Toxins and waste are expelled via the excretory system. Ammonia is a toxic by product of digestion, and it is converted to urea or uric acid which is less dangerous to cells. The excretory system also plays a role in homeostasis, specifically osmoregulation. This keeps the water and solute levels in the body constant, which can be a challenge for saltwater and freshwater fish. In humans, the excretory system is composed of the kidney, ureter, bladder, urethra, medulla, and cortex. Blood is filtered by sending it into the nephron in the kidneys. Helpful substances are reabsorbed into the bloodstream while toxins and wastes are secreted by the kidneys. This maintains pH and mineral levels. Antidiuretic hormone is produced by the hypothalamus and triggers changes in the osmolarity of the blood.
The excretory system is
responsible for the removal of nitrogenous wastes and for
The functional unit of the
kidney is the nephron. Filtration takes place in the glomerulus
where pressure drives water small molecules across the glomerular
Reabsorption of water and
substances such as NaCl, amino acids and glucose takes place in in
the proximal convoluted tubule. Other substances are secreted into
Aquaporins in the descending
loop of Henle allow for water reabsorption in this segment of the
The ascending loop of Henle is
impermeable to water. The ascending loop of Henle maintains the
concentration gradient from cortex to medulla by transporting NaCl
into the interstitium.
In the distal convoluted tubule
both reabsorption and secretion take place.
The collecting duct is
permeable to water when acted upon by antidiuretic hormone (ADH).
ADH stimulates the reabsorption of water by the kidney.
Aldosterone is released in
response to low blood pressure or blood volume and stimulates the
reabsorption of Na+ by the distal convoluted tubule.
The Excretory System
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