Human Excretory System Overview

Study Notes

The excretory system maintains internal equilibrium in humans by exchanging energy and matter with the environment.

The main functions are removing metabolic waste from the blood and excreting this waste from the body.

Various organs remove waste products.
Lungs: Remove carbon dioxide from cellular respiration.
Skin: Releases water, salts, and some urea as perspiration.
Liver: Removes bile pigment (from hemoglobin breakdown), removes lactic acid (from anaerobic respiration), and eliminates toxins (e.g., alcohol and heavy metals).
Kidneys: Remove ammonia, urea, and uric acid.

Ammonia is produced when excess dietary protein undergoes deamination in the liver.
Urea is the product in the liver when ammonia combines with carbon dioxide (this is a much less toxic form of the nitrogenous waste).
Uric acid is created after the breakdown of nucleic acids (DNA and RNA).

Kidneys maintain water balance in the blood.
Average adults lose roughly 2 liters of water per day (through sweat, urine, and exhalation).
To maintain balance, adults need to drink at least 2 liters of water a day to replace these loses.
A drop in blood fluid levels (as low as 1%) can lead to dangerous symptoms.
A 1% drop in fluid leads to thirst.
A 5% drop in fluid leads to extreme pain and collapse.
A 10% drop in fluid can lead to death.
The kidneys process around 180 liters of fluid per day. The average adult has around 5 liters of blood in their body.

Kidneys: Humans have two, and can function with one. If one kidney fails, the other increases in size to handle the extra load. They are located on either side of the spine in the lower back, protected by fatty tissue and lower portion of the rib cage; they filter blood to make urine.
Ureters: These are 28 cm long tubes that connect the kidneys to the bladder and move urine via peristalsis.
Bladder: A hollow, muscular organ that stores urine. The drainage from the bladder has two sphincters, the internal one being involuntarily controlled by the brain, and the external one being voluntarily controlled and trained during childhood.
Urethra: A tube connecting the bladder to the external environment. Urethra length varies by sex. Female urethra is about 4 cm; male urethra is about 20 cm and merges with the vas deferens. The different lengths are why females are more likely to experience urinary tract infections.

Renal pelvis: The inner layer of the kidney. The mouth of the ureter opens into a funnel-like structure. Urine from the renal tissue is funneled into the ureter—the cup-like extensions of the renal pelvis collect urine.
Renal medulla: The middle layer of the kidney, containing most of the nephrons—the functional units of the kidney.
Renal cortex: The outer layer of the kidney, protecting the inner structures and providing the kidney's general shape. It also has some nephrons.

Glomerulus: High-pressure capillary bed of small branches from the renal artery, supplying blood to the nephrons. Fluid flows to and from this high-pressure bed via the afferent (going to) and efferent (going away) arterioles. Located within Bowman's capsule.
Bowman's Capsule: Cup-like sac surrounding the glomerulus. Collects substances leaving the blood.
Proximal Tubule: Portion of the duct system, leading from Bowman's capsule to the loop of Henle.
Loop of Henle: Long, U-shaped portion of the tube.
Peritubular Capillaries: Surround the nephron, allowing substances to exchange between blood and the nephron solution.
Distal Tubule: Portion between the loop of Henle and the collecting duct.
Collecting Duct: Collects urine made by nephrons and moves it towards the renal pelvis and ureters.