What are Nephrons?

Nephrons = filtration units

Mammals have paired kidneys. In cats, each is composed of about 200,000 filtration units called nephrons. Individual nephrons can be subdivided into four different regions: the renal corpuscle; the proximal convoluted tubule and straight tubule; the descending and ascending thin limbs of the loop of Henle; the thick ascending Loop of Henle and distal convoluted tubule; and the collecting duct, which transports urine into the renal pelvis. The loop of Henle is formed by the proximal straight tubule, the descending thin limb, the ascending thin limb and the thick ascending limb. The primary role of the loop of Henle is to concentrate the urine and reclaim water for the body
(= reabsorption).

Construction of a Nephron

Renal corpuscle

The renal corpuscle is the place of origin of the initial urine (=primary urine). It is the site of filtration from the blood of certain substances and water to produce primary urine. The renal corpuscle consists of the glomerulus, which is enclosed by the Bowman’s capsule. The glomerulus resembles a ball of blood vessels. The afferent arteriole branches, entering the glomerulus where the blood is filtrated, removing substances for excretion in the urine in addition to water and solutes, including electrolytes.  The filtrated blood exits the renal corpuscle via the efferent arteriole. The Bowman capsule collects the filtrated substances (including electrolytes) and water, and conveys them to the proximal tubule, the longest part of the nephron.


Proximal convoluted tubule and straight tubule

This region of the nephron starts with the proximal convoluted tubule before becoming the proximal straight tubule, which is the first section of the loop of Henle. In the proximal tubule, beneficial substances such as sodium, sugar (glucose), bicarbonate (the blood’s buffer substance for maintaining pH) and amino acids are recovered (= reabsorbed) from the urine, and thus retained for the body. At the same time, substances are also secreted into the urine at the proximal tubule. These include uric acid.


Descending and ascending thin limbs of the loop of Henle

This region is responsible for recovering water (= reabsorption) and, hence, for further concentrating the urine. It forms the loop section of the loop of Henle, connecting the proximal and distal tubules.


Thick ascending limb and distal tubule

This region is a virtual mirror image of the proximal tubule and consists of the thick ascending limb (straight) of the loop of Henle and the distal convoluted tubule. It is responsible for recovering water and salt (sodium chloride = common salt) from the urine. It flows directly into the connecting tubule, which transports the urine onwards towards the renal pelvis.


The progression of the disease is unstoppable – CKD becomes progressively worse

Chronic kidney disease in cats has no cure. It is invariably progressive, leading to an increasing loss of nephrons (= kidney filtration units), which are then replaced by scar tissue. The result is increasing deterioration of kidney function. However, clinical symptoms are often first observed months or even years later.