Kidneys


Kidneys


       These produce urine and filter waste products dissolved in the in the blood and substances present in high quantity. Kidneys are in the bean shape and these are the most active glands of the body which are present on the posterior stomach wall behind each peritoneum in the both sides of vertebral column. They are present from twelfth thoracic vertebra to third lumbar vertebra (from t 12 to t3). The right side kidney is situated some below comparison to left side kidney because the liver covers excessive area. Each kidney is about 11 centimeter long, 6 centimeter broad and 3 centimeter thick. 

       Each kidney has upper and lower portions and upper portion is called superior pole and lower portion inferior pole. An adrenal gland is present on the superior poles of both kidneys. There are anterior and posterior surfaces of it and there are lateral and medial borders too. The lateral border is convex viz it is bulgy whereas internal border is concaved in which there is hilum in it too. Renal artery, renal vein, lymphatic vessels, nerves and both Ureters get enter into the kidneys and come out through this hilum.   

Each kidney is covered by the three layers of tissues.

  • The most internal layer is strong and made of fibrous substances which is called renal capsule. It gets dissolved in the surface layers of ureters.
  • Its middle layer is made of pararenal fat which is called adipose capsule. This padded layer avoids kidneys from jolts and shocks.
  • The external layer is fascia situated under the serum membrane and it is called renal fascia. There is another layer of fat too around the renal fascia which is called pararenal fat. Renal fascia is made of compact tissues which is covering the kidney and is holding it tightly by the posterior stomach wall.

Internal formation:

       Three separate portions appear from inside to outside in the sagittal section and these three parts are pelvis, medulla and cortex.  

Renal pelvis:

       It is a big collecting space inside the kidney which is made of by the broad fennel shaped upper portion of the ureter. The pelvis is divided into two small cavities which are called major calyces and minor calyces. Major calyces are two to three in number and minor calyces are in 8 to 18 in numbers. 

Renal medulla:

       It is the middle portion of kidney. There are 8 to 18 renal pyramids in it and it is in conical shape. The base of each pyramid is close by the renal cortex and papilla is made of by the apex which opens in the minor calyx. There are collecting ducts and tubules of nephrons in renal pyramids. Tubules of pyramids reabsorb to the filtered substances. The urine reaches first into the minor calyces by passing through the collecting ducts of pyramids and then reaches into major calyces and then finally into the pelvis and from this place the urine reaches in the urethra by passing through ureter because of the vibration of contraction in the cells of calyces.

Renal cortex:

       It is the most external part of the kidney which is divided into two parts. One is external cortical parts and the second is juxtamedullary parts which is close by the medulla. Cortex appears granular and it is extended from the external capsule to the base of renal pyramids. The granular form of cortex is because of bunches of cells and nephrons. Cortical tissue which emerges out from the middle portion of renal pyramids in the depth of medulla and it makes renal columns. Mainly these columns are made of collecting tubules which supply urine into minor calyces. The urine comes from a lobe of kidney in each minor calyces. Each lobe is made of a pyramid and a cortical tissue attached on it.

Blood supply of kidney:

       By getting entry into the hilum of each kidney, the renal artery brings blood from the abdominal aorata and supplies into the kidneys directly. The renal artery gets divided into interlobar arteries by reaching inside the kidneys which go through the renal columns. When these arteries reach on the meeting point of cortex and medulla, they get folded and increase parallel to the bases of pyramids. These arteries (folded like bow) are called arcuate arteries which make small arcs on the edges of cortex and medulla. The arcuate arteries get divided into interlobular arteries after separation and which go towards the cortex. These arteries get divided into small several afferent arterioles after separation which supplies blood to the filter point viz glomuralie. 

       Each afferent arteriole gets divided for making earring shaped bunch which are called glomerulus. This is the place where blood is filtered. Earring shaped glomerulus produce efferent arteriole by meeting altogether which supply blood into glomerulus. The efferent artirole is narrow comparison to afferent arterioles and because of this reason the pressure of blood gets increased which is essential to filter the blood.    

       Efferent arterioles make second capillary bed by getting divided again after coming out (first capillary bed is called glomerulus) which is made of peritubular capillaries around the tubules. The water and other important substances which come from after filtering blood in the glomerulus are reabsorbed by the by renal tubulus and thereafter these products reach into the peritubular capillaries. Peritubular capillaries make interlobular veins by meeting altogether which supply blood from cortex to arcuate veins. Small arcuate veins make big interlobar veins in the renal columns by meeting altogether which make renal vein by meeting altogether and supplies blood into the inferior vena cava from each kidney. 

       Efferent arterioles change into straight rectae by marching forward which cover to the loop of nephron or loop of Henle and open in the venae recta by reaching into the medulla. Only one or two percent blood of total renal blood flows through these conduits. Collectively these conduits are called vasa recta.  

Nephron:

       There are 10 lac nephron in each kidney and each nephron is a free unit for producing urine. These can be seen only by the microscope. In the form of working unit of the kidney, the nephrone completes early filtration of blood and absorbs those substances from the filtration which are selective reabsorption and unwanted substances excrete from the body.

       There are two kinds of nephrones-cortical and juxtamedullary. Cortical nephrones remain in the two third early portion of cortex of which tubular formations are only to the base of renal pyramid of medulla whereas long renal loop are emerging out in the depth of renal pyramid of juxtamedullary nephrons. Cortical nephrons are seven fold more comparison to juxtamedullary nephrons. Under normal conditions, function of kidney continues in the cortical nephrons but juxtamedullary nephrons are active only in the condition of stress. 

There are two below mentioned parts of each nephron.

1. Glomerular or Bowman’s capsule

2. Renal tubule

Glomerular or bowman’s capsule:

       It is a dilated cup shaped portion of the beginning of nephron which is covering to the glomerulus. Capsule and glomerulus both make renal or malpighian corpuscle. Glomerular capsule is situated in the cortex of kidney. Its internal and external walls make a cavity which is called capsular space. Parietal layer of glomerular capsule is made of   simple squamous epithelial cells which get dissolved in the renal tube and internal visceral layer special epithelial cells that are called podocytes which are covered by the glomerular cells.   

       Blood is filtered through three layers in the renal carpuscle. 

  • Its first layer is endothelium of glomerulus in which there are several minor holes which are called fenestrations.  
  • Its middle layer is basement membrane of glomerulus.
  • The third layer is visceral layer of glomerular and podosytes. Podocytes nuclei are big cells comparatively of which some processes are emerging out. These processes get divided into many small branches like fingers which are called foot processes or pedicels. Pedicels are the portion of podocytes which remain in the contact of glomerular cells. The small space of the center of pedicels leaves open to the lower side basement membrane which is called filtration slit. There is a thin slit membrane at the middle of pedicles close by the podocyte cells which check some special atoms by going through th filterationslit but allow other elements. It is called filtration barrier and it is made of fenestrated endothelium, basement membrane and filtration slits.          

       Comparatively, all the three layers of renal corpuscle make capsular membrane altogether. The process of filtration completes in complete membrane. Generally, cellular components of blood and atoms of big proteins don’t pass through this membrane but water and soluble minerals (electrolytes, glucose, urea, amino acids and polypeptides) reach into the capsular space of glomerulus capsule by filtering through blood. This blood filtered liquid is called glomerular filtrate. 

Renal tubule:

       It is situated in the continuity of glomerular or Bowman’s capsule.  

       There are four parts of it which have been given below.

  • Proximal convoluted tubule
  • Loop of henle
  • Distal convoluted tubule
  • Collecting duct

Proximal convoluted tubule:

       This portion of renal tubule is situated nearby glomerular capsule and it is twisted and appears in the shape of earring. Glomerular filtrate reaches in this portion from the glomerular capsule.  

     This tubule is made of cuboidal epithelium and its internal layer is covered by the small microbille. These increase the space of epithelium layer for the process of absorpotion. Several substances as water, electrolytes, glucose and some amino acids and polypeptydes are reabsorbed from the glomerulus in this portion of kidney. 

Loop of Henle or of the Nephron:

       It is a part of the English letter (U) in the continuity of near by circular tubule in which there are two ascending and descending arms which are made of cuboidal epithelium. The descending (thin) arm is inserted in the medulla of kidney and it becomes too much narrow. At this place it marches upwardly by taking a turn and it becomes enlarged again. This ascending (thick) arm reaches into the cortex by going through the medulla.

Distal convoluted tubule:

       This convoluted portion of renal tubule is situated distal from glomerulus capsule and is made of cuboidal epithelium cells like proximal convoluted epithelium membrane but there are a few microvilli on its internal lumen. 

Collecting duct:

       It is the last broad portion of renal kidney in which glomerulus filtration gather.

Collecting ducts are unstratified by two kinds of epithelium cells.

  • Light cell: There are microvilli and cilia on its apex. 
  • Dark cell: There are irregular flaps on its apex. Distal tubes of separate nephrons open in the collecting duct which goes through parallel to arms of the loop of nephrons by passing through the medulla. Collecting tubes make big tubes by attaching to each other which are called duct of bellini which reach to the minor calyx from where the last filtrate (which is called urine now) reaches into the pelvis which supplies urine in ureter and then into the bladder.

Juxtaglomerular apparatus:

       In the cortex, the distal convoluted tubule establishes privy contact with the afferent and efferent arterioles of glomerulus. There is cytoplasm in the cells of smooth muscles of tunica media of arterioles and in it there are many granules comparison to myofilaments. These special smooth muscles cells are called juxtaglomerular cells; these cells are arranged in the contact of the group of epithelium cells of distal tubule which is called macula densa viz dense spot. Cells of macula densa are much narrow and long comparison to the cells of epithelium of distal tubules and their nuclei are close by to each other. 

     Juxtaglomerular cells, macula densa and afferent and efferent arterioles make juxtaglomerular apparatus which is the source of an enzyme named rennin and helps in blood pressure.

Function of kidneys:

In spite of the production of urine, following mentioned are the deeds of kidneys.

  • Final products of metabolism of meal as water, urea and uric acid are excreted by the kidneys and besides these they expel out harmful medicines, poisons and chemical substances from the body.
  • They keep a balance between water and electrolytes in the body.
  • Kidneys govern the balance of water in the whole body and make the plasma volume of the blood.
  • They keep the chemical composition of the other liquids of the body and blood in control.
  • Kidneys control the balance in the acid and alkali in the body.
  • They help in sustaining the osmotic pressure of fluid in the body.
  • They keep the hydrogen volume thickness of the blood in the body stable by producing ammonia. 
  • Kidneys control the quantity of fluids, their strain and reaction of the body.
  • They produce an enzyme named rennin which helps in governing blood pressure.

Formation of Urine:

       Kidneys use three processes in producing urine.

  • Glomerular filtration
  • Tubular reabsorption
  • Tubular secretion
  • Glomerular filtration
  • Tubular reabsorption
  • Tubular secretion

Glomerular filtration:

       Glomerulus works as a filter. When the blood flows in glomerulus by passing through the afferent arteriole, its pressure is much (about 75 mm Hg).  Some portion of blood plasma reaches into the glomerulus capsule because of this pressure but the blood cells and big atoms of plasma proteins remain inside the glomerulus because these don’t go through the holes of semi permeable walls of capsule. This process is called glomerular filtration or filtration and originated liquid or fluid is called glomerular filtrate or filtrate.  

Tubular reabsorption:

       Glomerular filtrate pass through nephron or renal tubes, nefron tubes absorb useful products for the body such as water, sodium ions, glucose and amino acids whereas poisonous substances gathered in the blood and produced at the time of metabolism of the body such as urea, uric acid and keratin etc are not absorbed and they are excreted out from the body with urine. This process is called Tubular reabsorption.

Tubular secretion:

       For the body, some unnecessary ions and substances reach into the glomerular filtrate from the cells of tubular which pass through convoluted tubule. In this way, products as potassium ions, hydrogen ions, some medicines and carbonic composition come out from the body with urine. This process is called Tubular secretion.

       Glomerular filtrate reaches first in the medulla and then in the cortex in the internal portion of the lungs by passing through nephron. In this process, essential products as water and glucose are reabsorbed. Finally, glomerular filtrate reaches into the medulla again where it is called urine and reaches in the ureter. Filtered blood is supplied into the body again by the renal vein.