Friday, 27 February 2015

Human kidney

Human kidneys form an important organ of the excretory system. It is also a vital part of the urinary system. The soluble, non irritant and solid substances that are not utilized by the body are excreted by the kidney. Kidney also secretes out excess water. Apart from kidney, the human skin also excretes these substances, though to a smaller extent. The volatile substances are excreted out by the human lungs and the heavy metals are secreted out mainly by gastro intestinal tract. To a lesser extent liver and saliva also excrete out these substances. Fats are excreted out through skin (sebum) and liver (bile).

When the body is at rest, approximately 25% of the total output of blood from the heart is distributed in kidneys (through renal arteries). The kidney filters the blood and secretes out the unnecessary substances.
Human kidney is the main organ for excretion in the human body. It also performs homeostatic and endocrine functions. The urinary system, as a whole is comprised of those organs which produce urine and then carry out its elimination from the body.

Role of kidney as an organ of the urinary system- To absorb water etc. for formation of urine from the excretable material of the blood plasma that has been supplied to it by the renal artery.

There are also other organs in the urinary system. Their names and functions are described below.
Ureters- They are the ducts inside the human body for carrying the urine (formed in the kidneys) to the urinary bladder.
 Bladder- The organ acts as a urine reservoir. When you feel a pressure for urinating, it means that the urinary bladder is filled at a higher level.
Urethra- The passage through which the urine is passed from the urinary bladder to the external environment. In men, penis helps in urine excretion.
Formation of urine and its elimination are key functions through which the body maintains its homeostasis (regulation of the body’s internal environment).

LS (lateral section) of the Human Kidney
Kidney Anatomy External

The Renal Vein, Renal Artery and the Ureter enter the kidney through the opening known as Hilus. All the vessels and the kidney itself are embedded in peri-renal fat (a mass of fat tissues). This mass extends into the central cavity or the renal sinus.
Renal Vein- They connect the kidney to the inferior vena cava and carry the blood purified by the kidneys.
Renal artery- They arise out of the abdominal aorta and supply blood to the kidneys. The renal artery has a radius of approx. 0.25 cm. One third of the cardiac output of blood is carried to the kidney by these renal arteries for purification. The right renal artery is longer than the left renal artery.
Ureter- They are made of smooth muscle fibers. Each Ureter from each of the kidney carries urine from it to the urinary bladder. The diameter of the ureters is 3 to 4 mm. They are 25 to 30 cm long. They arise from the pelvis of the kidney and cross at the brim of the pelvis (in front the iliac arteries). They enter the urinary bladder from the backside after passing through the sides of the pelvis.
Kidney Anatomy- Internal

The main three internal parts of the human kidney are renal cortex, renal medulla and renal pelvis.
The Renal Medulla- It is composed of conical masses that are called renal pyramids. The apex regions of the entire renal pyramids combine together to form the renal papilla. The papilla projects themselves inside the lumen of minor calyces.

Renal Cortex- The renal cortex is located in between the medullary pyramids. These regions are also called as Renal Columns of Bertini.
Renal Pelvis- The short or minor calyces that surround the renal papilla have a cup shape. They combine and converge to form major calyces. All the major calyces then combine to form the renal pelvis.

The Renal Blood Vessels

 Both the cortex and medulla have a complex network of vessels that carry blood. Through the renal pyramids all the blood arteries pass. These arteries are called Interlobar Arteries. These Interlobar arteries arch around the renal pyramid’s base and form the Arcuate Artery.The Arcuate arteries form a series of branches and enter the renal cortex. They are then called the Interlobular Arterioles. These arterioles give rise to afferent arterioles. The afferent arterioles divide themselves to form the glomerulus (a capillary network). All the glomerulus combine with each other to form the efferent arteriole (afferent means towards an organ and efferent means away from an organ).
The efferent arteriole goes back in the medulla and is called the Vasa Recta.

The shape, size and location of human kidneys
The human kidneys have a bean shape. They are located just behind the vertebral column in the abdomen. More specifically, they are located in the 12th thoracic to the 3rd lumbar segment. The average size dimensions of human kidney are:
Length-10 to 12 cm.
Width- 5 to 6cm.
Thickness- 3 to 4 cm.
Weight- A healthy male kidney weighs 150 gm in an adult male and 135 gm in adult female.

In most of the cases, the right kidney is smaller than the left kidney by a fraction.
The renal artery, renal vein, nerves and the ureters pass through a concavity that is present at the medial border. The concavity is called Hilus.If we section the human kidney vertically, then we will find opposite the Hilus a dark reddish region that is called as Cortex. Rest of the lighter area is known as Medulla. The medulla is divided in 10-15 conical areas that are known as Renal Pyramids. The boundaries of these lateral pyramids are made of projection of the dark reddish cortex and are called Columns of Bertin. The brownish lines of the pyramid converge towards the apex which is also called Area Cribrosa.
At the renal medulla certain very important substances are formed which help in regulation of body’s blood pressure. These are PGA-2, PGF-2 and PGF-2 alpha.

Nephron- the smallest unit of Kidney

The functional microscopic unit of kidney is Nephron. Each human kidney contains about 1 million nephrons. The main function of homeostasis and excretion is performed by these nephrons only. The nephrons drain what they absorb into the Pelvis of Ureter. The drained fluid or urine passes down the ureters (ducts) and is stored in the urinary bladder.
Uriniferous Tubule is the name given to the combination of Nephron (secretory part) and duct (non secretory part).
According to their positions, Nephrons are divided in two categories:
The Superficial Nephron or Superficial Glomeruli- They form the major chunk of Nephrons (85%) and occupy the outer two third area of the Renal Cortex. They are smaller in size in comparison to the Juxtameduallary Nephrons and are fully functional under normal conditions.
Juxtamedullary Nephrons or the Juxtamedullary Glomeruli- They occupy the inner one third area of the renal cortex. They constitute the rest of the 15 % of the total Nephrons. They are larger in size than the superficial Nephrons and work only under stress.

Parts of Nephron

The human nephron is formed of different parts. They are described below.
1. Renal or Malphigian Corpuscle- The malphigian corpuscle is 200 µm in diameter. The corpuscle is only found in the renal corpuscle. It is comprised of 2 parts.
A. Glomerulus and, B.  Bowman’s capsule
A. Glomerulus- The capillary which invaginates the Bowman’s capsule is called Glomerulus. The Glomerulus lies within the Bowman’s capsule and has around 20 to 50 capillary loops. The afferent arteriole brings blood to the glomerulus and efferent arteriole takes out the filtered blood away from the glomerulus. The afferent arteriole is wide and short and the efferent arteriole is long and narrow. The arrangement is in such a way that the glomerular blood pressure is much higher (70 mm of Hg) than the capillary/artery blood pressure. This facilitates filtration of blood. The combined surface area of the glomerular capillaries or the total filtration bed of the human body is 1.73 sq meter.
B. Bowman’s Capsule
 The end of the Nephron is called the Bowman’s capsule. It has 2 layers viz. parietal and visceral.
The filtering layer/membrane of the Malphigian corpuscle consists of -
  1. The endothelial cell layer.
  2. A basement membrane.
  3. The epithelial cells of the visceral layer of the bowman capsule. After developing into the visceral layer the epithelial cells are also known as Podocyte or Glomerular Epithelial Cell. The podocytes give rise to tentacle like cytoplasmic processes called End Feet or Pedicles. These have large number of minute branches.
  4. The endothelium of the glomerulus also contains deep cells/stalk cells /mesangial cells. They are found between the glomerulus and the basal lamina. In certain pathological conditions, these cells turn into phagocytes and proliferative.

2. Renal Tubule- The glomerulus has 2 poles. The first pole is the vascular pole. At the vascular poles the blood vessels are attached. The renal tubule begins at the opposite pole or the tubular pole. The length of human renal tubule is about 3cm. it is 20-60 µm in width. If we measure the length of all the renal tubules put together in the 2 million nephrons that are present in the 2 human kidneys, then we will have a 65 km long renal tubule! The renal tubule has a neck just below the glomerulus. After the neck, the renal tubule is divided into following parts.

    1. Proximal Convoluted tubule/Pars convolute- The length of PCT is approx. 14mm. The external diameter of PCT is 60 µm and interior diameter is 15-25 µm. The pct has high amounts of twists and runs. The cells of PCT are very active and have large number of enzymes. Thus they play greater role in metabolism. Two third of water, all glucose and parts of sodium, phosphate and chloride are reabsorbed form the glomerular filtrate in the PCT.
    2. Henles loop-

It is a loop that appears after the PCT. It has a U shape and it dips to variable length inside the medulla. The parts of Loop of Henle are:
a. Descending limb
b. Ascending limb (thin walled)
c. Ascending limb (thick walled).

In the superficial nephrons, the length of Henle’s loop is very less. It also dips only slightly in the medulla in the juxta medullary nephrons; the loop is of considerable length and passes deep inside the renal medulla. The average length of Henle’s loop is 20 mm.
The membrane of descending loop is more permeable to water than it is to urea and NaCl. Thus more of the water is removed in this area. With the loss of NaCl, the fluid becomes less dense in the ascending limb. The thick portion of the ascending limb is also known as the functional proximal tubule. Active chloride re-absorption occurs in both the parts of ascending limb.

  1. Distal or second convoluted tubule- The DCT is located in the cortex region of the nephron. The average length of DCT ranges from 4.6 mm to 5.2mm.its diameter ranges from 20 µm to 50 µm. It is lined by cubical epithelial cells which do not have brush borders. The DCTs lumen diameter is larger than that of the PCT.
The central part of DCT comes in contact with the juxtaglomerular cells of the afferent vessel. This region of cells has closely packed nuclei and is termed as Macula Densa (as they have a dense structural network). The juxtaglomerular cells, macula densa and Lacis (extra glomerular mesangial cells) cells together form the juxtaglomerular apparatus. Vasopressin or ADH is a hormone that plays key role in determining the re-absorptive capacity of the DCT. Hence it also plays a vital role in deciding the urinary output.

Straight tubule or collecting tubule- The 20 mm long tubule is lined by cuboidal cells.
Duct of Bellini -Several straight tubules from the different nephrons join with each other to form the Duct of Bellini. The duct opens at the apex of the renal pyramid.
Juxtaglomerular Apparatus

The term juxtaglomerular apparatus was coined by Goormaghtigh. The apparatus includes
    1. The afferent arterioles granular juxtaglomerular cells.
    2. The DCTs macula densa, and
    3. Lacis Cells- found at the entry and exit points of afferent and efferent arterioles of each glomerulus.
The entire three parts combine together to form the juxtaglomerular complex. The apparatus plays a key role in control of blood pressure, in erythropoesis, in maintaining salt balance and renal flow.

Working process of human kidneys
The nephron is the smallest unit of kidney which does the filtering work. Each nephron filters a very small volume of blood. Much  of the material that is filtered by the glomerulus is reabsorbed by the tubules (including Proximal Convoluted Tubule and Distal Convoluted Tubule).

Selective Re-absorption
1. Protein- The glomerular filtrate has a small amount of protein. This protein is reabsorbed in the PCT.
2. Glucose- Glucose is completely reabsorbed in the PCT.
3. Water-The Glomeruli of all the nephrons in adult kidneys filter around 170 liters of water a day (both the   kidneys). Out of this approx. 1.5 liters is excreted out per day.
4. Blood cells- The nephrons do not allow passage to blood cells and large protein molecules. They are not absorbed by the permeable membranes of the nephron and are not excreted in urine.
5. Sodium and chloride- Both sodium and chloride are absorbed in the ascending limb of Henle’s Loop. The Glomeruli filter around 560 gm of Na+ per day. Out of this 5 gm is excreted in the urine. The rest is reabsorbed in the Proximal Tubule (490 gm) and distal tubule(65gm).
6. Potassium- Only 5 to 10% of the filtrate of potassium is excreted through the urine.  The rest is reabsorbed in the proximal tubule.
7. Bicarbonate- All the bicarbonate filtered by the glomerulus is reabsorbed in the renal tubule.
8. Phosphate- much of the phosphate is reabsorbed in the proximal tubule.
9. Re-absorption of other substances-  Other substances that are reabsorbed in the tubule are  ketone bodies, beta-hydroxybutyric acid, uric acid, sulphate, ascorbic acid, ketone bodies, creatin and some amino acids.
10. Urea- Is filtered in PCT and other parts.
The GFR or glomerular filtration rate is 125 ml per minute.

Functions of kidney

1.It helps to maintain the water balance in the body and regulates plasma volume.
The kidney performs various functions. The functions of human kidney are listed below.
2. It is the main excretory organ of the body. It excretes all the waste products. Of vital importance to excretion is the excretion of the nitrogenous and sulphur containing waste products that are formed in the body during protein metabolism. Such waste if gathered in the body can make the blood toxic.
3. It eliminates the drug residues and other toxic material from the body.
4. It maintains the hydrogen ion concentration and also maintains the electrolyte balance of the body.
5. The kidney also manufactures some new substances in the body during its waste processing process. These are inorganic phosphates, ammonia and hippuric acid. Ammonia preserves the acid base equilibrium of the body. Thus kidneys can be said to be the junk processor of the human body. They excrete the waste and also process some wastes into useful compounds.
6. During the condition of hypoxia, the kidneys regulate the blood pressure of the body. They do this by liberating rennin from the juxtaglomerular apparatus.
7. Kidneys help in metabolizing the Vitamin D inside the body.
8. Kidneys help in the regulation of erythropoiesis (process of RBC production).
9. The kidneys help in maintaining the osmotic pressure at desired levels in the tissues and blood of humans.
10. The kidney also selectively reabsorbs certain material from the blood. The process is vital for keeping the concentration optimum of certain constituents of blood. 

Diseases associated with the human kidney
Congenital kidney disorders
When a person is born with an impaired kidney or set of kidneys, then it is a case of congenital disease. The abnormalities include deformed kidneys, for example a single horseshoe kidney in which both the kidneys join at the base to form a single kindly that has a horseshoe shape. Other deformations include a single kidney, two ureters for a single kidney and both the kidneys on one side. A person can live with a single kidney though.
When the waste products accumulate in the body then the kidney fails or undergoes a functional abnormality. Heavy ingestion of toxic materials can lead to such an event. Also, over usage of certain drugs, malnutrition etc can also be the causes for kindly failure. The patient develops shortness of breath, weakness, confusion and lethargy as signs of kidney failure. The kidney fails to remove excess of potassium from the body because of which abnormality in heart functioning occurs. This may result in cardiac arrest and death. Sometimes the kidney failure is irreversible and cannot be cured.
Acute kidney diseases
  1. Pyelonephritis - Pyelonephritis is an infection of kidneys by any harmful microorganism. It is a common infectious disease and can be treated successfully with antibiotics. In this condition the symptoms include fever, pain in abdomen, and painful and frequent urination.
  2. Kidney stones- When stone occurs in the kidney they need to be broken to smaller particles. Then they flush out of the kidney through urine. They are made of hard crystals that separate in the urine formation process and accumulate in the kidneys.
  3. Acute kidney failure- It occurs when someone has an injury that significantly reduces his blood flow, in conditions of severe dehydration, stones, infections and exposure to toxic chemicals and drugs.
  4. Nephritic syndrome- the syndrome occurs when there is high level of protein the urine and small level of protein in bloodprotenuria. Te symptoms  include swelling across the eye, hands and feet. Diabetes mellitus and other diseases can give rise to this condition.
The diagnosis for kidney failure is done by blood tests for creatinine, BUN, and glomerular filtration rate. If you want to prevent failing of kidneys then you must take lifelong measures for controlling blood pressure and diabetes. Some measures include exercising regularly and eating healthy balanced diet full of fruits and vegetables.
When the kidneys fail permanently then the patient has undergo a kidney transplant or dialysis. In dialysis or hemodialysis, an artificial machine known by the name dialysis machine acts as an artificial kidney.
Renal hypertension
If a person has hypertension then he may have arteriolar sclerosis. In arteriolar sclerosis, the glomerular capillaries and arterioles thicken and harden. If the hypertension gets severe, then the sclerosis worsens and causes ischemia (inadequate blood supply), fibrosis (thickening and scarring) of the cortex and destruction of small vessels. In such condition, renal failure occurs.
Reno vascular hypertension
A renovascular disease may also cause malignant hypertension. This occurs when the renal artery’s diameter is decreased because of the accumulation of atherosclerotic plaque. This results in elevation of blood pressure. Because of increased blood pressure, the walls of the glomerular capillaries and small arterioles get damaged. The vessels break down (rupture) and cause excessive bleeding (hemorrhage) and scarring (infarction). Thus fibrosis of the renal cortex and atrophy (decrease in effectiveness) of cells occur in the renal cortex.
Atherosclerosis of renal artery
 There are three layers of the renal artery. They include the internal layer or intima, the middle layer or media and the external layer or adventitia. When arteriosclerotic plaque or lipid plaque gets deposited in the intima, it causes hardening of renal artery and fibrosis. Severe deposition results in hypertension.

Chronic disorders of the kidney
Chronic (long lasting) disorders are serious conditions as they result in kidney deterioration with time.
1. Glomerulonephritis- The Glomeruli of the kidneys become inflamed in a condition known as glomerulonephritis. High blood pressure and diabetes are a common cause of this disease.
2. Tumor or cancer in the kidney can result in kidney impairment and malfunctioning. When untreated, such conditions may result in and ESRD or end stage renal disease. When ESRD occurs, the kidneys fail to function and shut down completely.
 As we age, the functioning of Kidneys is reduced.

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