The kidney is the primary organ of excretion in the human body. A pair of kidneys are attached to the posterior abdominal wall. Each lie on either side of the vertebral column.
Gross Structure Of Kidney
A kidney is a bean-shaped organ that is reddish-brown in color. It has a dimension of 11 cm in length, 6 cm in width and 3 cm in thickness.
The average kidneys are about 150 gm in males and 135 gm in females. They are held in position by a mass of mass. A sheath of fibrous tissue called renal fascia encloses both the kidney and renal fat.
Kidney Structure by longitudinal section
kidney has 3 several layers of different tissues, that have different functions. These include
- Outer fibrous capsule
- The Cortex
- Medulla in the center.
Fibrous capsule (renal capsule)
This is the outermost cover of the kidney. It surrounds the kidney and helps to enclose the internal tissues.
This is the peripheral region of the kidney lying immediately below the kidney. It is also called the renal cortex. Cortex is highly vascularized and hence is reddish-brown in color.
Renal cortex has two parts:
- The Cortical arches or lobules, which form arches over the pyramids.
- The renal columns, which run in between the pyramids.
This is the innermost layer of the kidney. It is also called Renal Medulla. It has pale-colored with conical shaped striations called as pyramids. Each kidney has 9 to 10 renal pyramids.
The medulla is comparatively less vascularized than cortex and hence appears pale to the naked eyes.
The lobe of the kidney is formed by each pyramid and its overlying cortex.
Inner to the renal medulla is an open space, called sinus, or renal sinus.
These sinuses are the spaces that run from the hilum of the kidney up to the kidney functional parts.
Renal sinuses contain
- Renal Artery and its branches
- The tributaries of the renal vein
- Minor and Major and calyces.
These major calyces are the branchings of the renal pelvis. Renal pelvis divides into 2 to 3 major renal calyces. Each major calyces divide into 7 to 13 minor calyces.
The urine from the pyramid reaches minor calyces and then into major calyces. From there it passes into the renal pelvis.
The renal pelvis opens into the ureters.
The walls of the renal pelvis are made of smooth muscle with a lining of the epithelium. The peristaltic movements of originating in the smooth muscle cells of the pelvis proper the urine into ureters.
Blood Supply of Kidney
Kidneys receive blood from the Renal Arteries, and are direct branches of Abdominal Aorta.
The contents of renal hilum, the entry point of Kidney which are devoid of the renal capsule are
- Renal Artery
- Renal vein
Nephron and its structure
A nephron is the functional and structural unit of kidneys. Each kidney has 1 to 2 million functional nephrons. But the number of collecting ducts are lesser.
Each Nephron has four parts
I. Bowman’s Capsule
2. Proximal Convoluted Tubule or commonly called PCT
3. Loop of Henle; This again has two parts –
- Ascending loop of Henle
- Descending loop of Henle
4. Distal Convoluted Tubule or commonly called as DCT.
Collecting Ducts (CT): These are tubules where distal convoluted tubules empty their contents. These CT’s act as a common tube for many nephrons.
Bowman’s capsule is the thin layered unicellular lined closed chamber enclosing the glomerulus and continuing with the proximal convoluted Tubule. It encloses glomerulus, which is a tuft of capillaries arising from the afferent arteriole and emerging out of glomerulus as efferent arteriole. As we know, capillaries are one cell layer in thickness, and diameter of afferent arteriole is twice the diameter of the efferent arteriole, a tremendous pressure rises inside the glomerulus, which pushes the plasma along with its important contents like the glucose, ions, and also with excretory products like urea, creatinine, except the blood corpuscles, clotting factors and bigger proteins like the album, globulin, etc. in the Bowman’scapsule. This filtrate is called the primary filtrate.
An afferent arteriole is a branch of an interlobular artery, which is a branch of a renal artery that branches into the glomerular capillaries. These capillaries reunite to form the efferent arteriole, which then again divides into peritubular capillaries. Peritubular Capillaries are found only in the case of cortical Nephrons. In the case of Juxtaglomerullarynephrons, they are straight parallel to the loop of Henle, called vasa recta.
Special types of cells are found in Bowman’s capsule called podocytes which have gaps in between them to facilitate this ultrafiltration.
Proximal Convoluted Tubule
Proximal Convoluted Tubule, also called PCT has an average adult length of 15mm, with 55micron diameter. It is highly coiled and succeeds Bowman’s capsule. Proximal Convoluted Tubule reabsorbs about 67%, i.e. Two-third of the primary filtrate into the blood. These cells of Proximal Convoluted Tubule have several ion channels like sodium potassium cotransporter, divalent ion transporter, etc which actively reabsorbs these ions and has glucose absorbers which absorb 100% of the glucose from the primary filtrate in case of a healthy person. Passive water reabsorption follows due to osmotic pressure differences.
Loop of Henle
Loop of Henle is the site of absorption of urea according to osmotic pressure differences. It is the site of a highly complex mechanism not discussed here, which results in the particular absorption of urea according to body needs. Ions are also absorbed. Loop of Henle also is the site of water absorption according to body needs. It is acted upon by different cells, which in the course has hormonal effects on body fluid content.
Distal Convoluted Tubule
Distal Convoluted Tubule, also called DCT is the successor of Loop of Henle. Microvilli on the surfaces of its cells help in the absorbing of potassium ions and water according to body need.
Collecting Ducts are the successors of the Distal Convoluted Tubule. It has type I and P cells. These cells are involved in the active secretion of bicarbonate ions and urea into the fluid. There is further water reabsorption according to need.
The fluid leaving the collecting ducts is the final composition of urine. It is highly concentrated than the primary filtrate and is completely sterile. It leads to columns of Bertini which then leaves the urine into the renal pelvis, collected by the ureter via peristaltic movements, and carried forward towards the urinary bladder. It is then excreted outside the body via the urethra.
Nephrons are of two types:
1. Cortical Nephron
These Nephrons which constitute about 85% of all the Nephrons in the kidney. They are smaller in size with a shorter loop of Henle and penetrates less into the medulla. It is more confined in the cortical region of the kidney. Their glomerulus is located in the superficial parts of the renal cortex.
2. Juxtaglomerular Nephrons
These are the Nephrons which constitute the rest 15% of the Nephrons present in the kidney. They are larger in size, with a longer loop of Henle going deep down up to renal Medulla. They have their glomerulus located in the deeper part of the renal cortex.
Functions of the kidneys
Kidney is a very complex organ, maintaining the concentration of blood of our body. It is of immense importance as it is the primary excretory organ of our body, whose impairment will impair the whole excretory process. It has effects of hormones on it.
Secondary functions of kidneys include the secretion of hormones like renin and erythropoietin. Renin helps in the maintaining of blood pressure and volume. Erythropoietin boosts the production of RBCs which is secreted more in case of hypoxia.
Several diseases hamper the function of the kidney. They include renal calculi to the polycystic kidney. There may be hydronephrosis to complete renal failure. It is nor operable to a high extent and use of medicines preferred. If not controlled, regular dialysis up to 3times per week and even renal transplants are used. As per advice, lots of water and fruits are suggested for consumption, and avoiding rich and spicy foods are suggested.