There are a variety of causes to kidney failure in cats. For instance, some cats are born with poorly constructed or functioning kidneys and never reach totally optimum health. But to first understand why kidney failure occurs, you must first understand the components of the kidney.
Normal Kidney Physiology
The kidneys receive about 20 percent of the heart's blood output and play a vital role in keeping the cat in normal metabolic balance. When one or both kidneys are malfunctioning, it can lead to kidney failure. This condition can either be due to acute or chronic reasons.
The glomerular blood vessels have a large endothelial surface which allows for the active and passive transport of many chemicals into and out of the kidneys.
Normal kidney function involves the following responsibilities, among others:
- Regulating the amount of fluid in the spaces surrounding the body's cell. This is called extracellular fluid volume regulation.
- Regulating the amounts and types of solids in the blood in order to keep blood concentration within normal limits. This is called blood osmotic pressure regulation.
- Regulating the acid-base balance of the animal through retention or elimination of specific ions in the blood. This function keeps the pH (amount of acidity) of the blood and body fluids within strict normal ranges.
- Removing metabolic waste products such as uric acid and also molecular foreign substances detoxified by liver.
- Reacting to Aldosterone (ADH) produced in the adrenal glands. The major target of aldosterone is the distal tubule of the kidney, where it stimulates exchange ofwater back into the blood.
- Production of Erythropoetin, a chemical effecting red blood cell production.
The nephron is the structural and functional unit in the kidney. A nephron consists of a glomerulus in a capsule, proximal convoluted tubule, loop of Henle, and distal convoluted tubule which leads to a collecting duct. The collecting duct empties into the renal pelvis.
The functional unit of the kidney -- the real mechanism whereby the kidney does most of its prescribed tasks -- is called the nephron (pictured right). The nephron is a delicate, structurally complicated microscopic collection of tiny tubes (capillary beds) which are tasked with regulating the concentration of water and soluble substances like sodium salts by filtering the blood, reabosring vital components, and excreting the rest as urine.
The unit consists of the:
- Glomerulus - a ball of capillaries with a large surface area at which multiple interchanges of fluids and dissolved elements occurs.
- Bowman’s Capsule - the proximal end of a tubule that surrounds glomerulus.
- Proximal convoluted tubule - leads to the Loop of Henle, which is situated in the medullary area of the kidney. (There is an ascending limb and a descending limb, each of which has particular and unique functions.)
- Distal convoluted tubule - leads into collecting ducts.
- Pelvis - is an enlargement at the distal end of the collecting ducts that provides a common area of urine collection before the urine passes down the ureter into the bladder.
The glomeruli are found in the outer area of the kidney called the cortex. Each glomerulus is surrounded by a "Bowman's Capsule". Most of the fluid that passes into the Loop of Henle in the cortex is reabsorbed in the medulla back into the blood.
The medullary area of the kidney is fed by tiny arterioles. Any damage to glomeruli affecting efferent arteriolar blood flow will also cause damage in the tubules located in the medulla. Anything that adversely impacts the blood flow through the medulla can have serious consequences for the tubular structures.
The medulla is slightly less vascular than the cortex. The renal tubules, which are responsible for water loss and coservation, make up most of the medullary tissue have high metabolic rates and therefore high nutritional requirements. Filtered water containing waste products (urine) are then passed into the renal pelvis, followed by the ureter.
In addition to waste management the renal medulla assists in regulation of blood pressure, the elimination of toxins and the production of hormones such as erythropoietin.
The renal pelvis collects the kidney filtrate and funnels the urine fluid into the ureter that leads to the bladder. The pelvic area of the kidney often is the site of kidney stones and can be a reservoir of infection once microorganisms reach this area of the kidney.