Kidneys are mesodermal in origin and developed from nephrostomes of early embryo. A pair of metanephric kidneys is situated on each side of dorsal aorta, outside the coelom, in abdominal region. It is covered by peritoneum on the ventral side.
Right kidney is slightly lower than the left (reverse in rabbit, shown in the given figure) because of the large area occupied by the liver.
The concavity of kidney is called hilus, at these place blood vessels, nerves and lymphatics enter and leave the kidneys. Kidney is covered by a thin fibrous capsule of connective tissue.
Internally kidney is divided into two regions – outer cortex and inner medulla.
Cortex is covered by fibrous connective tissue forming a tough capsule. It is granular and darker in appearance. It contains malpighian corpuscles, proximal convoluted tubule and distal convoluted tubule.
Medulla is lighter and striated in appearance it has many conical shaped multilobular renal pyramids (composed of tubular parts of nephrons and blood vessels) with broader bases towards the outer side and pointed renal papillae towards pelvis side [Pelvis is large funnel shaped space towards the concavity]. Renal columns of bertini are invagination of cortex in the medulla.
Each kidney has about 1.0 million uriniferoustubule or nephrons embedded in the connective tissue. Nephron is the structural and functional unit of kidney. Nephrons open into collecting tubule/duct and duct of bellini which opens into the renal pelvis through renal papillae.
There are two types of nephrons according to their position in kidney.
1. Cortical nephron
2. Juxtamedullary nephron.
Basically nephron is formed of two parts –
I. Malpighian corpuscles
II. Tubule (which opens into collecting tubules)
Which lines the concavity of Bowman’s capsule. Each nephron gets blood supply by an afferent anteriole which breaks into the network of capillaries forming glomerulus in the Bowman’s capsule.
Blood from glomerulus is drained out by efferent arterioles which break up into peritubular capillaries around the rest of the nephron. These capillaries join and make the renal venule. This is a type of cortical nephron.
Tubule consists of proximal convoluted tubule, loop of Henle and distal convoluted tubule. Its walls are made up of simple cuboidal cells which become ciliated in the neck region. Neck (extension of Bowman’s capsule) leads to proximal convoluted tubule (PCT) which is lined by epithelial cells (cone shaped) with brush border due to the presence of abundant microvilli.
PCT is surrounded by peritubular capillaries and is the seat of reabsorption. Then it (PCT) goes into loop of Henle or U-shaped tubular part that lies in the renal medulla. Loop of Henle has 2 limbs-
Descending: Thin walled, lined by squamous epithelium and permeable to water
Ascending: Thick walled formed by cuboidal epithelium and impermeable to water. It reaches the glomerulus of the nephron from which the tubule arose and passes close to its afferent arteriole and efferent arteriole. The walls of the afferent arterioles contain the renin-secreting juxtaglomerular cells.
These cells are modified smooth muscle cells. At this point, the tubular epithelium is modified histologically to form the macula densa. The juxtaglomerular cells, the macula densa, and the lacis cells (Lacis cells = specialised glandular cells present at the vascular angle formed by the afferent and efferent arterioles whose significance at this location is unknown) near them are known collectively as the juxta-glomerular apparatus.
Macula densa cells have a basal polarization and monitor the chemical composition of fluid in the distal convoluted tubules. This apparatus is involved in events that regulate the volume and composition of blood (like salt concentration, fluid balance); consequently it is involved in the blood pressure regulation.
These limbs operate the counter current mechanisms to made urine hypertonic. After that the loop of Henle passes into distal convoluted tubule (DCT) which is a straight part, present in renal cortex. DCT lined by columnar cells without brush border leads into the collecting tubules.
Collecting tubule is formed by cuboidal columnar epithelium. Both (DCT in association with collecting tubules) constitute the area of tubular secretion of waste products from blood capillaries and interstitial fluid into the lumen of nephron.
Many collecting tubules of a number of nephrons open into a bigger duct called collecting duct, which unites to form still larger ducts called duct of Bellini. The latter runs through the renal pyramids and opens into the renal pelvis.
Another hormone ERYTHROPOIETIN, a circulating glycoprotein is produced by interstitial cells in the peritubular capillary bed of the kidney. It increases the number of erythropoietin-sensitive committed stem cells in the bone marrow that are converted to red blood cell precursors and subsequently to mature erythrocytes.
The usual stimulus for erythropoietin secretion is hypoxia, but secretion of the hormone can also be stimulated by cobalt salts and androgens.
Like renin secretion, erythropoietin secretion is facilitated by catecholamines via a b-adrenergic mechanism, although the reninangiotensin system is totally separate from the erythropoietin system. Erythropoietin production is also stimulated by adenosine and inhibited by the adenosine antagonist theophylline.
Ureters are narrow tubes with a thick muscular wall. Starting as a pelvis within the kidney, each ureter runs behind through the hilus and opens into a common urinary bladder. Peristalsis of ureter checks the regurgitation of urine as it undergoes peristalsis to pass urine from kidney to urinary bladder.
Ureters open in a urinary bladder which opens outside by urethra. Urethral opening is guarded by two pairs of circular sphincters.
Urinary bladder stores the urine temporarily. It has involuntary and smooth muscles and is lined by transitional epithelium.
It is a muscular tube that connects urinary bladder and external opening of urinary tract. Urethra is smaller in females and longer in males.