THE NEPHRON
Nephron is the structural and functional unit of the kidney. Kidney has millions of nephrons, and each nephron is made up of two parts: a renal corpuscle or Malpighian corpuscle, which is a blind end, and a tubular portion called the renal tubule.
The renal corpuscle is situated in the cortex of the kidney, either near the periphery or near the medulla. The renal corpuscle is formed by two portions: 1. Glomerulus 2. Bowman capsule.18.10.1 Glomerulus
Glomerulus is a bunch of capillaries surrounded by the Bowman capsule. It consists of glomerular capillaries interposed between the afferent arteriole and the efferent arteriole. Thus, the vascular system in the glomerulus is purely arterial. Glomerular capillaries are composed of a single layer of endothelial cells. The endothelium has many pores called fenestrae. The presence of fenestrae is proof of the filtration function of the glomerulus (Figure 18.4).
FIGURE 18.3 Blood flow to the kidney
FIGURE 18.4 Bowman capsule and renal corpuscles
18.10.2 Bowman Capsule
complete, leaving small cleft like spaces in between, called slit pore.
18.10.3 Tubular Portion of the Nephron
Tubular portion starts with the proximal convoluted tubules (PCT), which lie in the cortex of the kidney. The proximal tubule is continued by the loop of Henle, which dips into the medulla. The loop of Henle is composed of three segments: the thin descending limb, thin ascending limb, and thick ascending limb. The descending thin segment of each loop is continuous with the ascending thin segment at the hairpin curve. The ascending limb has a thickened segment in the cortex known as the macula densa. Beyond the macula densa, the tubule is known as the distal tubule, which resides entirely in the renal cortex. This is followed by the cortical collecting tubule, which leads to the cortical collecting duct. The cortical collecting ducts join to form a single medullary collecting duct and finally a larger collecting duct
18.10.4 Approximate Number of
Nephrons in Each Kidney
Approximate number of nephrons in each kidney of cattle and dogs is around 400,000. Nephron numbers vary considerably among species, while within a species, nephron numbers are relatively constant.
Bowman capsule is a capsular structure that encloses the glomerulus. It is formed by two layers; the inner layer is the visceral layer, and the outer one is the parietal layer. The visceral layer covers the glomerular capillaries, and the parietal layer is continuous with the wall of the tubular portion of the nephron. Bowman capsule resembles a funnel with filter paper. Epithelial cells of the visceral layer fuse with the basement membrane, but the fusion is not
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