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Chapter 11 Excretion

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Learning Objectives:

(a) define excretion and explain the importance of removing

nitrogenous and other compounds from the body

(b) outline the function of kidney tubules with reference to ultra-

filtration and selective reabsorption in the production of

urine

(c) outline the role of anti-diuretic hormone (ADH) in the

regulation of osmotic concentration

(d) outline the mechanism of dialysis in the case of kidney

failure

Prepared by, Ms Wong Fui Yen

Excretion is the removal of toxic

materials and the waste products of

metabolism from the body.

Egestion is the removal of undigested

food matter which have never been

involved in the metabolic activities of

cells, from the alimentary canal.

Faeces is NOT a product of metabolism!

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Metabolic Waste Products:

Excretory product Organ Mode of excretion

Carbon dioxide Lungs Exhalation

Urea and excess

mineral salts

Kidneys

Skin

Urine

Sweat

Excess water Kidneys

Skin

Lungs

Urine

Sweat

Expired Air

Bile pigments

(from breakdown of

haemoglobin)

Liver Faeces

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Excretory Organs in Man

Lungs: CO2, water vapour

Kidneys: excess water, excess salt,

Nitrogenous waste products (urea)

Liver: Bile pigmentsSkin: Urea, excess water, excess salt

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Ureter: thin tube carries urine from kidneys to bladder

Urinary bladder: elastic, muscular bag to store urinetemporarily. Sphincter muscle voluntarily relaxes to release urine from bladder

Urethra: passage of urine out of body.

The Urinary System

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Prepared by, Ms Wong Fui Yen

Blood reaches the kidney via the renal

artery, which branches into

arterioles

Blood from each arteriole flows into a

network of capillaries

(glomerulus) in the Bowman's capsule

Blood leaving the glomerulus flows

through the capillaries that run along the kidney

tubules

Blood from the capillaries enter the venules, which lead

to the renal vein

Blood vessels

Renal artery

Renal vein

Ureter

Nephron- smallest unit of kidney

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Urine formation

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•Nitrogeneous wastes and excess mineral salts are removed from the body

via the urine

•The renal artery carries oxygenated blood, nutrients, and wastes to the

kidney while the renal vein carries filtered blood away from the kidney.

•The pathway of blood flow in the kidney is shown below:

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Urine FormationTwo stages: 1. Ultrafiltration 2. Selective Reabsorption

1. Ultrafiltration:• Occurs in the Renal/Bowman's capsule. • Blood enters kidney via the renal artery• Renal artery divides to form a mass of arterioles, and

then capillaries which form a knot (glomerulus) in the Bowman’s capsule

• Afferent arteriole leading into the glomerulus is wider than the efferent arteriole that leads out, so creates a high blood pressure

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Urine Formation

•This pressure forces plasma containing small molecules to be filtered out of the blood.

• Red and white blood cells, large plasma proteins remain in the blood capillaries and continue into the proximal convoluted tubule.

•Glomerular filtrate: liquid that is filtered out contains glucose, amino acids, mineral salts, excess water nitrogenous waste (urea).

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2. Selective re-absorption:Starts at the proximal convoluted tubule.

This is the longest and widest part of the nephron.

Over 80% of the filtrate is reabsorbed into the blood.

Filtrate flows down the nephron, all useful materials

are reabsorbed back into network of capillaries.

Examples such as glucose, amino acids, salts, water

are reabsorbed.

Excess water and salts, and metabolic waste

products pass from the tubules into the collecting

ducts and then into the renal pelvis as urine.

Urine Formation

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Loop of Henle: mainly water (by osmosis).

Distal tubule: remaining fluid with more urea, excess useless substances e.g. sodium chloride, uric acid, ammonia, creatinine, water.

Collecting duct:Urine collected, passed into renal pelvis, leads to ureter.

Urine Formation

Blood containing

metabolic waste

products from

renal artery

Renal

capsule

(i) Ultrafiltration

(ii) Selective

Reabsorption

filtrate

Blood

capillaries

• amino acids • glucose• salts• water

••amino acids • glucose• salts• water• urea

Excess water, excess

mineral salts and urea

removed through urine

Purified blood to

renal vein

Concentration of fluid in different regions of a nephron,with and without presence of ADHPrepared by, Ms Wong Fui Yen

Prepared by, Ms Wong Fui Yen

Proximal convoluted reabsorptionTubule

All of the glucose and amino acids and most of the mineral salts( by diffusion and active transport )

Water ( by osmosis)

Loop of Henle reabsorption •Water (by osmosis)•Sodium ions ( Na+) (by active transport )

Distal convoluted reabsorptiontubule

•Water (by osmosis)•Sodium ions (Na+) (by active transport)

Collecting tubule reabsorption/ duct

•Water (by osmosis)

Glomerular filtrate

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Osmoregulation

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•Osmoregulation is the control of water and solute levels in

the blood to maintain a constant water potential in the body.

•The water potential of blood (concentration of water and salts

in the plasma) must be maintained at a relatively constantlevel. The water content is regulated via hormone vasopressin

(also known as anti-diuretic hormone, ADH)

•ADH is produced by the hypothalamus in the brain and is

released by the pituitary gland

•ADH increases water reabsorption of the kidneys by

increasing permeability of the distal convoluted tubule and

collecting duct to water.

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Kidney failure refers to the inability of the kidney to

function properly, causing accumulation of

nitrogenous waste in the blood.

Kidney failure may be caused of the following:

a) Diabetes

b) Hypertension

c) Excessive alcohol intake

d) Severe injury or bacterial infection

When both kidneys fail to function, the person will need

to get a kidney transplant, or undergo kidney

dialysis to get rid of wastes from patient’s blood.

Kidney Failure & Dialysis

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Kidney Dialysis Machine

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Larger molecules

(e.g. plates and

blood cells)

remain in the

tubing

The filtered blood

is returned to a vein

in the patient’s arm

artery

dialysis

tubingpump

filtered

blood

fresh dialysis

fluiddialysis

machine

dialysis

fluid

vein

1

2

3

6

4

5

23

Blood is drawn from an artery in the patient’s arm

1 Blood is pumped

through a tubing

to the dialysis

machine

2

The tubing is bathed

in a special dialysis

fluid and the tubing

is semi-permeable.

3

Small molecules (e.g. urea) and metabolic waste products diffuse out of the tubing

45

6

Prepared by, Ms Wong Fui Yen

Prepared by, Ms Wong Fui Yen

How does the kidney dialysis machine work?

1. Patient’s blood is drawn from the artery and allow to flow through the tubing in the dialysis machine.

2. Tubing which represents the partially permeable membrane is bathed in a specially controlled dialysis fluid.

They only allow small molecules, like urea and other waste products, to diffuse out of the tubing.

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3. The direction of blood flow is opposite to the flow of the dialysis fluid.

4. A concentration gradient is set up to allow diffusion of waste products from patient’s blood to the dialysis fluid.

5. The filtered blood is then returned to the patient’s arm vein.

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Comparing the Blood Composition & Dialysis Fluid

Partially permeable

membrane of tubing

Resources Online:The Urinary System:

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

Kidney Dialysis:

http://www.kidneypatientguide.org.uk/

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Prepared by, Ms Wong Fui Yen

• Factors affecting composition of urine

•Diet:

- Protein rich diet more urea being formed as proteins are deaminated in the

liver

- More water intake more urine formed

-Salty foods more salt secreted in urine

•Diseases:

-Diabetes: Glucose in urea

-Kidney Stone: glomerulus becomes fully permeable Red blood cells able to pass

through membrane, hence RBC found in urine

-Infection of kidney: White blood cells in urine

•Lifestyle:

-More exercise less water left in the body less water secreted in urine

•Climate:

-Cold weather sweat less more water left in the body urine more water

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Why you shouldn’t drink the salty sea water?

•Drinking sea water will increase

the concentration of dissolved

mineral salts in the blood, hence

causing water to diffuse from all

the body cells into the

bloodstream.

•This causes the cells to shrink

and malfunction.