After reading this article you will learn about the structure of the kidney. This will also help you to draw the structure and diagram of kidney.
The kidneys are two in number which are situated one on each side of the verteral column and in-front of the last ribs. They lie on the posterior abdominal wall. The right kidney is placed slightly lower- than the left due to the presence of liver which occupies much space on the right side. Kidneys are dark brown in colour and are embedded in a mass of fat. On the upper end of each kidney suprarenal glands are situated like a cap. Each kidney is about 10 to 13 cm (4- 5 inches) in long, 6 cm. (2 ½ inches) wide and 3 cm. (1 ½ inch) in thickness. The average weight of adult kidney is about 150 gms. in males and 135 gms in females.
Structure:
The kidneys have been bean-shaped appearance. The outer edge of the kidney is convex and the inner concave. In the concave side there is a depression in the middle point which is called as ‘hilum’ or ‘hilus’. Here the blood vessels of the kidney i.e. Renal artery enters and Renal vein comes out of the kidney. Ureter or the duct of the kidney also comes out through this hilum and pass downwards to open into the urinary bladder.
The bladder is a muscular bag lying-in the pelvis, in which the urine is collected. The urine is constantly poured into the bladder by the ureters and when it gets filled the bag contracts and the urine is passed through the urethra. This is a small tube opening to the outside of the body. The urethra is kept closed by a ring of muscular fibers which relaxes when urine is to be passed out.
Internal Structure of Kidney:
Human kidney is like that of a goat or sheep. If a longitudinal section of the kidney is made by cutting with a long knife from the outer convex surface to the hilum three layers are seen.
(1) Outer Cortex
(2) Medulla
(3) Pelvis
1. Outer Cortex:
This is the solid part of the kidney which is dark coloured and granular cortical portion.
2. Medulla:
This medullary part is made up of fifteen to sixteen pyramid- shaped masses which is called as pyramids of the kidney. A number of fine tubules are arranged in conical portions like pyramids. The base of each pyramid lies towards the cortex while its pointed end projected into the cup like branches of the pelvis. By squeezing a pyramid some urine may be seen to flow from the tubules.
3. Pelvis:
This is a funnel shaped structure. It is a large cavity. At its upper end it is divided to form some branches known as calyces. The average number of these calyces is 10. In the cavity of pelvis urine is collected and carried downwards by ureters. It can be represented from another point of view. The ureter widens out into a cavity with a number of short wide branches. The funnel shaped cavity is known as pelvis of the ureter and its branches calyces. In other words various calyces unite to from the pelvis.
Microscopic Structure of Kidney:
Under a microscope a thin section of the kidney is seen to be composed of the following parts:
(A) Malpighian Body or Renal Corpuscles
(B) Renal tubules.
(C) Blood vessels and supporting tissues.
(A) Malpighian Body:
It is found in the cortex of the kidney. Kidney is composed of a number of long coiling tubules. Every tubule begins in the cortex in a small cup like expansion called the malpighian body. After following a coiled course, finally it joins one of the larger tubes. Then it runs straight through a pyramid and opens into the funnel shaped pelvis.
The malpighian body consists of two parts:
(1) Glomerulus,
(2) Bowman’s capsule.
Glomerulus:
The small branches of renal artery run along the straight tubes in the medulla. Each gives a small branch to the malpighian body which breaks up in its interior into a rounded bunch of blood capillaries called glomerulus. It is a network of blood capillaries found by branching the afferent arterioles. The efferent vessels then emerge as a small efferent arteriole. It again breaks up to form a second capillary network around the urinary tubules. These capillaries reunite to form the Renal vein which carries blood from the kidney.
Bowman’s Capsule:
This is the expanded end of each tubule towards the cortex. Bowman’s capsule is a cup like double walled cavity, in which there is a network of blood vessels called glomerulus.
The function of Malpighian Body is filtration of the plasma.
(B) Renal Tubules:
The Renal tubule is about 3 cm long and 20-60 microns wide. There are a number of renal tubules in kidney.
The renal tubule consists of the following parts:
1. Bowman’s capsule the cup like part
2. Proximal coiling tubule
3. The loop of Henle
4. The distal coiling tubule
5. The junctional tubule
Nephron:
Nephron is the basic unit of kidney. The minute structure of the kidney is composed of a number of nephrons. Each human kidney possesses about 1 -2 millions of nephrons.
Each nephron is made up of two main parts:
(1) Malpighian Body,
(2) Renal tubule.
(C) Blood Vessels:
The two important blood vessels of the kidney are:
(1) Renal Artery
(2) Renal Vein.
Renal Artery enters into the kidney through hilum. It carries blood for filtration, i.e. separation of waste nitrogenous substances. Renal artery breaks up into afferent arterioles forming a network of capillaries in the malpighian body known as glomeruli. These glomerular capillaries unite to form efferent arterioles.
These are again united to form Renal vein which comes out of the kidney by bringing the blood back to the general circulation. Inside the kidney the small branches of renal artery and renal vein are united together by minute renal capillaries which surround the kidney tubules. The renal efferent vessels contain the purest blood in the body as the waste products have been filtered.