Function of the kidney

Part of the Kidney	Description
Renal hilus	An indentation near the center of the concavity of the kidney where the renal vein and ureter leave the kidney, and the renal artery enters the kidney.
Renal capsule	A tough, fibrous membrane surrounding the kidney. It consists of dense, irregular connective tissues that protect and help maintain the kidney's shape. It is also surrounded by fatty tissue to protect the kidney from damage.
Renal cortex	The outer reddish part of the kidney with a smooth texture. It houses the Bowman's capsule, glomeruli, proximal and distal convoluted tubules, and blood vessels.
Renal medulla	The inner striated red-brown part of the kidney.
Renal pyramids	Striped and triangular structures within the medulla, made of straight tubules and corresponding blood vessels.
Renal pelvis	The funnel-shaped cavity that receives urine drained from the nephrons through the collecting ducts and papillary ducts.
Renal artery	The blood vessel that delivers oxygen-rich blood to the kidney. It enters through the hilus and divides into smaller arteries that branch into afferent arterioles serving each nephron.
Renal vein	The blood vessel that receives deoxygenated blood from the kidney and returns it to the systemic circulation.
Afferent arteriole	The blood vessel that delivers oxygen-rich blood to the glomerulus under high pressure.
Efferent arteriole	The blood vessel that receives oxygenated blood from the glomerulus.
Kidney nephrons	The functional units where the kidney's main functions are performed. Each kidney contains about a million nephrons.
Collecting duct	Part of the nephron that collects urine and drains into papillary ducts, minor calyx, major calyx, and finally into the ureter and urinary bladder.
Ureter	A tubular structure that conveys urine from the pelvis of the kidney to the urinary bladder.

The nephron is the kidney's functional unit that is involved in production of urine in the process of removing waste and excess substances from the blood. Generally nephron is responsible for the filtration, excretion and re-absorption of most of the water and other materials. Each kidney has more than a million nephrons in the renal cortex, which gives it a granular appearance on sagittal section. Nephrons are used to separate water, ions and small molecules from the blood molecule, and filter out wastes and toxic materials, then it selectively returns the needed molecules to the blood. The most primitive nephrons (pronephros) are found in the kidneys of primitive fish, amphibian larvae, and embryos of more advanced vertebrates. The nephrons found in the kidneys of amphibians and most fish, and in the late embryonic development of more advanced vertebrates, are only slightly more advanced in structure (mesonephros). The most advanced nephrons occur in the adult kidneys, or metanephros of land vertebrates, such as reptiles, birds, and mammals.

There are two types of nephrons, namely cortical nephrons, (which are found deep in the renal cortex) and the juxtamedullary nephrons, which make up about 15 percent of total nephrons and lie close to the medulla. The nephrons consist of a renal corpuscle, a tubule, and a capillary network. These originate from the small cortical arteries. Each renal corpuscle is composed of a glomerulus (a network of capillaries) and a Bowman's capsule (the cup- shaped chamber that surrounds it).

The Bowman's capsule connects to a long convoluted renal tubule which is divided into three functional parts. These consist of the proximal convoluted tubule, the loop of Henle (nephritic loop) with its descending and ascending limbs, and the distal convoluted tubule, which empties into the collecting ducts.

Urea is the nitrogenous waste product of humans and other land living mammals.

The body is unable to store excess amino acids taken in the diet. Those not immediately needed for protein synthesis or making sugar must be removed by the process called deamination, which is followed by urea formation in the liver cells. The process of urea formation occurs in the urea cycle which is also called ornithine cycle and involves the following stages:

1. Formation of carbamoyl phosphate Before the cycle, ammonia (NH3) from metabolism of nitrogen containing compounds combine with carbon dioxide (CO2) gas from respiration in a solution form (ammonium ions and bicarbonate ions respectively) resulting into the formation of carbamoyl phosphate, by the help of the enzyme carbamoyl phosphate synthetase-I. The reaction occurs in the mitochondria of the liver cells, and requires 2 ATP molecules.

2. Synthesis of citrulline The carbamoyl phosphate formed in the first step enters the ornithine cycle and combine with ornithine resulting in the synthesis of citrulline, aided by an enzyme citrulline synthase or ornithine transcarbamoylase. In the reaction the phosphate group is released. Citrulline can easily pass through the mitochondrial membrane, thus it diffuses from the mitochondrion into cytosol (cytoplasm) of liver cells.

3. Synthesis of argininosuccinate In the cytosol, citrulline combines with the amino group of aspartate under condensation reaction catalyzed by enzyme argininosuccinate synthetase to form argininosuccinate. It requires ATP which is hydrolysed to adenosine monophosphate (AMP) resulting in the utilization of two high energy bonds. Magnesium ions (Mg2+) act as cofactors. This reaction incorporates the second nitrogen from aspartate.

4. Cleavage of argininosuccinate The cleavage of arginisuccinate involves the enzyme argininosuccinase, an intermediate enzyme in the urea synthesis pathway whose function is imperative to the continuation of the cycle. It acts reversibly to cleave argininosuccinate into a free arginine and fumarate. The arginine continues with the cycle in the other stage, whereas fumarate enters the Tricarboxylic Acid (TCA) cycle, which is also known as Kreb's cycle. The linkage between TCA cycle and urea cycle is known as the Kreb's bi- cycle.

5. Cleavage of arginine Arginine is hydrolysed into ornithine and urea under the influence of the enzyme arginase; hence, arginine is known as a semi-essential amino acid. Though it is synthesised in the body, it is not available for protein synthesis. Ornithine is regenerated in this step and the urea cycle completes by the formation of urea. The ornithine produced is transported back to the mitochondria to start the cycle again while urea is transported to the kidney through blood vessels to be excreted. Thus the urea cycle brings two amino groups (NH2) and hydrogen carbonate ions (HCO3-) together to form urea.

The urinary tract consists of the kidneys, ureters, bladder and urethra. Urinary disorders include any infections or conditions that affect any of these parts of the urinary system or their functions.

The following are some of the common urinary disorders in human beings:

1. Uremia This condition results from toxic effects of abnormally high concentrations of urea in the blood resulting into kidneys' failure to expel it via urine. The end products of protein metabolism accumulate in the blood but are normally filtered out when the blood passes through the kidneys. However, urea accumulation in the blood is comparatively high in uremia victims. In such patients, urea can be removed by the process called haemodialysis.

2. Renal failure (RF) or kidney failure Renal failure is a decrease or cessation of glomerular filtration in humans (it is a partial or complete loss of kidney function). Kidney failure can be either acute or chronic. In acute renal failure (ARF), both kidneys abruptly stop working. The main feature of ARF is either oligouria (scanty urine production) in which the daily urine output is less than 250 ml or anuria in which daily urine output is less than 50 ml. The causes of renal failure may be low blood volume (atherosclerosis), decreased cardiac output, damaged renal tubules, kidney stones, nephritis, and some excessive use of antibiotics.

3. Urinary tract infections (UTI) In humans, UTI is caused by the invasion of microorganisms, usually bacteria, into the urethra and bladder. The most common UTI cases that affect the bladder and urethra are.

   • Infection of the bladder (cystitis). This type of UTI is usually caused by bacteria, normally Escherichia coli (E. coli); a type of bacteria commonly found in the gastrointestinal tract (GIT). All women are at risk of cystitis because of their anatomy, specifically the short distance from the urethra to the anus and the urethral opening to the bladder.

   • Infection of the urethra (urethritis). This type of UTI can occur when GIT bacteria spread from the anus to the urethra. Also, because the female urethra is close to the agina, sexually transmitted infections such as herpes, gonorrhea, chlamydia and mycoplasma can cause urethritis.

4. Urinary tract obstruction This is due to blockage or constriction at any point in the urinary tract. This impedes the normal flow of urine and causes urine to be retained in the bladder or kidneys.

   Obstruction causes urine to become blocked up into the kidneys, the condition is known as hydronephrosis. Obstructions in the urinary tract causes distension of the walls of the bladder, ureter, urethra, and kidneys. This condition may stem from Sexually Transmitted Diseases (STDs) such as syphilis and gonorrhoea. Its preventive measures include prevention against STDs and seeking for medical assistance whenever sensing signs of STDs.