Page 471 - Concise Pathology for Exam Preparation ( PDFDrive )

456 SECTION II Diseases of Organ Systems

and is lined by cuboidal cells with a brush border (presence of microvilli), acidophilic
granular cytoplasm and central nuclei.
2. Loop of Henle: The PCT continues as the straight part of loop of Henle. The loop of
Henle begins near the corticomedullary junction; it is U shaped and has a thin descend-
ing and a thick ascending portions. The thin portion is lined by flat epithelium with
nuclei projecting into the lumen. The thick portion is identical in structure to the DCT
and ends at the corticomedullary union.
3. DCT: The thick part of loop of Henle becomes tortuous, enters the cortex and contin-
ues as the DCT. It is lined by flatter cells which are smaller in size as compared to cells
lining the PCT, are less acidophilic and do not have a brush border. The lumina of
the distal tubules are larger due to the smaller size of the lining cells. DCT touches the
vascular pole of the renal corpuscle of its nephron close to the point of entry of the
afferent arteriole. Here, the lining epithelium gets modified to become columnar with
closely packed nuclei (thereby appearing darker). This area is called the macula densa.
4. Collecting tubules: The collecting ducts join to form the larger straight ducts called
the papillary ducts of Bellini. Collecting tubules form the major bulk of the medulla.
The smaller ducts are lined by cuboidal epithelium; however, as they dip into the me-
dulla the lining epithelium becomes columnar. The cytoplasm of the cells is uniformly
pale staining.

Juxtaglomerular Apparatus (JGA)
The JGA is located in the vascular pole of the glomerulus and has three parts:
1. Juxtaglomerular cells—These are epithelioid cells with granular cytoplasm located in
the media of the afferent arteriole and secrete rennin.
2. Macula densa
3. Extraglomerular mesangial or Lacis or Polkissen cells—Lightly staining cells whose
function is not clearly understood.

Vascular Supply
The kidneys receive approximately 20% of the cardiac output from the paired renal arter-
ies which enter into the renal hilum. The anterior half of the kidney can be divided into
upper, middle and lower segments, each supplied by a segmental branch of the anterior
division of the renal artery. The posterior half of the kidney is divided into apical,
posterior and lower segments, each supplied by branches of the posterior division of the
renal artery. The segmental branches branch into interlobar arteries, which travel
between the major calyces to branch further into arcuate arteries. The arcuate arteries
run between the cortex and medulla across the bases of the renal pyramids. They then
radiate into interlobular arteries, extend into the cortex of the kidney to finally become
afferent arterioles, each of which supplies a single glomerulus. From the glomerulus arise
the efferent arterioles. The efferent arterioles supply the peritubular capillary plexus
which anastomoses with the capillary plexus of another nephron. Some of the terminal
branches of the interlobular arteries become perforating radiate arteries, which supply
the renal capsule.

FUNCTIONS OF THE KIDNEY

1. Maintenance of electrolyte levels and acid–base balance
2. Regulation of blood pressure and maintenance of salt and water balance
3. Removal of water soluble wastes from the blood, eg, urea and ammonia and reabsorption
of water, glucose and amino acids
4. Production of hormones like calcitriol, erythropoietin and rennin

URINE FORMATION

• Kidney maintains water and electrolyte balance and contributes to acid–base homeostasis.
• Composition of urine varies with water, salt and protein intake.

mebooksfree.com