightness of the signal. There is increased correlation between increased echo-genicity of the kidney and chances of underlying kidney disease. The kidney hyperechogenity in foetus can be secondary to multiple microscopic kidney cysts, kidney dysplasia or tubular dilation. Other causes of foetal hyperechogenic kidneys include polycystic kidney diseases.

Kidney dysplasia/multicystic dysplastic kidney

is a condition where the internal structures of one or both fetus’ kidneys don’t develop normally during pregnancy. Babies with severe kidney dysplasia affecting both kidneys may not survive birth. Children with dysplasia in only one kidney have normal kidney function if the other kidney is normal. Those with mild dysplasia of both kidneys may develop impairment of kidney function and may need kidney replacement therapy.

Hydronephrosis

is the swelling of the kidneys when urine outflow is obstructed in any of part of the urinary tract, for example due to subpelvine stenose or urethral valve. Hydronephrosis implies that a ureter and the kidneys pelvis (the connection of the ureter to the kidney) are over-filled with urine.

Ureteropelvic junction obstruction (subpelvic stenosis)

is a condition where the outflow of urine from the renal pelvis is obstructed/blocked. This blockage occurs at the junction between the ureter and the kidney and prevents the normal flow of urine out of the kidney into the bladder, this can lead to hydronephrosis and further to kidney damage.

Ureterocele

is a congenital abnormality found in the ureter. In this condition the distal ureter balloons at its opening into the bladder, forming a sac-like pouch.

Horseshoe kidney

is the fusion of both kidneys together during prenatal development. Their lower parts fuse together forming a ‘U’ shape which gives it the name “horseshoe”. Horseshoe kidney can occur alone or associated with other disorders. The fused part is the isthmus of the horseshoe kidney. The abnormal anatomy can affect kidney drainage resulting in increased frequency of kidney stones and urinary tract infections.

Crossed fused kidney ectopia/malposition

occurs when both kidneys develop on the same side of the spine. In many cases, the two kidneys are fused together, but they maintain their own vessels and ureters.