In this article we will discuss about:- 1. Meaning of Nephrons 2. Number of Nephrons 3. Structure 4. Types.
Meaning of Nephrons:
Nephrons are structural and functional units of the kidneys.
Number of Nephrons:
Each human kidney contains about one million (ten lakhs) nephrons.
Structure of Nephron (= Uriniferous Tubule):
It consists of Malpighian corpuscle and renal tubule.
1. The Malpighian Corpuscle (= Renal corpuscle; Fig. 19.7):
It comprises glomerulus and Bowman’s capsule.
It is a tuft of capillaries. This capillary network consists of a complex anastomosing plexus of vessels and not of independent capillary loops. Blood enters the glomerulus through an afferent arteroile and leaves it through an efferent arteriole. Glomerular filtration takes place in the glomerulus.
(ii) Bowman’s Capsule (= Glomerular capsule):
It is a double cup-shaped structure. The lumen of the capsule is continuous with the narrow lumen of the renal tubule. The two layers of the Bowman’s capsule are outer parietal layer and inner visceral layer. The parietal layer consists of squamous cells. The visceral layer surrounds the glomerulus and is composed of special type of cells, the podocytes (Fig. 19.8)
Functional structure of Glomerular Membrane:
It has three layers:
(a) The endothelium is perforated by small holes called fenestrae.
(b) The basement membrane is present outside the endothelium and is mainly composed of a meshwork of collagen and proteoglycan fibrillae that also have large spaces through which fluid can filter,
(c) The epithelium (= epithelium of visceral layer) has cells which are of peculiar shape and are called podocytes (foot cells).
The podocytes are so called because they possess foot like processes (projections), the pedicels. The space between pedicels are called slit pores (= filtration slits) through which the glomerular filtrate filters. The permeability of the glomerular membrane is from 100 to 500 times as greater as that of the usual capillary.
2. The Renal Tubule. It consists of the following parts:
(i) Proximal Convoluted Tubule (PCT):
The Bowman’s capsule leads into the proximal convoluted tubule that is lined by cuboidal epithelial cells bearing a brush border of tall microvilli (finger-like processes) at free end which increase the surface area. The cells have numerous mitochondria near the basolateral surface, which allows the reabsorption of salts by active transport.
(ii) Loop of Henle:
It starts at the end of proximal convoluted tube and its major part lies in the medulla. It consists of a descending limb and an ascending limb. The descending limb comprises thick segment and thin segment. The upper part of the descending limb is the thick segment that has the same diameter as that of the proximal convoluted tubule. It is also lined by cuboidal epithelium.
However, its cells have much less microvilli and mitochondria compared to the cells of proximal convoluted tubule. The distal part of descending limb is the thin segment and is lined by flat epithelial cells (low cuboidal or squamous cells) with thinly scattered microvilli and very few mitochondria. Presence of very few cell organelles indicate a passive rather than active role in ionic movements.
The ascending limb also has thin segment and a thick segment. The thin segment forms the major part of the loop of Henle. The thick segment of the ascending limb is lined by cuboidal epithelial cells with short apical microvilli. The cells have numerous mitochondria.
(iii) Distal Convoluted Tubule (DCT):
The thick segment of the ascending limbs opens into the distal convoluted tubule lying in the cortex. It is lined by cuboidal cells that have few, small and irregularly spaced microvilli (no brush border). The terminal part of the distal convoluted tubule is straight and is called the junctional tubule (= connecting tubule).
The distal convoluted tubule joins collecting duct that is lined by cuboidal or columnar epithelial cells with a few microvilli. Many distal convoluted tubules of number of nephrons open into a bigger duct termed as the collecting duct. Collecting ducts unite to form still larger ducts called the ducts of Bellini. The latter run through the renal papilla.
Blood supply. The renal artery arises from the dorsal aorta. In the kidney, renal artery divides into the afferent arterioles. An afferent afteriole enters each Bowman’s capsule to form the glomerulus. An efferent arteriole arises from the glomerulus. The efferent arteriole has narrower lumen than that of afferent arteriole.
The efferent arteriole divides to form the peritubular capillary network around the proximal and distal convoluted tubules of the nephrons. From the peritubular capillary network arise the capillaries of vasa recta (sing, vasa rectum), which extend parallel to the loops of Henle and the collecting ducts in the medulla.
The vasa recta consist of descending capillaries and ascending capillaries. All the capillary networks join to form renal venules which join to form a renal vein that opens into the inferior vena cava.
The efferent arterioles and peritubular capillaries technically constitute a portal system.
The Juxtaglomerular Apparatus (JGA):
(a) The smooth muscle cells of both the afferent and efferent arterioles are swollen and contain dark granules. These cells are called Juxtaglomerular (L. Juxta- near, glomerular- glomerulus) cells. The granules are composed mainly of inactive renin. It means renin (an enzyme, which acts as hormone) is secreted by juxtaglomerular cells. Renin converts angiotensinogen (present in the blood) into angiotensin.
The latter increases blood pressure. Angiotensin also stimulates the secretion of aldosterone by the adrenal cortex, thus influencing the reabsorption of sodium ions by the distal convoluted tubule and that of water through the collecting duct.
(b) The epithelial cells of the distal convoluted tubule that come in contact with the afferent and efferent arterioles are more dense than the other tubular cells and are collectively called the macula densa (L. macula- a spot, densa- dense).
The cells of macula densa are columnar (rather than cuboidal as in the rest of the tubule). The cells of the macula densa lie in close contact with the juxtaglomerular cells. The cells of macula densa may function as chemoreceptors; feeding information to the juxtaglomerular cells. The juxtaglomerular cells and macula densa together form the juxtaglomerular apparatus or complex.
(c) In addition to the juxtaglomerular cells and macula densa, the juxtaglomerular apparatus has a third component, the lacis cells. These cells are so called as they bear processes that form a lace like network.
Laci cells are located in the interval between the macula densa and the afferent and efferent arterioles. The function of laci cells is unknown. The juxtaglomerular apparatus is only found in juxtamedullary nephrons and not in cortical nephrons.
Types of Nephrons:
On the basis of location, the nephrons are of two types:
1. Juxtamedullary Nephrons:
They form about 15 percent of total nephrons. Their glomeruli are found in the inner margin of the cortex (near its juction with medulla). They are large in size. The loops of Henle are long and are found deep into the medulla. They are associated with vasa recta. They control plasma volume when water supply is short.
2. Cortical Nephrons:
They form about 85 per cent of total nephrons. They mainly lie in the renal cortex. Their glomeruli are found in the outer cortex. The loops of Henle are short and extend a short distance into the medulla. They do not have vasa recta.
Pressures in the Renal Circulation:
(i) In the small arteries and afferent arterioles, the pressure is 100 mm Hg.
(ii) Glomerular Hydrostatic Pressure (GHP) is the blood pressure in the glomerulus. It is about 60 mm Hg.
(iii) Blood Colloidal Osmotic Pressure (BCOP) is exerted by plasma proteins in the glomeruli. The plasma proteins are not filtered through the glomerular capillaries. This pressure opposes filtration. It is about 32 mm Hg.
(iv) Capsular Hydrostatic Pressure (CHP) is the pressure exerted against the filtration membrane by the filtrate in Bowman’s capsule during filtration. This pressure also opposes filtration and represents a back pressure of about 18 mm Hg.
Thus both BCOP and CHP oppose the glomerular filtration and are called pressures opposing filtration.
(v) Effective Filtration Pressure (EFP). The effective filtration pressure (EFP) is the total pressure that promotes filtration, is determined as follows:
EFP = GHP — (BCOP + CHP)
= 60 – (32 + 18)
= 60 – 50 = 10 mm Hg
Thus, a pressure of 10 mm Hg causes a normal amount of blood plasma to filter from the glomerulus into the glomerular capsule.
Glomerular Filtration Rate (GFR):
The quantity of glomerular filtrate formed each minute in all the nephrons of both kidneys is called glomerular filtration rate. In the normal person, the glomerular filtration rate is 125 ml per minute or about 180 litres per day.
The fraction of the (part of the) renal plasma which becomes the filtrate is called filtration fraction. It is the ratio between renal plasma flow and glomerular filtrate. It is expressed in percentage.
Thus filtration fraction = Glomerular filtration rate/Renal plasma flow x 100
125ml/min/650ml/min x 100 = 19.2%
The normal filtration fraction varies from 15 to 20%.