13 Urinary system

The urinary system is specialized for the removal of unnecessary and no longer needed substances from the body. It regulates water and salt balance, removes nitrogenous waste and other byproducts of metabolism. It also removes the extraneous substances that have happened to make it into the organism, such as drugs or food colorings.

The urinary system consists of the kidneys, the organ of urine production, and a set of muscular tubes that transport urine to the outside of the body. Starting from the kidneys these tubes are:

  • ureters
  • the urinary bladder
  • the urethra.

Kidney

The structural unit of urine production in the kidney is a nephron. The nephron is a microscopic structure that is made of blood capillaries and a set of tubules made of kidney epithelium. Capillaries bring blood to the nephron where is gets filtered through the filtration membrane into the capsular space. From the capsular space “future urine”, called primary filtrate, will pass through the system of microscopic tubules where the composition of urine will be adjusted to the current needs of the organism. Once urine passes to the renal pelvis urine cannot be adjusted anymore. It can only be transported and removed.

Kidney structure

On the longitudinal section, the kidney has two characteristic layers:  the cortex and the medulla. The cortex is located on the outside and is fairly uniform, while the medulla is organized into renal pyramids spaced by renal columns. The bottom of the pyramid is turned toward the cortex. At the tip of each pyramid are holes where the papillary ducts open to the renal pelvis.

The renal cortex contains most of the glomeruli and numerous cross sections of the tubules. The nephron tubules are made of simple cuboidal epithelium. A distinct, often visible on the histological sections, basement membrane surrounds each tubule.

The medulla only has tubules-mostly loops of Henle and collecting ducts-and does not contain glomeruli.

Figure 5: Two layers in the kidney.

Nephron

The structural and functional unit of urine production in the kidney is a nephron. The nephron is a microscopic structure that is made of blood capillaries and a set of tubules made of kidney epithelium. Capillaries bring blood to the nephron where it gets filtered through the filtration membrane at the renal corpuscle.

The renal corpuscle has two parts, the glomerulus of capillaries in the middle, and a glomerular (Bowman’s) capsule that wraps up around the glomerulus like a chalice. Between the glomerulus and the capsule is the capsular space where the primary filtrate (future urine) gathers before continuing into the system of microscopic tubules where the composition of urine will be adjusted to the current needs of the organism. Once urine passes to the renal pelvis it cannot be adjusted anymore.

The proximal convoluted tubule detaches from the capsular space and continues in a “convoluted way” until it becomes the loop of Henle. From the loop of Henle, the filtrate continues to the distal convoluted tubule and then the collecting duct. The distal part of the collecting duct, called the papillary duct, releases urine to the renal pelvis.

All parts of the tubular system have a network of accompanying capillaries that pick up the substances that have been reabsorbed.

 

Figure 1: Illustration indicating the parts of the nephron.

 

Below are the parts of the nephron listed in order of urine flow.

    • Renal corpuscle (glomerulus surrounded by Bowman’s capsule)
    • Proximal convoluted tubule
    • Proximal straight tubule
    • Loop of Henle (descending limb and ascending limb)
    • Distal convoluted tubule
    • Collecting duct (cortical and medullary)
    • Papillary duct
      Figure 2: A renal corpuscle as seen under the microscope

Organs Of Urine elimination

Once urine enters the renal pelvis, it will be delivered by the ureters to the urinary bladder, and then to the outside by the urethra. All organs for urine elimination are hollow with three-layered walls consisting of:

  • mucosa
  • muscularis
  • adventitia

The inside layer, the mucosa, is made of transitional epithelium and a wide lamina propria.

The muscularis layer is made of smooth muscle arranged in two layers (inner longitudinal and outer circular).

The outer layer is adventitia, which is made of connective tissue, and wherever it touches peritoneum it is called serosa.

Figure 6: Three layers of the urinary bladder wall. 1. Mucosa 2. muscularis, 3. adventitia

Transitional epithelium

Transitional epithelium is multi-layered but lacks the organization of stratified epithelia. Cells in the upper layers have polyhedral shapes, becoming cuboidal in deeper regions. Transitional epithelium cells can change shape, in extreme cases becoming flat, as the walls stretch to accommodate the flow of stored urine.

 

Figure 7: Transitional epithelium in the urinary bladder.

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Histology Copyright © by Malgosia Wilk-Blaszczak. All Rights Reserved.

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