Welcome back to our systems blogs! Take a quick trip to the bathroom before reading this one, its going to be long!
The excretory system has the major function of excretion, which is the loss of nitrogen containing waste and byproducts of metabolic processes in the animal body. Major organs involved in this system include the lungs, skin, colon, kidneys, and the liver. The kidneys are the most complex of the organs, and contain nephron units.
- Filtration: an excretory tubule collects a filtrate from the blood and the water and solutes are pushed by blood pressure across a membrane of capillaries into the excretory tubule
- Re-absorption: the transport epithelium regains useful substances from filtrate and sends them to the body fluids
- Secretion: the toxins and excess ions are extracted from the fluids and re-sorted into the excretory tubule
- Excretion: the filtrate leaves the body and system
Several different excretory systems exist. For example, flatworms have a protonephridia which is a system made up of a network of dead end tubules that lack internal openings. The tubules are branched throughout the body and the smallest of these are compacted into a flame bulb. Metanephridia are also tubular systems that have internal openings to collect fluids. These are present in annelids. Lastly, Malipighian tubules are tasked with removing nitrogenous waste and are key in osmoregulation, the regulation of osmotic pressure in fluids of organisms. Arthropods and insects have these organs.
Kidneys in vertebrates are excretory organs made of tubules and are compact. They are key to water balance and solute stability. Renal arteries supply kidneys with blood and the renal veins drain used, circulated blood. Urine is removed from kidneys through a duct known as the ureter which moves urine to a urinary bladder. The urine then exits the body from the bladder through the urethral tube.
The nephron has a long tubule and a cluster of capillaries called the glomerulus. A Bowman’s capsule is at the blind head of the nephron and it surrounds the glomerulus. Blood pressure forces glomerulus fluid into the Bowman’s capsule and begins their filtration pathway. The filtrate passes the proximal tubule, then the loop of Henle, and the distal tubule to finally be thrown into the collecting duct. This travels to the renal pelvis and down the ureter. Two types of nephrons are in the kidney. Cortical nephrons are looped as well and are in the renal cortex while juxtamedullary nephrons are more looped and enter the bottom of the renal medulla. This nephron allows urine production and conserves water. Blood is supplied to the nephrons by afferent arterioles from the renal artery and as these leave glomerules they are efferent arterioles. Capillaries entangled and tasked with filtration in the loop of Henle are part of the Vasca Recta and peritubular capillaries surround the tubules.
This is the steps of the transformation of blood to urine:
- The proximal tubule secretes and absorbs the filtrate and secretes substances like ammonia in acidic filtrates. Drugs and poisons are secreted across the epithelium of this tubule. Salt and water are reabsorbed to lower loss of these important nutrients.
- As the filtrate goes down into the loop of Henle, more water is reabsorbed because of its high osmolarity, which also creates a high solute concentration in the filtrate.
- The filtrate moves up the loop, but on this side the membrane is permeable to salt not water. Sodium is reabsorbed to conserve salts, and this dilutes the filtrate.
- The filtrate goes through a secretion of hydrogen and potassium and reabsorbs water, salt, and bicarbonate like the proximal to increase regulation of pH.
- The collecting duct is permeable to urea, which is a waste product, but it is reabsorbed for it is key in water re-absorption.
Thank you, and stay tuned for the next talk!
Written By: Neil 5/27/2019