CHAPTER XIEXCRETORY SYSTEM

Objective

• Define the term excretion and describe how this

process helps maintain homeostasis.

• Name the major metabolic wastes and the

processes by which they are formed.

• Describe the types of vertebrate kidney.

• Explain the parts of urinary system and the

processes of urine formation.

• As an organism carries out its life processes,

• Waste products build up in the body fluids.

• If these metabolic wastes are not removed from

the body, the organism would die.

• Therefore, the organism must be able to remove

metabolic waste and other excess substances

that build up over time.

Excretion

• Excretion is the process by which these waste

and excess substances are removed from the

organism.

• The process of excretion also removes excess

heat from the body, thus helping to keep the

temperature of the body constant.

• In human and other complex animals,

• The organs of excretion are the lungs, kidneys, liver, and skin.

• These organs work with the circulatory, nervous, and endocrine system to keep the body’s internal environment constant.

• On other words, these organ systems maintain homeostasis.

Major Metabolic Wastes AndTheir Excretory Organ

• The most important of the metabolic wastes

1. Carbon dioxide

2. Waterare

formedduring cellular respiration

3.Nitrogen compounds ,such as ammonia,

urea, and uric acid ,are produced by the

breakdown of amino acid.

4.Mineral salts, such as sodium chloride and

potassium sulfate, build up during metabolism.

-All of these wastes are poisonous in highconcentrations.

• Many people confuse excretion with elimination.

• Elimination, or defecation, is the removal from

the digestive tract of unabsorbed and

undigested food in the form of feces.

• Since these materials have never entered the

body cells, they are not metabolic wastes.

• The liver removes harmful substances, such as bacteria, certain drugs , and hormones from the blood.

• Within the liver, these substances are changed into inactive or less poisonous forms.

• Thus, the liver purifies or detoxifies, the blood.

• The inactive substances formed in the liver are returned to the blood stream and

• Are finally excreted from the body by the kidneys.

Type of vertebrate kidney

- Vertebrate kidneys, or nephroi, are built in accordance with a basic structural pattern consisting of

1. Glomeruli , usually incorporated in renal corpuscles;

2. Tubules, surrounded by peritubular capillaries and

3. A pair of longitudinal ducts.

- Variations in the details from fish to man are primarily in the number and arrangement of glomeruli and in the relative length of the tubules.

- In present –day vertebrates

- The uriniferous tubules develop antero-posteriorly in two or three stages

- In succession these stages are

-pronephros,

-mesonephros, and

- metanephros.

- These stages have evolved from the original archinephros.

Pronephros

- Pronephros develops in the anterior most part of the nephrotome

- There are only 1 to 13 uriniferous tubules in each, one pair to each segment.

- Near each tubule is a glomerulus but Bowman’s capsule and peritoneal funnel are lacking.

- The glomeruli are called external glomeruli

- The uriniferous tubules of each pronephros open into a common pronephric duct

- Which runs backward to enter the embryonic cloaca.

- A pair of pronephroi become functional only in some cyclostomes and embryonic of all anamniote.

- In other vertebrates, they degenerate during development .

Mesonephros

- Mesonephros develops from that part of the nephrotome which lies behind the pronephros.

- At first it consists of paired segmental uriniferous tubules,

- Each with a peritoneal funnel opening into the coelom,

- and a glomerulus enclosed in a Bowman’s capsule.

- These mesonephricuriniferous tubules join the existing pronephric duct on each side,

- Which is called mesonephricduct or Wolffian duct.

- Later, the mesonephrictubules undergo budding to form hundred of tubules,

- So that their segmental arrangement is lost.

- The later tubules have no peritoneal funnels.

- Mesonephroi form the adult functional kidneys in some fishes and amphibians

- They form the kidneys of embryo of amniotes, which they degenerate in the adult.

Metanephros

• The first embryonic hint of a metanephros is the formation the metanephric duct that appear as a ureteric diverticulum arising at the base of the preexisting mesonephric duct.

-The ureteric diverticulum grows dorsally into

the posterior region of the nephric ridge.

-Here it enlarges and stimulates the growth of

metanephric tubules that come to make up the

metanephric kidney .

• The metanephros becomes the adult kidney of amniotes ,and the metanephric duct is called the ureter.

The Urinary System

-The urinary system is made up of the

-kidneys

-ureters

-bladder

-urethra

-The two kidneys are the organs that produce

urine.

-Urine passes from each kidney through a tube

called a ureter to the urinary bladder, where it

is stored.

-During urination ,the stored urine travels from

the bladder to the outside of the body through

the uretra.

Urine Formation

Urine is made in the nephrons in two stages:

1.filtration

(𝑓𝑖𝑟𝑠𝑡 𝑠𝑡𝑎𝑔𝑒)→ both useful substances

and wastes are removed

from the blood .

2.Reabsorption𝑠𝑒𝑐𝑜𝑛𝑑 𝑠𝑡𝑎𝑔𝑒

→ some of useful

substances reenter the

blood to be used by the

body .

Filtration

Filtration takes place in the glomeruli and Bowman’s capsules.

The blood that enters a glomerulus is under pressures.

The pressure forces the filtrate, which include

1.Water

2.Urea

3.Glucose

4.Amino acid

5.Various salts

through the thin walls of the glomerulusinto Bowman’s capsule.

- Blood cells and blood proteins, however, are too large to pass through the walls of the glomerulus.

- These substances remain in the blood.

- The filtrate that enters Bowman’s capsule is like blood plasma, but it does not contain proteins.

- If all of the filtrate that is formed were excreted.

- The body would lose too much water along with important nutrients and salts dissolved in that water.

These substance

Remain in capillary

These substance

Enters into

Bowman’s capsule

plasma protein

cells

platelets

glucose

water

urea

amino acid

various salts

Reabsorption

- After the filtrate has left Bowman’s capsule, reabsorption occurs in the renal tubule.

- It is the process of reabsorption that reducethe volume of filtrate and

- return various important substances to the blood.

- Normally , as the filtrate passes through the renal tubules of the nephrons,

- about 99 percent of the water,

- all of the glucose and amino acid and

- many of the salts are reabsorbed.

- These substances are reabsorbed in to the blood by the capillaries that surround the tubules.

- The reabsorption of water from the renal

tubules is an important means of water

conservation in mammals.

- Since most of the water is reabsorbed, the

substances left in the filtrate are highly

concentrated.

- While water is reabsorbed by osmosis,

- Glucose

Amino acid and

salts

- ATP, the energy source for active transport, is

supplied by the many mitochondria found in the

cell of the renal tubule.

need active transport to be

reabsorbed

- The tubules are lined with microvilli that

greatly increase the surface are through which

reabsorption can occur.

- The large area allows the reabsorption of huge

amounts of water and other substances.

s

After reabsorption - The fluid remaining in the

tubule is urine - The urine is made up of 1. Water 2. Urea and 3. Various salts.- Urine flows from the

tubule into the collecting ducts

- It passes out of the kidneys through the ureters to the bladder, which is emptied from time to time through the urethra.

- Amino acids are the breakdown products of proteins.

- Because excess amino acids cannot be stored in the body, they are broken down in the liver.

- The parts of the amino acids are charged into other substances.

Formation of Urea

Form each amino acid

The amino group is changed into ammonia

(N𝐻3)

- The remainder of amino acid molecule either

is changed into pyruvic acid and used as an

energy source in cellular respiration or is

changed into glycogen or fat storage.

- Because the ammonia produced from the amino group is very poisonous

- It is changed into the less harmful substance urea by a series of enzyme-catalyzed reactions.

-The urea diffuses from the liver into the blood stream.-The bloodstream, then carries the urea to the kidneys.-The kidneys filter urea from the blood, and -It is finally excrete from the body in the urine