The kidneys are a pair of bean-shaped organs located behind the abdominal cavity that filter the blood and produce urine to remove waste and regulate fluid balance. Each kidney contains approximately one million nephrons, the functional units that filter blood. Urine is formed through a three step process - filtration in the glomerulus, reabsorption of nutrients and water in the renal tubules, and secretion of waste products. The kidneys maintain electrolyte and acid-base balance, produce hormones, and regulate blood pressure. Kidney diseases can damage the nephrons and impair waste removal, leading to a buildup of toxins.

1. Kidney System:
The Kidney serve the body as a natural filter of the blood and remove wastes that
are excreted through the urine. They are responsible for the reabsorption of
water, glucose and amino acids and maintain the balance of these molecules in
the body. In addition, the kidneys produce hormones including calcitriol,
erythropoietin, and the enzyme renin, which are involved in renal and
hematological physiological processes.
The kidneys are a pair of bean-shaped, brown organs about the size of fist. They
are covered by the renal capsule, which is a tough capsule of fibrous connective
tissue. Adhering to the surface of each kidney there are two layers of fat to help
cushion them.

2. The renal cortex, renal medulla, and renal pelvis are the three main internal
regions found in a kidney.
Nephrons- the functional unit of kidney located in the medulla and receive
fluid from the blood vessels. Nephrons are the urine-producing structures of
the kidneys.
The renal cortex produces erythropoietin.
The kidneys are made up by three external layers:
i) The renal fascia (the outermost layer)
ii) The perirenal fat and
iii) The renal capsule.
The renal capsule is the blood-filtering region of the nephron.
The renal pelvis contains a hilium in which the renal artery, renal vein and
renal nerves enter the kidney and the ureter leaves the kidney.

3. • The kidneys are at the back of the abdominal cavity, with one sitting on each
side of the spine.
• The right kidney is generally slightly smaller and lower than the left, to make
space for the liver.
• Each kidney weighs 125–170 grams in males and 115–155 grams in females.
• The adrenal glands lay on top of the kidneys.
• Inside the kidneys there are pyramid-shaped lobes. Each consists of an outer
renal cortex and an inner renal medulla. Nephrons flow between these
sections.
• Blood enters the kidneys through the renal arteries and leaves through the
renal veins.
• The kidneys are relatively small organs but receive 20–25 percent of the heart's
output.
• Each kidney excretes urine through a tube called the ureter that leads to the
bladder.

7. Renal medulla: The inner-most region of the kidney, arranged into pyramid-like structures,
that consists of the bulk of nephron structure.
Renal cortex: The outer region of the kidney, between the renal capsule and the renal
medulla, that consists of a space that contains blood vessels that connect to the nephrons.
Nephron: The basic structural and functional unit of the kidney that filters the blood in
order to regulate chemical concentrations and produce urine.

10. Functions of the kidney:
The primary function of the kidney is to make urine and purify the blood. Each
kidney removes waste materials, and other chemicals which are not required by
the body. Most important functions of the kidney are described below.
a) Removal of waste products:
• Purification of blood by removal of waste products is the most important
function of the kidney.
• The food that we consume contains protein. Protein is necessary for the
growth and repair of the body. But as protein is utilized by the body it
produces waste products. Accumulation and retention of these waste products
is similar to retaining poison inside the body. Each kidney filters blood, and
toxic waste products which are eventually excreted in the urine.
• Creatinine and urea are two important waste products that can easily be
measured in the blood. Their “values” in blood tests reflects the function of
the kidney. When both the kidneys fail, value of creatinine and urea will be
high in blood test.

11. b) Removal of excess fluid:
• The second most important function of the kidney is the regulation of fluid balance by
excreting excess amount of water as urine while retaining the necessary amount of
water in the body, that is essential for living. When the kidneys fail they lose the ability
of removing this excess amount of water. Excess water in the body leads to swelling.
c) Balance minerals and chemicals by reabsorption:
• The kidneys play another important role of regulating minerals and chemicals like
sodium, potassium, hydrogen, calcium, phosphorus, magnesium and bicarbonate and
maintains normal composition of body fluid.
• Changes in the sodium level can affect person’s mental state, while changes in the
potassium level can have serious adverse effects on the rhythm of the heart as well as
functioning of the muscles. Maintenance of normal level of the calcium and phosphorus
is essential for healthy bones and teeth.
d) Control of blood pressure
• The kidneys produce different hormones (renin, angiotensin, aldosterone, prostaglandin
etc.) which help to regulate water and salt in the body and plays vital roles in the
maintenance of good blood pressure control. Disturbances in hormone production and
regulation of salt and water with kidney failure can lead to high blood pressure.

12. e) Red blood cells production:
• Erythropoietin is another hormone produced in the kidneys, it plays an
important role in the production of red blood cells (RBC). During kidney failure,
production of erythropoietin is decreased, which in turn leads to decreased
production of RBC resulting in low hemoglobin (anemia). This is the reason why
in patients with kidney failure, the hemoglobin count does not improve despite
supplementation with iron and vitamin preparations.
f) To maintain healthy bones:
• The kidneys convert vitamin D into its active form which is essential for the
absorption of calcium from food, growth of the bones and teeth and keep the
bones strong and healthy. During kidney failure, decreased active vitamin D
leads to decreased growth of bones and become weak. Growth retardation
may be sign of kidney failure in children.

13. Purification of blood and formation of Urine:
• In the process of blood purification, the kidneys retain all necessary
substances and selectively remove excess fluid, electrolytes and waste
products.
• Every minute, 1200 ml of blood enters the kidneys for purification, which
is 20% of the total blood pumped by the heart. So, in one day, 1700 liters
of blood is purified.
• This process of purification occurs in small filtering units known as
nephrons.
• Each kidney contains about one million nephrons, and each nephron is
made up of glomerulus and tubules.
• Glomeruli are filters with very tiny pores with the characteristic of
selective filtration. Water and small-sized substances are easily filtered
through them. But larger-sized red blood cells, white blood cells,
platelets, protein etc. cannot pass through these pores. Therefore, such
cells are normally not seen in the urine of healthy people.

14. • The kidney’s chief function is to remove waste and harmful products and excess
water in the form of urine.
• The first step of urine formation occurs in the glomeruli, where 125 ml per
minute of urine is filtered. In 24 hours, 180 liters of urine is formed. It contains
not only waste products, electrolytes and toxic substances, but also glucose and
other useful substances.
• Each kidney performs the process of reabsorption with great precision. Out of
180 liters of fluid that enters the tubules, 99% of fluid is selectively reabsorbed
and only the remaining 1% of fluid is excreted in the form of urine.
• By this intelligent and precise process, all essential substances and 178 liters of
fluid are reabsorbed in the tubules, whereas 1-2 liters of fluids, waste products,
and other harmful substances are excreted.
• Urine formed by the kidneys flow to the ureters, and passes through the
urinary bladder and is finally excreted out through the urethra.

15. Formation of Urine:
Filtration, Reabsorption, Secretion: The Three Steps of Urine Formation
The kidneys filter unwanted substances from the blood and produce urine to excrete them.
There are three main steps of urine formation: i) glomerular filtration, ii) reabsorption
iii) secretion.
1. The Glomerulus Filters Water and Other Substances from the Bloodstream:
Each kidney contains over 1 million tiny structures called nephrons. Each nephron has
a glomerulus, the site of blood filtration. The glomerulus is a network of capillaries surrounded
by a cuplike structure, the glomerular capsule. As blood flows through the glomerulus, blood
pressure pushes water and solutes from the capillaries into the capsule through a filtration
membrane. This glomerular filtration begins the urine formation process.
2. The Filtration Membrane Keeps Blood Cells and Large Proteins in the Bloodstream:
Inside the glomerulus, blood pressure pushes fluid from capillaries into the glomerular capsule
through a specialized layer of cells. This layer, the filtration membrane, allows water and small
solutes to pass but blocks blood cells and large proteins. Those components remain in the
bloodstream. The filtrate (the fluid that has passed through the membrane) flows from the
glomerular capsule further into the nephron.

16. 3. Reabsorption Moves Nutrients and Water Back into the Bloodstream:
The glomerulus filters water and small solutes out of the bloodstream. The
resulting filtrate contains waste, but also other substances the body needs:
essential ions, glucose, amino acids, and smaller proteins. When the filtrate
exits the glomerulus, it flows into a duct in the nephron called the renal tubule.
As it moves, the needed substances and some water are reabsorbed through
the tube wall into adjacent capillaries. This reabsorption of vital nutrients from
the filtrate is the second step in urine creation.
4. Waste Ions and Hydrogen Ions Secreted from the Blood Complete the
Formation of Urine:
The filtrate absorbed in the glomerulus flows through the renal tubule, where
nutrients and water are reabsorbed into capillaries. At the same time, waste
ions and hydrogen ions pass from the capillaries into the renal tubule. This
process is called secretion. The secreted ions combine with the remaining
filtrate and become urine. The urine flows out of the nephron tubule into a
collecting duct. It passes out of the kidney through the renal pelvis, into the
ureter, and down to the bladder.

18. Composition of Urine: The nephrons of the kidneys process blood and create
urine through a process of filtration, reabsorption, and secretion. Urine is
about 95% water and 5% waste products. Nitrogenous wastes excreted in
urine include urea, creatinine, ammonia, and uric acid. Ions such as sodium,
potassium, hydrogen, and calcium are also excreted.

19. Kidney disease:
Diseases: A number of diseases can affect the kidneys. Environmental or medical
factors may lead to kidney disease, and they can cause functional and structural
problems from birth in some people.
a) Diabetic nephropathy: In people with diabetic nephropathy, damage occurs to the
capillaries of the kidney as a result of long-term diabetes.
Symptoms do not become clear until years after the damage starts to develop.
They include: i) headaches ii) tiredness iii) nausea iv) swollen legs v) itchy skin
b) Kidney stones: Stones can form as a solid build-up of minerals in the kidney. They
can cause intense pain and might affect kidney function if they block the ureter.
c) Kidney infections: These tend to result from bacteria in the bladder that transfer to
the kidneys.
Symptoms include lower back pain, painful urination, and sometimes fever. Changes in
the urine may include the presence of blood, cloudiness, and a different odor. Kidney
infections are more common in women than in men, as well as in women who are
pregnant. The infection often responds well to antibiotics.

20. d) Renal failure: In people with renal failure, the kidneys become unable to filter out
waste products from the blood effectively. If an injury causes kidney failure, such as the
overuse of medication, the condition is often reversible with treatment. If the cause is
a disease, however, kidney failure often does not have a full cure.
e) Hydronephrosis: This means "water on the kidney." It usually occurs when an
obstruction prevents urine from leaving the kidney, causing intense pain. In time, the
kidney might atrophy, or shrink.
f) Duplicated ureter: Two ureters might form between a kidney and the bladder, rather
than one. There are few complications, but it can increase the risk of urinary tract
infections and, in females, incontinence. Duplicated ureter affects around 1 percent of
people.
g) Interstitial nephritis: A reaction to medications or bacteria can inflame the spaces
within the kidney. Treatment usually involves removing the cause of inflammation or
changing a course of medication.
h) Kidney tumor: These can be benign or malignant. Benign cancers do not spread or
attack tissue, but malignant cancers can be aggressive. The most common
malignant kidney cancer is renal cell carcinoma.
i) Nephrotic syndrome: Damage to the kidney function causes protein levels in the
urine to increase. This results in a protein shortage throughout the body, which draws
water into the tissues.

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