The kidney is the primary organ of excretion found in the upper chordates. It is an abdominal organ, situated just anterior to the posterior abdominal wall. Being two in number, they are located one on each side of the vertebral column.
It is a retroperitoneal organ, i.e., it has no peritoneal covering, peritoneum gets reflected from in front of the organ. We will see the complete details as below.
Kidney Structure and Functions
Location: Anatomically speaking, the kidney is located in the hypochondriac, epigastric, umbilical and lumber regions of the abdomen. The length of the average adult kidney is 11 cm, extending upper up to the upper border of twelfth rib, up to the center of the third lumbar vertebra below. The right kidney is slightly smaller than the left one and located slightly below the left one just to accommodate the huge massive liver. Average adult kidneys are 6cm broad and 3cm in thickness. Average kidneys are moderately weighed- 150gm in males and 135gm in females.
Deep red is the color of kidneys because of the high supply of blood.
Structure: The kidney has several layers of different tissues, that are designated to have different functions. The layers or parts of kidneys are
A. Cortex: The outer most layer kidney includes the cortex, also called renal cortex. It is highly vascularized and is red in color.
Renal cortex has two parts:
- There are Cortical arches or lobules, which form arches over the bases of pyramids.
- There are renal columns, which goes in between the pyramids.
B. Medulla: The layer inner to the cortex is the Medulla, also called Renal Medulla. It is also highly vascularized, but not as much as the renal cortex. So it is somewhat pale to naked eyes.
The lobe of the kidney is formed by each pyramid of the kidney and its overlying cortex.
The renal medulla is composed of about 9 to 10 conical masses, which is called renal pyramids.
C. Sinus: The inner to the renal medulla is an open space, called sinus, also called renal sinus.
Renal sinuses are the spaces that run from the hilum of the kidney up to the kidney functional parts.
Content of Renal sinuses include
- Renal Artery and its branches
- The tributaries of the renal vein
- Major and minor calyces.
These major calyces are the branchings of the renal pelvis. Renal pelvis divides into 2 to 3 major renal calyces. These each major calyces divide into 7 to 13 minor calyces.
Blood Supply of Kidney: Kidneys receive its blood from the Renal Arteries, which are supplied one to each kidney and are direct branches of Abdominal Aorta itself. The renal Artery gives 5 segmental divisions- four from the anterior part and one from the posterior part.
The content of renal hilum, the entry point of Kidney only which is 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 Nephron has four parts:
I. Bowman’s Capsule
2. Proximal Convoluted Tubule or commonly called PCT
3. Loop of Henle popularly called LH which again has two parts –
- Ascending loop of Henle
- Descending loop of Henle
4. Distal Convoluted Tubule or commonly called as DCT.
Collecting Duct also called as CT. It is a common tube for many nephrons.
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.
Bowman’s capsule: 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 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 album, globulin, etc. in the Bowman’scapsule. This filtrate is called primary filtrate.
An afferent arteriole is a branch of an interlobular artery, which is a branch of a renal artery which 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 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 has 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 need. Ions are also absorbed. Loop of Henle also is the site of water absorption according to body need. 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 Duct: Collecting Ducts are the successors of the Distal Convoluted Tubule. It has typical I and P cells. These cells are involved in 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.
Conclusion and Functions: 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 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.