93735583 excretion

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Excretion: removal of waste products of metabolic pathways from the body

I. Waste Products

A. Oxygen (photosynthesis)

B. CO2 (cell respiration)

C. Nitrogen-containing compounds (breakdown of protein and nucleic acids for energy or converted to carbohydrates or fats)

1. Ammonia: a very small and very toxic molecule (due to its basicity)

II. Animals

A. Freshwater fish

1. Gets rid of nitrogenous waste as ammonia because they have access to a lot of water (toxic ammonia will thus be diluted)

2. Ammonia is lost as ammonium ions (NH4+) across the epithelium of the gills

3. Gill epithelium takes up Na+ from the water in exchange.

4. Kidneys exrete only minor amounds of nitrogenous wastes

B. Mammals

1. Cannot excrete ammonia due to:

a. its toxicity, and can only be transported and excreted in large columes of very dilute solutions,

b. most mammals do not have access to enough water

2. Instead, mammals (as well as adult amphibians, many marine fishes, and turtles) excrete mainly urea

a. Urea: substance produced in the vertebrate liver by a metabolic cycle that combines ammonia with CO2

i. Advantages

• Low toxicity, about 100,000 times less than that of ammonia

• Safely carried

• Reduces amoung of water required for nitrogen excretion

-ex. some desert animals have an adaptation that allows them to produce a very concentrated urine (a very long Loop of

Henle)

ii. Disadvantage= animals must expend energy to produce it from ammonia

3. The circulatory system carries urea to the kidneys (excretory organ)

C. Birds (as well as land snails, insects, and many reptiles)

1. Excrete uric acid

a. characteristics

i. Relatively nontoxic (like urea)

ii. Insoluble in water (unlike urea or ammonia) and can be excreted as a semisolid paste with very small water loss

b. Advantage for animals with little access to H20

c. Disadvantage= uric acid is even more energetically expensive to produce than urea, requiring a lot of ATP for synthesis from ammonia

III. Kidney (function: excretion and osmoregulation) structure

A. Major structures

1. Outer renal cortex

2. Inner renal medulla

3. Renal Pelvis: collecting region for all the urine produced by nephrons

B. Nephron: microscopic excretory tubule; kidney’s unit of function

1. Single long tubule

2. Glomerulus: ball of capillaries; 20% blood plasma escapes

a. Bowman’s capsule or glomerular capsule: cup-shaped swelling that acts as a filter for the blood being carried by the afferent arteriole

b. Endothelium: single layer of cells which has pores (fenestrations)

i. Restricts the passage of red blood cells and other formed elements

ii. Basement membrane: consists of fibrils in a glycoprotein matrix; restricts the passage of molecules with a molecular mass over

68,000 (plasma proteins)

iii. lies between the endothelium and a layer of the Bowman’s capsule

c. Podocytes: specialized epithelial cells that form a layer of the glomerulus (Bowman’s capsule)

i. Provide support

ii. Filtration slits that restrict the passage of medium-sized proteins

• Slit membrane: thin membrane that extends across filtration slits to restrict passage of molecules

iii. 1000s of footlike structures called pedicels extend from each podocyte and cover the basement membrane, except filtration slits

3. Each kidney contains a million nephrons, with total tubule length of 80 km.

C. Blood and Urine Flow

1. Blood enters through the renal artery (branch of the aorta) and exits the kidney via the renal vein)

2. 20% of the resting cardiac output goes to the kidney

3. Urine exits the kidney through a duct called the ureter, and both ureters drain into a common urinary bladder

4. During urination, urine is expelled from the urinary baldder through a tube called the urethra

IV. Urine Production

A. Ultrafiltration

1. Bowman’s capsule

a. Fitration

i. Nonselective

ii. occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule

b. Pressure is high because the efferent arteriole (vessel taking away from the glomerulus) is narrower than the afferent arteriole

c. Porous capillaries (fenestrated) and podocytes

i. Permeable to: water and small solutes

ii. NOT permeable to: large molecules like plasma proteins

d. The filtrate in Bowman’s capsule contains salts, glucose, vitamins, nitrogenous waste (like urea), and other small molecules.

e. The filtrate moves on into the next part of the nephron: the proximal convoluted tubule.

2. Proximal Convoluted Tubule

a. General

i. Located in the cortex of the kidney (like the glomerulus)

ii. Secretion and selective absorption change the volume and composition of the filtrate

iii. Initial filtrate is about 1 liter every 10 minutes in the two kidneys.

iv. Substances are taken out of the plasma that are needed by the body

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Excretion: removal of waste products of metabolic pathways from the body I. Waste Products A. Oxygen (photosynthesis) B. CO2 (cell respiration) C. Nitrogen-containing compounds (breakdown of protein and nucleic acids for energy or converted to carbohydrates or fats) 1. Ammonia: a very small and very toxic molecule (due to its basicity) II. Animals A. Freshwater fish 1. Gets rid of nitrogenous waste as ammonia because they have access to a lot of water (toxic ammonia will thus be diluted) 2. Ammonia is lost as ammonium ions (NH4+) across the epithelium of the gills 3. Gill epithelium takes up N ...
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