Biology 12: Excretory System Summary

Introduction

-The diagram right illustrates the products of our body’s breakdown of the food we eat.

-Some of these products are very useful (ATP in particular), but others are wastes that we need to deal with.

-As we saw in our last unit the respiratory system deals with the CO2

and H2O produced, but now we need to look at another system to see how NH3

(Ammonia) is dealt with.

Protein and Nucleic Acid Digestion

-Protein and Nucleic Acids are very important parts of our diet, but they can’t be stored by our bodies. Instead, extra amino acids are turned into carbohydrate and fat storage molecules. However, these molecules don’t use the amino group (NH2), creating nitrogenous wastes.

-Animals deal with the NH2 waste product in different ways depending on the environment they live in and their own lifestyles:

a) Fish: Aquatic animals can rely on the water in their environment to dilute ammonia, which is very toxic.

b) Mammals: Need to convert ammonia into a less toxic form and then excrete it out of the body.

c) Reptiles and Birds: Convert ammonia into uric acid, which is less toxic and does not need large quantities of water for excretion.

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-The liver converts the toxic ammonia into urea, which is less toxic. The urea will travel through the blood stream to the kidneys’ where it will put into urine.

*You do not need to know the steps involved in the formation of urea from ammonia! The diagram right is for interest only!

Kidney Structure and Function

-Kidneys are made of three components:

a) Renal Cortex: Contains the Bowmans Capsule

b) Renal Medulla: Contains the Loops of Henle and Collecting Ducts

c) Renal Pelvis: Space lined with smooth muscle where urine collects before being funneled into the ureters.

-Within the cortex and the medulla there are nephrons that work to reabsorb useful components of the urea (such as water) back into the blood stream.

-The process of urine formation can be broken down into three steps:

a) Pressure Filtration

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-Blood pressure causes small molecules (water, salts and nutrient and waste molecules) to leave the glomerulus and enter the Bowman’s Capsule.

b) Tubular Reabsorption

-Nephrons have a high surface area because they are so long and twisted. This gives them longer to actively reabsorb nutrient (glucose, amino acids) and salt molecules into the capillary network. It is here that the nephrons can also work to regulate the blood’s pH.

-Convoluted tubule cells are also cuboidal and have cilia, again facilitating reabsorption.

-Another advantage of the twisting shape of the nephrons is that substances secreted in one part of the nephron can subsequently help to extract other substances (See the ascending and descending Loops of Henle).

c) Tubular Secretion

-The Collecting Duct is permeable to urea in the medulla, contributing to the high salt concentration here that is used to reabsorb water in the descending Loop of Henle.

-Some waste materials (excess H+) are also actively transported from the peritubular capillaries into the collecting duct.

d) Excretion

-Urine passes into the renal pelvis and then is carried by the ureters to the bladder before leaving the body through the urethra.

*Important Idea: The nephrons return most of the water put into them from the bloodstream. Your kidneys are very improtant in water conservation!

Hormone Regulation of Kidney Functioning

-Your kindeys play a very important role in ensuring that your blood volume remains constant. This can be accomplished in a couple of ways:

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a) Reabsorption or excretion of water in the nephrons.

-Antidiuretic Hormone (ADH) is produced by the Hypothalamus and released by the Posterior Pituitary Gland when the water content of blood is too low.

-ADH triggers the feeling of thirst and makes the collecting duct walls more permeable to H2O so it can be reabsorbed.

b) Reabsorption of Na+ and H2O and arteriole contraction.

-The Juxtaglomerular Apparatus (JGA) responds to low blood volume or pressure by releasing the enzyme renin.

-Renin breaks down angiotensinogen into Angiotensin I in the blood stream. This is further broken down into Angiotensin II in the lungs.

-Angiotensin II causes:

i) Constriction of the arteriole leading into the Bowman’s capsule to keep pressure constant for pressure filtration.

ii) The adrenal glands to secrete Aldosterone, causing the reabsorption of Na+ and H2O from the distal convoluted tubules.

-Blood pressure should increase, inhibiting the producting of renin.

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