Functional Morphology of Kidney

The mammalian urinary system has a pair of bean-shaped kidneys located on the dorsal side of the lower abdominal cavity on either side of the vertebral column (Fig. 9.1). Both kidneys together constitute 0.4% of the body weight.

There are two tubular structures called ureters carrying urine from the pelvis to the urinary bladder for temporary storage. Urine is removed from the urinary bladder periodi­cally through the urethra.

The nephrons are the basic functional units of the kidney. Several nephrons of a kidney bind together by connective tissue. The number of nephrons in each kidney varies in different species of mammals, for example, about 4 million in cattle, 1.25 million in pigs, 1 million in humans, 0.5 million in dogs and 0.25 million in cats. Each nephron has a renal corpuscle and a renal tubule. The renal corpuscle comprises a tuft of capillaries called glomerulus surrounded by a cup-shaped Bowman's capsule (Fig. 9.2). The Bowman’s capsule encloses a space between the two walls

Fig.

Fig. 9.2 Schematic diagram showing the major features of the renal corpuscle. (a) Renal corpuscle, (b) structure of glomerular capillary, (c) enlarged structure of glomerular capillary wall showing the intercellular pores (fenestrations) between the endothelial cells lining the glomerular

capillary, acellular basement membrane and filtration slits situated between the foot processes of the podocytes constituting the visceral layer of Bowman’s capsule

(visceral and parietal) of it called Bowman’s space, which continues as the renal tubule.

The renal tubule has three structurally and functionally different divisions; the proximal convoluted tubule (PCT) located in the cortex, the loop of Henle (with its thin descending limb, thin ascending limb and thick ascending limb) located in the medulla and the distal convoluted tubule (DCT) present in the cortex. In mammals, up to 8 DCTs coalesce to form a collecting duct (CT), which runs back to the medulla (Fig. 9.3). The portion of the CT that lies in the cortex is the cortical collecting duct, which dips into the medulla called the medullary collecting duct. The collecting ducts plunge deep into the medulla to evacuate their contents to the renal pelvis.

In mammals, there are two types of nephrons: cortical nephrons and juxtamedullary nephrons (Fig. 9.4). The glo­merulus of cortical nephrons lies in the peripheral cortex, and that of the juxtamedullary nephrons lies in the cortex’s inner layer, adjacent to the medulla. The arrangement of glomeruli in the cortex gives the cortex a granular appearance.

The position of the loops of Henle varies in two types of nephrons; the terminal hairpin loop of Henle dips slightly into the medulla, whereas the longer loops of juxtamedullary nephrons pass deep into the medulla.

Fig.

Fig. 9.4 Relative positioning of cortical and juxtamedullary nephrons. The glomeruli of both the nephrons are placed in the cortex, but the loop of Henle of juxtamedullary nephrons dips deep into the medulla

9.3.1 Renal Blood Supply

In a human being with a 70 kg body weight, 1100 mL of blood flows through both kidneys per minute, constituting 22% of cardiac output. The arterial blood reaches the kidney through the renal artery through the hilum. It then branches progressively to form the interlobar arteries, arcuate arteries, interlobular arteries (radial arteries) and short afferent arterioles, one of which supplies blood to the capillary tuft of each nephron. The capillaries of the glomerulus reunite at the exit point as efferent arteriole. The efferent arteriole then subdivides into the second capillaries called peritubular capillaries surrounding the proximal and distal convoluted tubules in the renal cortex. These capillaries ultimately rejoin to form the vessels of the venous system, which run parallel to the arteriolar vessels and progressively form the interlobu­lar vein, arcuate vein, interlobar vein and renal vein, which leaves the kidney at the hilus. The peritubular capillaries of juxtamedullary nephrons form hairpin loops in close associa­tion with the loops of Henle. These vascular hairpins are known as vasa recta. When passing through the medulla, the collecting ducts of both cortical and juxtamedullary nephrons always run parallel in proximity to the ascending and descending loops of Henle of juxtamedullary nephrons and vasa recta.

9.3.2 Juxtaglomerular Apparatus

The thick ascending limb of the loop of Henle enters the cortex from the medulla, passes between the afferent and efferent arterioles and continues as a distal tubule (Fig. 9.5). The epithelial cells of the tubules that come in contact with the afferent and efferent arterioles facing the angle between the blood vessels are collectively known as macula densa. The presence of epithelial cells in the densa marks the begin­ning of the distal tubule. The specialised granular smooth muscle cells of afferent arterioles that contact macula densa are called juxtaglomerular (JG) cells. From these cells, pro­teolytic enzyme renin is produced. Mesangial cells and the matrix secreted by the mesangial cells occupy the space between the afferent arteriole, efferent arteriole, macula densa and glomerular capillaries. Those cells located between the arterioles and macula densa are lacis cells or extraglomerular mesangial cells. Mesangial cells exhibit con­tractile property to aid blood flow through glomerular capillaries. Besides, they also secrete prostaglandins, main­tain basement membrane and possess phagocytic properties. The macula densa, juxtaglomerular cells and lacis cells are together known as the juxtaglomerular (JG) apparatus. This apparatus has functional importance in renal haemodynamics and glomerular filtration rate (GFR).

9.3.3 Nerve Supply

Sympathetic fibres of the autonomous nervous system enter the kidneys through the hilus along with the renal artery and vein and innervate blood vessels of the kidney, nephron segments and JG cells. Motor activity of these nerves produces alterations in renal haemodynamics and composi­tion of the tubular fluid. The kidney contains afferent sensory nerve fibres sensing stretch, located mainly in the renal pelvic wall. The renal pelvic pressure increases due to the accumu­lation of urine, and it causes stretching of the pelvis wall that stimulates the mechanoreceptors present in between the smooth muscles.

Fig. 9.5 Juxtaglomerular apparatus. It comprises specialised granular afferent arteriolar JG cells, macula densa of DCT and extraglomerular mesangial cells or lacis cells. JG apparatus is concerned with controlling the dynamics of renal circulation and tubular reabsorption of sodium ions

Fig. 9.6 Glomerular membrane. (a) Filtration surface of the glomerular membrane with podocytes, glomerular basement membrane and endothelial cells. (b) The structure of a glomerular capillary, which is placed in close apposition with the visceral layer of Bowman’s capsule

diminished efferent renal sympathetic nerve activity in the contralateral kidney causes excess sodium ions and water excretion as a compensatory mechanism. The renorenal reflex coordinates the functions of the two kidneys and thus facilitates the physiological regulation of sodium and water balance to maintain homeostasis.

9.3.4 Ultrastructure of Glomerulus

The glomerular capillary membrane has three layers, the innermost highly fenestrated endothelial layer, next to that a basement membrane and an outermost epithelial layer made of podocytes (Fig. 9.6). Podocytes are flattened epithelial cells with foot processes, which interdigitate with adjacent foot processes, leaving filtration slits in between and encircling the complete glomerular tuft. The basement mem­brane is composed of collagen and glycoprotein, and the collagen provides structural strength to the membrane. The glycoproteins repel the plasma proteins, if present, with their negative charge. Less than 1% of albumin is completely excluded from the filtrate usually.

9.4