EXCRETORY SYSTEM

excretory system anatomy

• made up of kidneys, ureteres, bladder, urethra

• urine produced by each kidney exits through a duct called the ureter; the two ureter drain into a common sac called the bladder

• during urination, urine is expelled from the bladder through a tube called the urethra, which empties to the outside

• sphincter muscles near the junction of the urethra and bladder regulate urination

kidney function (3)

kidneys play a critical role in removing wastes, balancing blood pH, and maintaining body’s water balance

kidney anatomy

• each kidney has an outer renal cortex and inner renal medulla; both regions are supplied with blood by a renal artery and drained by a renal vein

• within the cortex and medulla are excretory tubules and associated blood vessels

processes in kidneys

• excretory tubules carry and process a filtrate produced from the blood entering the kidney

• nearly all the fluid in the filtrate is reabsorbed into the surrounding blood vessels and exits the kidney in the renal vein

• remaining fluid leaves excretory tubules as urine; it is collected in a hollow cavity, the renal pelvis, that connects the kidney to the ureter, through which urine passes to the urinary bladder

nephron

• weaving back and forth across the renal cortex and medulla are the nephrons, the functional units of the kidney

• essential for the production of urine that is hyperosmotic to body fluids, a key adaptation for water conservation in mammals

nephron anatomy

• at one end is the Bowman’s capsule, a small folded structure that encircles a group of blood capillaries, the glomerulus, in the cortex

• blood is supplied to the glomerulus by the afferent arteriole and, after being filtered, exits via the efferent arteriole and is carried to the peritubular capillaries

• unfiltered blood passes through Bowman’s capsule into a proximal convoluted tubule, which forms the loop of Henle before rising to form a distal convoluted tubule

• distal tubule drains urine into collecting ducts that lead to the renal pelvis, which empties through the ureter to the bladder

Bowman’s capsule

• filtrate forms when fluid passes from the bloodstream to the lumen of Bowman’s capsule

• higher pressure of the blood in the glomerulus drives fluid that contains these molecules and ions into the capsule

• result is the formation of glomerular filtrate, which is similar to plasma but protein-free

selective permeability of Bowman’s capsule cells

• the cells of the Bowman’s capsule are selectively permeable to water, ions, small nutrients (glucose, amino acids) and nitrogenous waste (urea)

• blood cells, platelets, and plasma proteins are too large to pass through and are retained in capillaries

proximal convoluted tubule

• reabsorbs salt, water, and valuable nutrients from the filtrate

• the inner walls of the tubule are covered with microvilli which increases the total surface area that is available for the reabsorption of solutes

• purpose of the tubule is to conserve essential substances and prevent their loss in urine

processes in proximal convoluted tubule

• salt leaves via active transport pumps

• as salt leaves from the filtrate to the interstitial fluid, water follows by osmosis; this reduces filtrate volume

• glucose, amino acids, and other essential substances are actively transported from the filtrate to the interstitial fluid and then into the peritubular capillaries

• urea and other unwanted compounds are not reabsorbed

• as filtrate passes through the proximal tubule, materials to be excreted become concentrated

interstitial fluid

• the fluid that surrounds cells, and originates from the plasma that leaks out of blood capillaries

• the fluid drains into lymphatic capillaries, which is returned to the bloodstream via lymphatic vessels

unwanted substances in filtrate

• urea: made in the liver through the urea cycle, and helps the body safely remove toxic ammonia

• drugs and toxins processed in the liver pass from peritubular capillaries into the interstitial fluid, and are then actively secreted by transport epithelium into the lumen of the proximal tubule

descending loop of Henle

• upon leaving the proximal tubule, filtrate enters the loop of Henle

• since the interstitial fluid is hyperosmotic to the filtrate in the tubule, water flows out of the tubule into the fluid through aquaporin channels

• the filtrate loses water and increases in solute concentration

• no channels for salt and other small solutes

hyperosmotic vs hypertonic

• hyperosmotic: a solution with a higher concentration of solutes compared to another solution, impacting the movement of water across a membrane due to osmotic pressure

• hypertonic: a solution that specifically causes a cell to shrink due to water moving out of the cell; used when referring to the effects of a solution on a cell

ascending loop of Henle

• ascending limb lacks water channels (impermeable to H₂O)

• two specialized regions: a thin segment near the loop tip and a thick segment adjacent to the distal tubule

• in the thin segment, NaCl, which became highly concentrated in the descending limb, diffuses into the interstitial fluid through passive transport

• in the thick segment, the movement of NaCl out of the filtrate continues via active transport (thus requires energy)

• high concentration of salt in kidney interior, enabling the kidney to form concentrated urine

distal convoluted tubule

• the fluid enters the distal convoluted tubule, where additional water and salts are removed

• more solutes move out of the fluid than into it; this causes the further transport of water out of the tubule by osmosis through the aquaporins

collecting duct

• urea and wastes flow into collecting ducts which further concentrate urine

• ducts descend from the kidney cortex and through the medulla

• they are permeable to water, but not to salt

collecting duct processes

• concentration of solutes increase as the fluid descends into the medulla

• causes further removal of water through the ducts, increasing the concentration of urine

• collecting duct carries urine to the renal pelvis

• near the bottom of the medulla, the walls of collecting ducts contain passive urea transporters, which allows some of the nitrogenous waste to pass from the duct into the interstitial fluid

• urea adds significantly to the concentration gradient of solutes in the medulla

antidiuretic hormone (ADH)

• key hormone of the kidney; also called vasopressin

• ADH molecules released from the posterior pituitary bind to and activate membrane receptors on the surface of collecting duct cells

• more aquaporin channels result in more water capture

homeostatic regulation of the kidney (blood osmolarity)

• as blood osmolarity rises, such as eating salty food or losing water through sweating, osmoreceptors in the hypothalamus trigger increased release of ADH from posterior pituitary

• the resulting increase in water reabsorption in the collecting duct concentrates urine, reduces urine volume, and lowers blood osmolarity back toward set point