Excretion

Excretion is the removal of the waste products resulting from metabolic process

in an organism

Lungs : removal of excess carbon dioxide

Liver : produces urea and uric acid as a by-product of the breakdown of proteins

Skin : removal of excess water, salt, urea and uric acid

Urinary System : kidneys filter the blood to form urine, which is excess water, salt, urea and uric acid

The Human Excretory System

Waste products include : Urea Ammonia Uric acidThese products contain nitrogen Nitrogen wastes are a by product of protein

metabolism. Amino groups are removed from amino acids prior to energy conversion. The

NH2 (amino group) combines with a hydrogen ion (proton) to form ammonia (NH3).

Excretion also involves the removal of other waste products of metabolism

such as carbon dioxide from respiration

Mammalian kidney

The function of the kidneys include:

Removal of urea from the blood Regulation of the water content

of the body fluids.

Roles of the kidney excretion homeostasis osmoregulation regulation of salts in the

body regulation of pH production of a hormone

(EPO)

The human urinary system The two kidneys lie at the back of the

body ,at about waist level

The kidneys : Receive blood through the

renal arteries and blood is taken away by the renal veins.

Have the largest supply of blood of any organ ,about 1200cm3

of blood each minutes.

Require a blood supply at high pressure if they are to function properly ;they fail if the pressure drops.Urine made by the kidneys flows in to the ureter and pushed by

peristalsis into bladder where it is stored until it is convenient to release it. When full, sphincter muscles are relaxed allowing the urine

to pass in to the urethra and then out of the body.

http://www.youtube.com/watch?v=r7m5IyzQzAM&feature=related

Each kidney has three major regions:The cortex, medulla and pelvis. The renal artery take

blood into kidney where it is filtered to remove potentially toxic material.

Useful substances leave the blood as well as toxic ones but are reabsorbed back into the blood .Toxic substance such as urea leave the kidneys and enter the bladder ,via the

ureters

What is the role of kidneys in

body ?

Each nephron is divided into a number of distinct regions:

Renal capsule First convoluted

tubule Loop of henle Second convoluted

tubule Collecting duct

Each our kidneys contains between 1 and 2 million microscopic multicellular structures called nephrons. each nephron is long tube

about 3 cm in length.

It is important to know the position of the nephrons in relation to the overall structure of

the kidneys.

The collecting ducts deliver urine into a space called pelvis

From pelvis the urine drains into the ureter. Ureter connects the kidney to the bladder. Blood enters each kidneys by renal artery, which

progressively branches. ,finally into arterioles. Each arterioles divides in to knot of capillaries

called the glomerulus. the glomerulus is found in the cup of each renal capsule.

How does the nephron function?

Blood arrives at the glomerulus from the renal artery. The blood pressure is very high as a result of:

- contraction of the left ventricle of the heart - the arteriole leading to glamerular capillaries is wider than

the venule leaving them - high resistance of the interface between glomerular capillaries

and the inner wall of the renal(Bowman’s)capsul.• Glomerular filtrate is forced in to the nephron; this is known as

ultrafiltration.• Glomerular filtrate includes urea, glucose, water, amino acids

and mineral ion • Selective reabsorption takes place in the Urea continue

through the tubule to the collecting duct and finally down a ureter to be excreted from the bladder.

• proximal tubule resulting in substance such as gulocose being returned to the blood.

• 100% of glucose and 80% of water are reabsorbed at the proximal tubule.

• Further reabsorption of substance can take place at the distal tubule.

UltrafilrationAlso known as pressure filtration ,ultra filtration relies on the properties of the

capillaries the inner wall of the renal(Bowwman’s)capsul.

Capillaries lie very close to the inner capsular membrane

The capillaries membrane consist of podocytes

Podocytes help to support the basement membrane of capillaries.

It is the basement membrane that is the selective high pressure sieve.

Only molecules which are small enough can pass through.

Reabsorption capillaries from the glamerulus extend to a network across both proximal and distal tubules. The close contact between capillary and tubule is important

Substance ,such as glucose, urea and water travel along the tubule.

Each tubule is one cell thick, consisting of epithelia cells with microvilli on the outer membrane.

Microvilli give a high surface area to allow the efficient transport of substance to cross to the capillaries.

Carrier protein on the microvilli reabsorbs glucose from the filtrate in to the tubules.

Glucose molecules are then actively transported in to the fluids surrounding the capillaries.

Glucose molecules finally enter the capillaries and so have re-entered the blood

By the end of the tubule all glucose has been returned to the blood.

How do the kidney conserve water?

Water molecules which pass into tubule and reach the kidney pelvis continue down a ureter and are lost in urine.

Such water loss is carefully controlled; some is always reabsorbed.

This control involves both the nervous system, the endocrine system and structures along a nephron.

The diagram opposite outlines the role of the countercurrent multiplier in the control of water content in the body.

The role of the loop of henle

Na and Cl ions are actively transported into medulla from the ascending limb of the loop of henel.

The ascending limb is thicker than the descending one, and impermeable to the outward movement of water so only the ion leave.

The Na and Cl ions slowly diffuse into the descending limb resulting in their greater concentration towrads the base of the loop.

A high concentration of Na and Cl ions in the medulla cause water to leave the collecting duct by osmosis.

Additionally ,water leaves the descending limb by osmosis due to the ions in the medulla.

Water molecules pass into the capillary network and are successfully.

The role of the distal tubule

The structure of the distal tubule is similar to the proximal tubule, however its specific roles are:

Maintenance of a constant blood plasma PH at around 7.4.

If blood plasma falls below a PH of 7.4 then ionic movements take place

(H- ions) plasma → filtrate

(HCO3- ions) filtrate → plasma

If blood plasma rises above a PH of 7.4 then more ion movements take place

(OH- ions) plasma → filtrate

(HCO3- ions) plasma → filtrate

The control of water balance

It is necessary to control the amount of water in the blood. The kidneys can help to achieve this with their ability to intercept water before it can reach the ureters. There are , however, problem

to overcome. In hot condition we lose a lot of water by

sweating; too much loss would lead to dehydration problems.

In cold condition much less water is lost by sweating, giving a potential problem of too much water being retained in the blood.

A balance must be achieved!

scenario 1: warm environmental conditions:

Osmoreceptors in the hypothalamus detect an increase in the solute concentration of the blood plasma.The hypothalamus the produces ,by neurosecretion ,the hormone ADH.The ADH is secreted in to posterior lob of the pituitary gland.From here it passes in to the blood and finally reaches the target organs, the kidneys.Here it increases permeability of:

(i) the collecting ducts (ii) the distal tubules.

The effect is that more water can be reabsorbed back in to the solute.

Control is achieved with the help of antidiuretic hormone (ADH), produce by the hypothalamus and secreted by the

posterior lobe of the pituitary gland.

Hormonal control of the kidney-the role

of ADH

The events outlined above give a maximum effect of the countercurrent multiplier. Too much water would be lost by sweating so the water component of the urine must be drastically limited. The resulting urine is therefore low in

water content and high in solutes.

scenario 2: cold environmental conditions:• Osmoreceptors in the hypothalamus detect an increase in

the solute concentration of the blood plasma.• The hypothalamus then produce less ADH.• Less ADH leaves the posterior lobe of the pituitary gland.• Less ADH reaches the target organs, the kidneys.• The collecting ducts and the distal tubules are not so

permeable.• Less water can be reabsorbed back.

The urine is of greater volume due to greater water content. No wonder we urinate more in cold weather!

Diuresis

Diuresis is a condition in which excessive amount of watery urine are produced in a healthy person

this is avoided with the secretion of ADH.

Sometimes people are prescribed a drug equivalent to ADH to cure the

symptoms reabsorption can take place efficiently so urine at a correct

solute concentration is excreted.

Sources

Water ingested drink and food / metabolic water

Protein ingested food / tissue breakdown

Glucose ingested food / glycogen / other compounds

Urea deamination / urea cycle

Uric acid metabolism of nucleotide bases

Creatinine metabolism of creatine (creatine phosphate)

Ammonia deamination

Sources

Where do these come from? Sodium extracellular cation Potassium intracellular cation Chlorideextracellular anion Phosphate bone / tissue fluid Sulphate amino acids

Bibliography

John parker &Ian Honeysett, revise biology, Letts educational, chapter 3