Kidneys are two bean shaped organs in the human body. They are situated on both sides of the lumber spine (back bone), on the posterior abdominal wall as shown in figure No.1.
Gross anatomy of the kidney
The size of the kidney is 9-12cm in length, 4-6cm in width and 4-5 cm in thickness. The average weight of the kidney is 200-250gm. It is larger in size and weight in males as compared to females. Each kidney takes its blood supply from a major artery in the abdomen called the Abdominal Aorta via a blood vessel called the Renal Artery.
The blood after passing through the kidney passes back to the Inferior Vena Cava through the Renal Veins. On the medial side of each kidney, there is a depression called the Hilum, from where a space known as the renal sinus extends into the substance of the kidney.
All the blood vessels i.e. the renal artery and vein enter and leave the kidney through the Hilum. The Hilum also bears the excretory duct called the Ureter which transfers the urine from the kidneys to the urinary bladder.
Urinary bladder is situated at lower part of the abdomen, just behind the pubic bone. It is shaped like an inverted flask. It stores the urine brought to it by the ureters from the kidneys, for a specific time and then excretes it through the urethra.
The following portions of the kidney are visible on naked eye examination as shown in Figure No.2.
Longitudinal section of kidney
Cortex: This is the outer portion of the kidney which is more pinkish in color.
Medulla: This is the inner portion of the kidney, comparatively lighter in color and is mainly formed by the tubules. The cortex surrounds the medulla like a cup with inverted margins.
Renal Pelvis: This is a sac like structure situated medial to the medulla. At the lateral border of the renal pelvis there are finger like projections called Major Calyces which divide into Minor Calyces which extends into the renal medulla.
Renal pelvis inferiorly continues into a tube like structure called the ureter. Urine formed by the kidney cells (nephrons) passes through the renal pelvis to the ureter which transmits it to the urinary bladder.
Microscopic Structure of the Kidney
The structural unit of the kidney is the Nephron. Each kidney is made up of more than one million nephrons. Each nephron can be divided into two major parts, the Renal Corpuscles (malpighian corpuscles) and the Tubules. See Figure No.3.
The renal corpuscles are mainly located in the cortex and are formed by a tuft of blood capillaries called the Glomerulus, which invaginates into the proximal dilated part of the nephron called the Bowman’s capsule.
Bowman’s capsule extends into the second part of the nephron known as the Tubule. Blood is brought to the glomerulus by a very small branch of the renal artery known as the Afferent Artery and after filtration of the blood, it returns to the venous system through the Efferent Artery and then through the renal vein to the Inferior Vena Cava.