Overarching Function

The excretory system removes unnecessary substances, or wastes, from the body.

Basic structures and functions

Macroscopic structures

  1. Lungs

    The lungs excrete carbon dioxide from our bodies. Inhaled oxygen moves into capillaries and is transported to body cells, whereas carbon dioxide will be exhaled from the lungs.

  2. Kidneys (and parts of the urinary system)

    The kidneys are the major organs in the excretory system. The renal artery transports nutrients and wastes to the kidneys. Then, the kidneys filter out water, salt, and wastes such as urea from the blood. Larger substances such as proteins remain in the bloodstream. The ureters then carry urine from the kidneys to the bladder, where urine is stored. Urine then exits the body through the urethra.

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Microscopic structures

  1. Nephron

    There are approximately 1 million nephrons in each kidney acting as filtering units. Blood enters each nephron through a long tube that is surrounded by a ball of capillaries called the glomerulus, which is then surrounded by a structure called the Bowman’s capsule. The renal artery transports nutrients and waste to the kidney and branches into gradually smaller blood vessels, eventually reaching the glomerulus. Due to the high pressure, water and substances dissolved in the water will be pushed into the Bowman’s capsule, whereas larger molecules like red blood cells will remain in the bloodstream.

  2. Urea

    A nitrogenous waste product that is the main component of urine.

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Connection to other systems

The excretory system works with many other organ systems. For example, the skin, which is part of the integumentary system, plays a role in excreting water through the production of sweat by the sweat glands. The lungs in the respiratory system help remove waste products such as carbon dioxide. The blood vessels in the circulatory system deliver nutrients and waste to the kidneys. The kidneys then filter out the waste and turn it into the urine.

Differences between a frog and human

Frogs have a cloaca, an organ that collects waste and opens to the outside of the body. The cloaca gathers both solid and liquid waste and pushes them out together. On the other hand, human only has an embryonic cloaca. It is, later on, split up into different tracts during the development of reproductive organs and the urinary system.