The Nephron
The nephron is a fundamental structural and functional unit of a kidney that regulates water and soluble substances in the bloodstream. It carries out this function through blood filtration, reabsorption of essential substances or water, and excretion of the waste as urine. The main parts of a nephron are the renal corpuscle and the renal tubule.
The Renal Tubule
The function of this segment is the conversion of the filtrate to urine. The renal tubule consists of a tube that starts from the glomerular capsule to the top of the medullary pyramid. This segment has a length of three centimeters and is divided into four parts namely the proximal convoluted tubule, nephron loop, distal convoluted tubule, and the collecting duct.
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The proximal convoluted tubule is the longest part. It is long and coiled with microvilli on the simple cuboidal epithelium. It performs most of the absorption.
The nephron loop lies in both the cortex and medulla. It is U-shaped with an ascending and descending limb that are close and almost parallel. The nephron loop contains thick and thin segments. The thick segment is partially formed by the initial part of the descending limb and partially by parts or the whole ascending limb. The thick segments in this part have numerous mitochondria responsible for producing energy used in the active transport of salts. The thick segments contain a simple cuboidal epithelium. The thin segment is formed by a large part of the descending limb, the bend, and a small part of the ascending limb. The simple squamous epithelium in this segment is highly permeable to water hence reabsorption occurs.
The distal convoluted tubule has a smooth surface with no microvilli. It is shorter and straighter than the proximal convoluted tubule. It is found at the end of the nephron.
The collecting duct serves many nephrons by collecting fluid from their distal convoluted tubules. The ducts converge near the top of the medullary pyramid to form a papillary duct, which is larger. The pores of papillary ducts drain urine into the minor calyx that surrounds each papilla (Saladin et al., 2018) .
Capillary Systems Adjacent to the Nephron
Three major capillaries are adjacent to the nephron. These are the glomerular capillaries, the peritubular capillaries, and the vasa recta.
Vasa Recta
The vasa recta is a capillary network that transports blood to and from the medulla and has a highly permeable membrane through which salts and water diffuses in and out. The vasa recta stem from nephrons in the deep cortex next to the medulla and are parallel to the loop of Henle. These capillaries arise from efferent arterioles that descend into the medulla. The vasa recta capillaries then connect to venules that rise and empty into the arcuate and cortical radiate veins. The vasa recta capillaries are lodged in the small spaces between the medullary parts of the renal tubule.
The vasa recta transport the water that has been reabsorbed by the collecting duct. The vasa recta perform its function through a countercurrent exchange system where the adjacent capillaries transport blood flowing in opposite directions at a very slow rate to improve the absorption of water and salt. The vessels that transport blood downwards towards the medulla diffuse out water while diffusing in salt. The vessels that transport blood upwards towards the cortex diffuse out salt while diffusing in water. The salts that are diffused out and absorbed by the vasa recta are urea and sodium chloride. The amount of water that is absorbed by the vasa recta is more than the one that is lost during the downward flow into the medulla. In this way, they transport water absorbed by the kidney but do not affect the osmolarity of the medulla. The absorption of water by the vasa recta is important in the control of urine concentration.
The blood in the vasa recta contains oxygen and nutrients that are used by the medulla. The medulla uses these elements to produce energy that is used in the transportation of salts. Through this flow, the osmolarity of the medulla is maintained at all times (Saladin et al., 2018) .
References
Saladin, K., Gan, C., & Cushman, H. (2018). Anatomy & physiology (8th ed.). McGraw Hill Education.
