BIOLOGY Volume 3 - A Guide to General Biology - 2004

20. EXCRETION AND OSMOREGULATION

20.5. The Human Kidney

20.5.2. General Structure and Blood Supply of the Nephron

The basic Structural and functional unit of the Kidney is the nephron (Fig. 20.13), along with its associated Blood Vessels. A single human kidney contains about one million nephrons, each measuring approximately 3 cm in length. The total length of the tubules in one kidney approaches 120 km, providing a massive surface area for metabolic exchange. During one complete circulatory cycle, roughly one-fifth of the total blood volume passes through the Kidneys, filtering out ~125 ml of fluid per minute. About 99% of this Water is subsequently reabsorbed into the bloodstream, meaning urine is produced at an average rate of -1 ml/min, although this value fluctuates considerably depending on various factors, including the volume and composition of fluid intake.

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Fig. 20.13. Diagram of the nephron Structure (the scale of individual parts is not maintained).

Each nephron consists of six distinct regions that differ in structure and physiological function:

1) the renal corpuscle (Malpighian corpuscle), comprising Bowman's capsule and the glomerulus;

2) the proximal convoluted tubule;

3) the descending limb of the Loop of Henle;

4) the ascending limb of the loop of Henle;

5) the Distal convoluted tubule;

6) the collecting duct.

The structural relationships among these PARTS OF THE nephron are shown in Fig. 20.13.

There are two types of nephrons: cortical and juxtamedullary. Cortical nephrons are located within the renal cortex and possess relatively short loops of Henle that extend only a short distance into the medulla. In juxtamedullary nephrons, the renal corpuscles lie near the border of the cortex and medulla. They feature long loops of Henle that penetrate deeply into the renal medulla (Fig. 20.14, A). The Significance of these Two Types of nephrons is related to their functional differences. When the body has a normal water balance, plasma volume is regulated by the cortical nephrons, whereas during water deprivation, enhanced water reabsorption takes place in the juxtamedullary nephrons.

Blood enters the kidney via the renal artery, which branches into progressively smaller vessels and ultimately forms the afferent glomerular arterioles. These enter Bowman's capsules to form capillary glomeruli. Blood leaves the glomeruli via efferent arterioles and then flows through a network of peritubular capillaries located in the cortex, which surround the proximal and distal convoluted tubules of all nephrons as well as the loops of Henle of cortical nephrons (Fig. 20.14, D). Branching off from these capillaries are the vasa recta, which run through the medulla parallel to the loops of Henle and collecting ducts. The function of both vascular networks is to return blood containing valuable substances back to the general Circulatory system. Considerably less blood flows through the vasa recta than through the capillary networks surrounding the proximal and distal convoluted tubules; this maintains a high osmotic pressure in the interstitial fluid of the medulla, as described below.

Fig. 20.14. A. Cortical and juxtamedullary nephrons. B. Blood supply of a juxtamedullary nephron.