How are unwanted substances produced inside the body excreted? Let us examine the functions of the kidneys and liver.
Basic Knowledge
Kidneys: a pair of fist-sized organs located in the upper back. They filter the blood and produce urine.
① Blood is filtered in the glomerulus (a cluster of capillaries), becoming filtrate
② Glucose, water, and salts are reabsorbed in the renal tubules
③ The remaining urea, waste products, and excess water are stored as urine in the bladder and excreted
Liver: the largest internal organ in the body (about 1〜1.5kg). It has many functions:
① Detoxification: breaks down alcohol and drugs
② Converts ammonia into urea (changes harmful ammonia produced by protein metabolism into less toxic urea)
③ Produces bile (helps digest fats)
④ Stores nutrients (stores glucose as glycogen)
⑤ Maintains body temperature (produces heat through metabolic reactions)
Thus, the liver is called a “chemical factory” and the “silent organ.”
Kidney (an organ that filters blood and makes urine; a pair of organs)
Urea (a waste product produced by protein metabolism; made from ammonia in the liver)
Glomerulus, Bowman's capsule, and renal tubule (the parts of the kidney's filtration apparatus)
Ureter, bladder, and urethra (the passage, storage organ, and exit for urine)
Liver (a multifunctional organ responsible for detoxification, urea production, bile secretion, and nutrient storage)
Ammonia (NH₃) (a harmful substance produced by protein breakdown; converted into urea in the liver)
Further Study (Background and Significance)
The kidneys filter about 180L of blood per day, but most of it is reabsorbed in the renal tubules, leaving only about 1.5L of final urine. Reabsorbing more than 99% is an advanced control system that prevents excessive loss of water and sugar.
If the kidneys stop functioning, kidney failure occurs, and dialysis (a treatment that filters blood with a machine) becomes necessary. Many dialysis patients receive treatment three times a week for four hours each time, greatly affecting their daily lives.
The liver has a very strong ability to regenerate and can return to its original size even after more than half is removed. However, because symptoms are difficult to notice, diseases are often discovered late, which is why it is called the “silent organ.” It is important to detect disease early through health checkups before it progresses to fatty liver, hepatitis, cirrhosis, or liver cancer.
Summary of excretion pathways:
・Urea, excess water, and salts → kidneys → urine
・CO₂ → lungs
・Water and salts → sweat (skin)
These multiple pathways maintain homeostasis in the body.
- Kidneys=filter blood and make urine
- Filtration occurs in the glomerulus; reabsorption occurs in the renal tubules
- Liver functions: detoxification, urea production, bile, storage, and body temperature
- Ammonia (harmful)→liver→urea (harmless)
- Urine pathway: kidney→ureter→bladder→urethra
- Daily filtration volume 180L, urine volume 1.5L
- Excretion occurs through the lungs, kidneys, and skin
Points of Caution (Easily Confused)
① The organ that changes ammonia (harmful protein-metabolism product) into urea (harmless) is the liver, not the kidneys. The kidneys excrete urea as urine. ② The liver makes bile, while the gallbladder stores it. These locations are different. ③ Do not confuse the ureter (kidney→bladder) with the urethra (bladder→outside the body). ④ Excretion (removing waste products from the body) and defecation (expelling feces) are related but strictly different.
Practice
- Name the organ that changes ammonia into urea.
- What is the structure in the kidneys where unwanted substances in the blood are filtered called?
- Name the organs that make and store bile, respectively.