STRUCTURE AND FUNCTIONS OF THE KIDNEYS

The kidneys are organs whose functions are excretion, osmoregulation and regulation of pH. Kidneys are located at the back of the abdominal cavity. Each kidney receives oxygenated blood from renal artery, while deoxygenated blood leaves through the renal vein.

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Urine is carried by the ureter from the kidney to the bladder, which temporarily stores it. From the bladder, the urine is released to the outside via the urethra. The opening from the urethra is controlled by a ring-like sphincter muscle.

A longitudinal section of the kidney shows three distinct regions: a darker outer cortex, a lighter inner medulla and the pelvis. The pelvis is a collecting space leading to the ureter which takes the urine to the bladder from where it is eliminated through the urethra.

The Nephron

A nephron is a coiled tubule at one end of which is a cup-shaped structure called the Bowman’s capsule. The capsule encloses a bunch of capillaries called the glomerulus. The glomerulus receives blood from an afferent arteriole a branch of the renal artery.

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Blood is taken away from the glomerulus by efferent arteriole leading to the renal vein. The Bowman’s capsule leads to the proximal convoluted tubule that is coiled and extends into a U-shaped part called loop of Henle. From the loop of Henle is the distal convoluted tubule that is also coiled.

This leads to the collecting duct which receives contents of many nephrons. Collecting ducts lead to the pelvis of the kidney.

Mechanism of Excretion

Excretion takes place in three steps:
Filtration, reabsorption and removal. Filtration
The kidneys receive blood from renal artery a branch of the aorta.
This blood is rich in nitrogenous waste e.g. urea.
It contains dissolved food substances, plasma proteins,hormones and oxygen.
Blood flow in capillaries is under pressure due to the narrowness of the capillaries.
The afferent arteriole entering the glomerulus is wider than the efferent arteriole leaving it.
This creates pressure in the glomerulus.
Due to this pressure, dissolved substances such as urea, uric acid, glucose, mineral salts and amino acids are forced out of the glomerulus into the Bowman’s capsule.
Large sized molecules in the plasma such as proteins and red blood cells are not filtered out because they are too large.
This process of filtration is called ultra-filtration or pressure filtration and the filtrate is called glomerular filtrate.

Selective Reabsorption

As the filtrate flows through the renal tubules the useful substances are selectively reabsorbed back into the blood. In the proximal convoluted tube all the glucose, all amino acids and some mineral salts are actively reabsorbed by active transport. The cells lining this tubule have numerous mitochondria which provide the energy needed.

Cells of the tubule have microvilli which increases the surface area for re-absorption. The tubule is coiled, which reduces the speed of flow of the filtrate e.g. giving more time for efficient re-absorption. The tubule is well supplied with blood capillaries for transportation of reabsorbed substances.

The ascending loop has thick wall and is impermeable to water. Sodium is actively pumped out of it towards the descending loop. As glomerular filtrate moves down the descending loop, water is reabsorbed into the blood by osmosis in the distal convoluted tubule and in the collecting duct. Permeability of the collecting duct and proximal convoluted tubule is increased by anti-diuretic hormone (ADH) whose secretion is influenced by the osmotic pressure of the blood.

The remaining fluid consisting of water, urea, uric acid and some mineral salts is called urine. The urine is discharged into the collecting d ct and carried to the pelvis. The loop of Henle is short in semi¬-aquatic mammals, and long in some mammals like the desert rat.

Removal

The urine is conveyed from the pelvis to the ureter.
The ureter carries the urine to the bladder where it is stored temporarily and discharged to the outside through the urethra at intervals.

Common Kidney Diseases

Uraemia

This is a condition in which concentration of urea in the blood.
It may be due to formation of cysts in tubules or reduction in blood supply to the glomeruli as a result of contraction of renal artery.

Symptoms

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Symptoms include yellow colouration of skin, smell of urine in breath, nausea and vomiting.
Treatment includes dialysis to remove excess urea and a diet low in proteins and salts especially sodium and potassium.

Kidney Stones

Kidney stones are solid deposits of calcium and other saIts.
They are usually formed in the pelvis of the kidney where they may obstruct the flow of urine.

Causes: the stones are formed due to crystallization of salts around pus, blood or dead tissue.

Symptoms: include blood in urine, frequent urination, pain, chills and fever. Severe pain when urinating.

Treatment