Chapter 19: Excretory Products and Their Elimination

The Henle’s loop and vasa recta play an important role in the formation of concentrated urine. The transport of sodium chloride in the ascending limb of Henle’s loop results in an osmolar gradient. The creation of a hypertonic medulla is facilitated by the renal tubules and vasa recta in the following manner: There is an active absorption of urea that occurs from the collecting tubule. In the vasa recta, absorption of water into the blood takes place and return of the absorbed solute to the interstitium follows. This is called the countercurrent exchange mechanism.

The functional unit of the kidney is nephron where the urine is formed and followed by the urinary bladder. The urine will be stored until a voluntary signal is given by the central nervous system. his signal stretched the urinary bladder as it gets filled with urine. In response, the stretch receptors on the bladder walls would now send signals to the CNS that passes on motor messages that would lead to contraction of the muscles of the bladder leading to relaxation of the sphincter. This releases the urine and this process is called micturition.

Disorders of the excretory system:

1. Glomerulonephritis

It is the inflammation of the glomeruli of the kidney

2. Uremia

It is the condition where due to the malfunctioning of kidney some toxic urea will be accumulated in the blood which causes uremia

3. Renal calculi

This disorder is due to the formation of stone-like calcium oxalate which results in kidney stones.

Tubular secretion is crucial as it helps in the maintenance of ionic balance in the body fluids.

During urine formation, Proximal Convoluted Tubule maintains the pH and ionic balance of the body fluids by selective secretion. Distal Convoluted Tubule is where conditional reabsorption of sodium ions and water takes place and maintain the ionic balance and pH. Collecting Duct: From this long duct, large amounts of water could be reabsorbed to produce concentrated urine.

Reabsorption is minimum in its ascending limb which plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid. The descending limb of the loop of Henle is permeable to water but almost impermeable to electrolytes so water can flow out. This concentrates the filtrate as it moves down. The ascending limb allows transport of electrolytes and impermeable to water. So as the concentrated filtrate passes through the ascending limb, it gets diluted due to the passage of electrolytes to the medullary fluid.

The kidney is a reddish-brown coloured organ which is a bean-shaped located between the last thoracic and third lumbar vertebra near the abdominal cavity. The hilum is a notch which is present in the inner side of the kidney through which ureter, blood vessels and nerves enter. Columns of Bertini and an inner medulla divided into medullary pyramids projecting into the calyx which is inside the kidney. The glomerulus and the renal tubule are the two functional units of the nephron. The renal tubule starts with a cup-like structure called Bowman’s capsule, by the glomerulus is enclosed and together they are called the renal corpuscle. Then it is followed by proximal convoluted tubule (PCT) followed by a hairpin shaped Henle’s loop having a descending and an ascending limb. The ascending limb continues as another highly coiled tubular structure called distal convoluted tubule (DCT). The DCTs open into a tube called a collecting duct.