Session 2: Excretory System Flashcards
Overview: excretion in mammals.
Main organs are kidneys, though skin, gut, and lungs play important roles. Main jobs are to rid the body nitrogenous wastes, kidneys also excrete many unwanted poisons and drugs, and they regulate water balance. Excretion is not to be confused with elimination or egestion of food material from the gut.
Describe osmoregulation (overview)
All organisms, whether living on land or in water, must maintain their water and solute (salt) levels at correct levels for life processes to occur. Osmoconformers cannot regulate their water and solute concentrations, their body fluids fluctuate with the changing solute concentration of their external environment. Osmoregulators maintain (relatively) constant water and solute concentrations irrespective of the solute concentrations in their environment.
Describe nitrogenous wastes in animals.
Nitrogenous wastes are produced from the breakdown of amino acids. The simplest breakdown product is ammonia, a highly toxic molecule that cannot be retained in the body long. Some animals convert ammonia to a less toxic form (either urea or uric acid) that can remain in the body for a longer period of time. Aquatic invertebrates and fish excrete ammonia directly with no energy cost, straight into the water. Some fish, mammals, and amphibians turn ammonia into urea (produced in the liver). Urea is 100,000 times less toxic than ammonia and is soluble so it can be excreted in concentrated form. Birds, insects, land snails, and some snails excrete ammonia as uric acid (as a white paste). Uric acid has a very low solubility and is virtually non-toxic.
What do the kidneys do?
Filter the blood, produce urine to rid the body of wastes (acids, nitrogenous wastes), and maintain homeostasis (water and salt balance, and produce the hormone that stimulates red blood cells production)
What are Bowman’s capsule, glomerulus and podocytes?
The Bowman’s capsule is a double-walled cup, lying in the cortex of the kidney and forms the start of the nephron. It encloses a capillary network called the glomerulus. The Bowman’s capsule comprises specialized epithelial cells called podocytes. The plasma filtrate passes through the filtration slits between them, but large molecules such as plasma proteins and blood cells remain in the capillaries.
Describe filtration & the nephron.
Filtrate flows from the glomerulus through the tubules that make up the rest of the nephron in this order:
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
Water and solutes are reabsorbed into surrounding blood vessels (vasa recta). The remaining filtrate flows into the collecting duct and leaves the kidney via the ureters.
What happens in the proximal convoluted tubule in a nephron?
Water and bicarbonate is passively reabsorbed. Glucose, amino acids, Na+ and Cl- (NaCl) are actively reabsorbed. H+ is actively secreted into the tubule. K+ may be passively reabsorbed
Describe the loop of Henle (in a nephron).
Descending loop: Permeable to water but
impermeable to ions. Water is passively reabsorbed.
Ascending loop: Impermeable to water but permeable to ions. Na+ and Cl- (NaCl) is actively and passively reabsorbed.
These processes start to concentrate the filtrate.
What happens in the distal convoluted tubule of a nephron?
Na+ and Cl- (NaCl) and bicarbonate are actively reabsorbed. Water is passively reabsorbed. K+ and H+ is actively secreted
What happens in the collecting duct of a nephron?
Water is passively reabsorbed. Na+ and Cl- (NaCl) is actively reabsorbed. Some urea may be passively reabsorbed.
Describe the cortex and medulla.
The three regions of the kidney tissue (cortex, inner and outer medulla) enable movement of substance in and out of the nephron due to the differences in concentrations in the surrounding tissue. This establishes a salt gradient across the medulla which is needed to concentrate the filtrate. Water will follow the salt and is transported away by the capillaries which maintains the gradient.
Describe ADH and aldosterone.
Osmoreceptors in the hypothalamus detect a fall in the plasma volume. They stimulate neurosecretory cells in the hypothalamus to synthesize and secrete the hormone ADH (antidiuretic hormone). ADH passes from the hypothalamus to the posterior pituitary where it is released into the blood. ADH increases the permeability of the kidney collecting duct to water so that more water is reabsorbed and urine volume decreases. Aldosterone is produced by the adrenal glands and acts on the distal tubules and collecting ducts of the nephron, causing conservation of sodium ions and secretion of potassium ions.
What factors inhibit ADH release?
Low solute concentration, high blood volume, low blood sodium levels, high fluid intake, alcohol consumption.
Water reabsorption decreases. Urine output increases.
What factors cause ADH release?
high solute concentration, low blood volume, high blood sodium levels, low fluid intake, nicotine and morphine.
Water reabsorption increases. Urine output decreases.
What factors cause the release of aldosterone?
low blood volumes also stimulate secretion fo aldosterone from the adrenal cortex, this is mediated through a complex pathway involving the hormone renin from the kidney.
Na+ absorption increases, water follows, blood volume is restored.
