What are kidneys made up of ? || Components of Nephron

 Kidneys

The kidneys are responsible for filtering toxins, wastes, ingested water, and mineral salts out of the bloodstream. Kidneys are also responsible for regulating the acidity of the blood by excreting alkaline salts when necessary.

The paired kidneys are compact, reddish brown, oval or bean shaped structures located on the dorsal wall of the abdominal cavity, one on either side of the vertebral column behind the peritoneum and below the diaphragm, so also called peritoneal kidneys and extend between the twelfth thoracic and third lumbar vertebrae. The right kidney is slightly lower than the left one. 

It is due to the fact that the right side of abdominal cavity is occupied by the right liver lobe. Each kidney is about 4-5 inches (10cm.) long, 3 inches (6cm.) wide and about two inches (4cm.) thick, weighing about 150 grams in an adult male and 135 grams in adult female.

The left kidney is slightly larger than right one. Because of the high degree of vascularity (presence of blood vessels).

Each kidney is enclosed in a thin, tough, fibrous, white capsule called fascia which encloses a semi liquid fatty tissue, the adipose tissue which keep the kidney in position. Each kidney has an outer convex surface and inner concave called the hilus renalis, through which the renal artery, the renal vein, nerves, and the ureters enter or leave the kidney. The kidneys of mammals are metanephric type. The adrenal gland is present at the tip of each kidney.

Histology of Kidney

The longitudinal section of a kidney shows that it consists of two regions, the outer granular darker region called the cortex, and the striated, inner lighter colored medulla. The cortex includes a large number of tiny heads of nephrons and with many capillaries and connective tissue. 

The medulla includes about 6 to 15 conical elevated areas, the renal pyramids (multilobular) which contain the collecting and discharging tubules (which carry the urine from cortex to the pelvis of kidney) of nephron. 

The renal cortex extends into the medulla in between the each two pyramids is called renal column of Bertini. The broad base of each pyramid lies towards the cortex and the narrow end, the enal papilla towards the pelvis. The renal papilla extends into minor calyces. The minor calyces are joining to form 2 or 3 major calyces which are leads into a large funnel like collecting space called the pelvis where the ureter leaves the kidney.

Nephron

The nephron, an evolutionary modification of the nephridium, is the kidney's functional unit or the nephron is the basic excretory unit of the kidney; there are over a million in each kidney.

 Each nephron is about 6cm long and consists of a glomerulus, Bowman's capsule, proximal convoluted tubule and associated renal tubules (descending and ascending tubules with a Henle's loop in between) and a distal convoluted tubule leading to a large collecting duct (also called the duct of Belini). The collecting duct passes through the medulla and opens into the pelvis of the kidney at the apex of a pyramid.

The Malpighian body, proximal and distal convoluted tubules lie in the cortex, while Henle's loop and main collecting ducts are present in the medulla. Blood flows into the kidney through the renal artery, which branches into capillaries associated with the glomerulus. Arterial pressure causes water and solutes from the blood to filter into the capsule. 

Fluid flows through the proximal convoluted tubule, which include the loop of Henle, and then into the distal proximal convoluted tubule The distal tubule empties into a collecting duct. Fluids and solutes are returned to the capillaries that surround the nephron tubule.

Components of the Nephron

1. Glomerulus: 

The glomerulus is a small knot of blood vessels formed by a capillary network about 50 capillaries) of the renal artery. The smaller efferent vessels take the blood away from the glomerulus and enter the capillary network around the tubule of the nephron.

The capillaries unite to form the venules to form the renal vein which joins the inferior vena cava. The glomerular wall has numerous minute pores which mechanically filters the blood.

2. Bowman's capsule: 

The glomerulus is surrounded by a double walled cup-shaped structüre called Bowman's capsule. It is lined by a thin and semi permeable single layer of squamous epithelial cells supported over basement membrane. The outer wall is called parietal layer while the inner layer is called visceral layer. The space between the two is called capsular space. 

The visceral layer is made of specialized epithelial cells called podocytes. The spaces between the podocytes are called filtration slits. The inner membrane of the Bowman's capsule is apposed closely to the walls of glomerular capillaries. 

The Bowman's capsule and the glomerulus together are called the Malpighian body which was first reported by M. Malpighi (1966). It is only permeable for glucose, water, salt, urine, uric acid, etc.

3. Proximal convoluted tubule: 

The proximal convoluted tubule is a long and coiled tube arises from posterior end of Bowman's capsule. It is present in the cortex of kidney. It is lined by ciliated cubical epithelial cells. Its wall is lined by a single layer of columnar cells bearing microvilli on the free surface.

4. Loop of Henle: 

It runs straight in the renal medulla. It is a 'U' shaped tubular structure formed by a thin descending limb and a thick ascending limb. The descending limb is continuation of proximal convoluted tubule into the renal medulla and an ascending limb enters the cortex and oins the distal convoluted tubule. 

The ascending limb is lined by cuboidal epithelium but the descending limb is lined by squamous epithelial cells. It reabsorbs 75% of the water, salts, glucose, and amino acids.

5. Distal convoluted tubule: 

It is a convoluted part and is again present in the cortex. It is lined by cuboidal epithelium. It leads into the collecting tubule.

6. Collecting duct: 

The collecting ducts are larger tubes each receiving about eight uriniferous tubules and each collecting duct pass into renal medulla and join to each other, forming still larger ducts of Bellini. 

These run through the renal pyramids and open into calyces which ultimately leads into pelvis. The collecting ducts are internally lined by ciliated cuboidal and columnar epithelium in different regions.

7. Renal blood supply: 

Each kidney is supplied by renal artery directly from the dorsal aorta. Inside the kidney renal artery divides into a number of major branches called interlobar arteries. Each artery divides into numerous afferent arterioles. 

An afferent arteriole enters the cavity of Bowman's capsule and forms a tuft of capillaries called glomerulus.Blood from glomerulus is drained by the efferent arteriole which immediately breaks up into peritubular network of capillaries and lies around the proximal, distal convoluted tubule and loop of Henle. These capillaries then form venules which join together to form the branch of renal vein which removes the venous blood of kidney.