Structural Biochemistry/Cell Signaling Pathways/Excretory System
The excretory system in organisms is a system that helps with the homeostasis of water and fluid regulation within each organism by absorbing useful fluids such as water and excreting out wastes and excess fluids. Although various animals have distinct and unique excretory systems, most systems carry out the same processes of filtration, reabsorption, and finally secretion.
The Vertebrate Excretory System 
Most vertebrate organisms contain an organ called the kidney that helps with both osmoregulation as well as the excretion process. Fluids enter the kidney where fluids and nutrients are reabsorbed, and the waste product is stored in the kidney until the organism is ready for excretion. Each kidney then pushes out the stored waste product (urine in vertebrates) into ducts known as ureters. These ureters then drain the waste product into urinary bladder. The urine stored in the bladder is then finally excreted through the urethra out of the organism’s body system.
The Kidney 
The kidneys are the major organ that helps aid in osmoregulation/excretion. The kidneys themselves are separated into various segments. The renal cortex and renal medulla are constantly provided with blood via the renal artery, in which the blood is drained out of the kidney via the renal vein. Upon filtering and reabsorbing the essential nutrients, the kidneys produce the urine waste, which is transferred to the renal pelvis via excretory tubules, until the urine is finally released into the ureters. Within each kidney are many functioning units of the kidney known as nephrons, which weave back and forth through the renal cortex and renal medulla.
Major Kidney Functions
- Regulate volume of fluid in the body
- Regulate content of body fluid through osmotic concentration, ionic content, acid/base balance
- Excrete wastes (especially nitrogenous, but also water-soluble waste)
A single kidney is made up of several layers including the renal cortex (the outer layer), the renal medulla (the inner layer) and the renal papillae (strands that connect to the renal medulla. In order to carry out the specific functions named above, the kidneys work to filter out a large amount of blood and process the filtrate. The kidneys make up a profound part of the body, especially in function.
In vertebrates, nephrons are the functioning unit of the kidney that performs the various tasks associated with the excretory system. They are responsible for the filtration, reabsorption, and secretion steps associated with excretion. The nephrons function starts with the glomerulus, which is a capillary cluster that delivers blood/fluid to the nephrons. The glomerulus is encompassed by Bowman’s capsule; and blood pressure squeezes fluids out from the blood in the glomerulus out to Bowman’s capsule, and the filtrate then travels to the proximal tubule. The proximal tubule marks the beginning point of reabsorption. The proximal tubule then utilizes methods of active and passive transport to reabsorb nutrients from the filtrate. Active transport is utilized to reabsorb salts and other nutrients, while passive transport allows for the reabsorption of potassium ions, water, and bicarbonate. After the filtrate passes through the proximal tubule, it is then transported into the loop of Henle, which is divided into two distinct portions. The filtrate first passes through the descending loop of Henle, in which water is passively transported out of the nephron back into the body (in the outer medulla). Upon looping back up, the ascending loop of Henle passively transports salts in the inner medulla, and actively transports it in the outer medulla. Upon shooting up on the ascending loop of Henle, the filtrate enters the distal tubule, in which salts and carbonate are actively transported out, and water is passively diffused outwards. In the final part of the nephron, remaining salts are finally transported outward to the body actively, as well as traces of urea and water.
Filtration, Reabsorption and Secretion 
Arterial pressure drives fluid and small solutes across the walls of the glomerular capillaries. From this, filtration occurs. The most proximal part of the nephron is where filtration occurs. All of the filtered material enters the Bowman's capsule through special filtration slits between structures called pedicels. Reabsorption and secretion occur in the proximal tubule.