Chapter 11 Excretion

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

Excretion is the removal of toxicmaterials 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!

skin lungs liver kidneys

Waste products excreted by:

• urea• excess water• excess salts

• carbon dioxide• excess water

• bile pigments

• urea• excess water• excess salts

in sweat

in expired air in faeces through the intestines

in urine

Metabolic Waste Products:

Excretory product Organ Mode of excretion

Carbon dioxide Lungs Exhalation

Urea and excess mineral salts

KidneysSkin

UrineSweat

Excess water KidneysSkin Lungs

UrineSweat Expired Air

Bile pigments (from breakdown of haemoglobin)

Liver Faeces

Kidneys - produce urine

Ureters- connect the kidneys to the bladder

Bladder- stores urine temporarily

Urethra- passage through which urine is discharged from the bladder

Components of the human urinary system

Ureter: thin tube carries urine from kidneys to bladder

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

Urethra: passage of urine

out of body.

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

11.3

Learning Outcome

After this section, you should be able to:

• outline the function of the kidney tubules in urine formation with reference to ultrafiltration and selective reabsorption.

Urine Formation

Nephron- smallest unit of kidney

i. Ultrafiltration (occurs at the renal corpuscle)

ii. Selective reabsorption (occurs at the renal tubule)

Urine formation involves two main processes within the kidney tubule:

1. Ultrafiltration

i) Force:

a high hydrostatic blood pressure at the glomerulus, and

ii) Filter:

basement membrane around the glomerular capillaries is partially permeable and works like a fine filter.

afferent arteriole (larger)

Bowman’s capsule

glomerulus

efferent arteriole (smaller)

filtrate

The mechanical filtration that occurs at the renal corpuscle is called ultrafiltration.

Ultrafiltration occurs because of:

What is in the filtrate?

• Water• Small molecules such as glucose, amino

acids, mineral salts, urea.afferent arteriole (larger)

Bowman’s capsule

glomerulus

efferent arteriole (smaller)

What is not filtered out?

• Large molecules such as plasma proteins and red blood cells

• 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.

2. Selective Reabsorption

• 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.

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 reabsorption tubule

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

Collecting tubule reabsorption/ duct

•Water (by osmosis)

Glomerular filtrate

Blood containing metabolic waste products from renal artery

Renal capsule

(i) Ultrafiltration

(ii) Selective Reabsorption

filtrateBlood 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

What are the

substances carried by

renal artery and renal vein?

11.4

Learning Outcome

After this section, you should be able to:

• outline the role of anti-diuretic hormone in the regulation of osmotic concentration.

Osmoregulation

(Imagine if you are stranded in an island for weeks and have no supply of water and food like the young Indian boy in the story of Pi...)

• Why are you not recommended to drink the sea water?

• What are the consequences of drinking sea water?

Spark Interest

Osmoregulation

• The water potential of blood plasma needs to be kept relatively constant.

red blood cell

dilute plasma

concentrated plasma

burst red blood cell

crenated red blood cell

• If there are large fluctuations in the water potential, numerous problems can occur.

• For example, if the blood plasma is too concentrated, the red blood cells (rbcs) would dehydrate and undergo crenation.

• If the blood plasma is too dilute, the RBCs would absorb water, expand and burst.

• The water potential of blood plasma is controlled by anti-diuretic hormone (ADH).

hypothalamus

pituitary gland

• ADH increases reabsorption of water by the kidney tubules.

• The regulation of the water potential of blood plasma is called osmoregulation.

• The water potential of blood plasma depends on the amount of water and solutes in the plasma.

• ADH is produced by the hypothalamus and released by the pituitary gland.

• Since kidneys help to regulate the concentration of water and salts in blood, they are called osmoregulators.

• Osmoregulation maintains the concentration of water and salts within an organism. It is a type of ‘homeostasis’.

• As blood volume is controlled by removing excess salts and water, blood pressure is also indirectly regulated by osmoregulation. High blood pressure can cause blood vessels in the brain to burst, and this would result in a stroke.

Loss of water through

sweating

Pituitary gland secretes

more ADHWater

potential of plasma

decreases

Kidney tubules reabsorb

more water

Water potential of plasma returns to

normal level

Osmoregulation (dehydration)

hypothalamus

stimulates

causes

Less urine produced and hence, more concentrated

Kidney tubules reabsorb less water

Pituitary gland secretes less ADH

Water potential of plasma increases

Large intake of water

(e.g. drinking)

Water potential of plasma returns to

normal level

hypothalamus

stimulates

causes

Osmoregulation (excess water)

More urine produced and hence, more diluted

Concentration of fluid in different regions of a nephron,with and without presence of ADH

Kidney Failure & Dialysis

• 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 Dialysis Machine

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 tubing

pump

filteredblood

fresh dialysisfluid

dialysismachine

dialysisfluid

vein

1

2

3

6

4

5

30

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

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.

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.

Comparing the Blood Composition & Dialysis Fluid

Partially permeable membrane of tubing

• 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

Spark Interest:(Imagine if you are stranded in an island for weeks and have no supply of water and food like the young Indian boy in the story of Pi...)

• Why are you not recommended to drink the sea water?

• What are the consequences of drinking sea water?

11.4

• The water potential of blood plasma needs to be kept relatively constant.

red blood cell

dilute plasma

concentrated plasma

burst red blood cell

crenated red blood cell

• If there are large fluctuations in the water potential, numerous problems can occur.

• For example, if the blood plasma is too concentrated, the red blood cells (rbcs) would dehydrate and undergo crenation.

• If the blood plasma is too dilute, the RBCs would absorb water, expand and burst.

Osmoregulation

Resources Online:• The Urinary System:

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

• Kidney Dialysis:

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