Topic 7: Excretion

Question 1

Excretion

Answer 1

Excretion is a process by which metabolic waste products and toxic substances are removed from the body of an organism.

Question 2

Metabolism

Answer 2

The total sum of all chemical reactions occuring in the cells of an organism (metabolism = anabolism + catabolism)

Question 3

Anabolism

Answer 3

Metabolic reactions where simple molecules are built up into complex molecules with a net intake of energy.

Examples: photosynthesis (in plants), conversion of excess glucose to glycogen for storage (in liver and muscles)

Question 4

Catabolism

Answer 4

Metabolic reactions where complex molecules are broken down to simpler molecules with a net release of energy.

Examples: hydrolysis, tissue respiration, deamination of excess amino acids

Question 5

Metabolic waste products

Answer 5

• Unwanted products from metabolic reactions (e.g., carbon dioxide from respiration)
• Harmful if allowed to accumulate in the body
• Must be removed (e.g., removal of carbon dioxide by the lungs and removal of oxygen through stomata in plants) or deposited as harmless insoluble substances

Question 6

Difference between Excretion and Egestion (Defecation)

Answer 6

• Egestion is the removal of waste substances, mainly undigested matter, from the alimentary canal. These undigested materials (e.g. cellulose) have never been absorbed into cells and hence they are not produced as a result of metabolic reactions within cells.
• Excretion is also the removal of waste substances but these waste substances are produced by metabolic reactions that occur within cells.

Question 7

Methods of Excretion

Answer 7

• Unicellular organisms excrete their metabolic waste products into surrounding water by diffusion.
• Multicellular organisms (e.g. mammals with impermeable skin) require special excretory organs to remove our metabolic waste products.

Question 8

State the respective source, excretory organ, and what it’s excreted as.

Carbon dioxide

Answer 8

tissue respiration; lungs; gas in exhaled air

Question 9

State the respective source, excretory organ, and what it’s excreted as.

Nitrogenous waste

Answer 9

deamination of excess amino acids; kidneys & skin; components of urine

Nitrogenous waste products are also found in sweat albeit in small amounts.

Question 10

State the respective source, excretory organ, and what it’s excreted as.

Excess mineral salts

Answer 10

diet, neutralisation reactions; kidney & skin; components of sweat & urine

Question 11

State the respective source, excretory organ, and what it’s excreted as.

Bile pigments

Answer 11

breakdown of haemoglobin; liver; components of faeces

Question 12

State the respective source, excretory organ, and what it’s excreted as.

Excess water

Answer 12

respiration, other chemical reactions in body; kidneys, skin & lungs; main component in urine, sweat and water vapour in exhaled air

Question 13

Functions of healthy kidneys

Answer 13

1. Removes mainly nitrogenous waste products (e.g. urea) and excess water and mineral salts in the form of urine (as an excretory organ)
2. Regulates the salt and water balance of blood plasma (as an osmoregulator)
3. Maintain the pH and composition of the blood plasma

Question 14

Parts of the Urinary System

Hilum

Answer 14

a depression where renal artery, renal vein and nerves are connected to the kidney

Question 15

Parts of the Urinary System

Kidney

Answer 15

• bean-shaped organ
• remove urea and extra water from the blood (as urine)
• keeps chemical concentrations (such as sodium, potassium, and calcium) balanced in the blood

Question 16

Parts of the Urinary System

Ureter

Answer 16

narrow tube that emerges from the hilum, brings urine from each kidney to the urinary bladder

Question 17

Parts of the Urinary System

Urinary bladder

Answer 17

elastic, muscular bag in front of the rectum that stores urine

Question 18

Parts of the Urinary System

Sphincter muscle

Answer 18

found at the bottom of the bladder; controls urine flow into the urethra and out of the body

Question 19

Parts of the Urinary System

Urethra

Answer 19

duct through which urine passes from the urinary bladder to outside of body

Question 20

Associated Blood Vessels in the Urinary System

Renal arteries

Answer 20

transports oxygenated blood containing urea and excess water from the heart to the kidneys

Question 21

Associated Blood Vessels in the Urinary System

Renal veins

Answer 21

bring deoxygenated blood (from which urea and excess water have been removed) away from the kidneys to the heart

Question 22

Internal Structure of Kidneys

Cortex

Answer 22

the outer darker red region of the kidney, enclosed by fibrous capsule

Question 23

Internal Structure of Kidneys

Medulla

Answer 23

• the inner, thicker, paler red region
• contains 12-16 conical structured called pyramids

Question 24

Internal Structure of Kidneys

Renal pelvis

Answer 24

• funnel like space where pyramids project into
• enlarged part of the of ureter inside the kidney

Question 25

Structure of Nephrons

Answer 25

Nephrons: * basic functional units of the kidney * microscopal tubules where urine is formed * each kidney has about one million nephrons * each nephron has 4 main parts: Bowman’s capsule, proximal convoluted tubule, loop of Henlé, distal convoluted tubule * several nephrons open into a tube known as the collecting duct

Question 26

Blood flow through the nephron

Answer 26

1. Blood enters a kidney via the renal artery 2. A branch of renal artery splits into many arterioles 3. An arteriole further divides into a network of capillaries (glomerulus) 4. Blood leaves the glomerulus and enters a network of capillaries surrounding the rest of the nephron (PCT, DCT, loop of Henle) 5. Blood flows into venules from capillaries 6. Blood flows into the renal vein from venules 7. Blood leaves the kidney via the renal vein

Question 27

Urine Production

Answer 27

urine is formed in the nephron through the processes of ultrafiltration and selective reabsorption

Question 28

Ultrafiltration

Answer 28

Occurs in the glomerulus (plural: glomeruli) Function: * mechanical filtration removes small molecules from the blood * most of the blood plasma (except large molecules) is forced out of the glomerular blood capillaries into the Bowman’s capsule to form the glomerular filtrate

Question 29

Conditions for Ultrafiltration

Answer 29

High hydrostatic blood pressure in the glomerulus * Difference in diameter between the afferent and efferent arteriole creates the high hydrostatic pressure in the glomerulus. i.e. the afferent arteriole is wider than the efferent arteriole (main force in ultrafiltration) Partially permeable membrane * The basement membrane that wraps the glomerular blood capillaries has very small pores to allow only water and very small molecules to pass through

Question 30

Filtered & Unfiltered Substances

Answer 30

Water, mineral salts, glucose, amino acids, nitrogenous waste products (e.g. urea) are filtered off during ultrafiltration. Blood cells, platelets and large molecules like proteins and fats are not filtered off

Question 31

Selective Reabsorption

Answer 31

Useful substances are selectively reabsorbed from the filtrate formed by ultrafiltration. This occurs through the walls of the tubule (proximal convoluted tubule, loop of Henlé, distal convoluted tubule and collecting duct) into the surrounding blood capillaries.

Question 32

# State the processes and reabsorbed substances. Proximal Convoluted Tubule

Answer 32

Active transport: all glucose, all amino acids, most mineral salts (sodium) Osmosis: most of the water

Question 33

# State the processes and reabsorbed substances. Loop of Henle

Answer 33

Osmosis: some of the water

Question 34

# State the processes and reabsorbed substances. Distal Convoluted Tubule

Answer 34

Active transport & diffusion: some mineral salts

Question 35

# State the processes and reabsorbed substances. Collecting Duct

Answer 35

Osmosis: some water

Question 36

Urine

Answer 36

Mixture passed out of the collecting duct and into the renal pelvis, containing excess water, excess mineral salts and nitrogenous waste products such as urea, uric acid and creatinine

Question 37

Kidney Failure & Treatment

Answer 37

Common causes for kidney failure are: * high blood pressure * diabetes * alcohol abuse * damage to the kidney(s) due to accidents * complications from a major surgery A person can survive on just one kidney. If both kidneys fails to function, the person will die unless prompt medical intervention is given (due to the buildup of toxic waste products in the body) Options for treatment of kidney failure includes: kidney transplant, hemodialysis and peritoneal dialysis.

Question 38

Haemodialysis

Answer 38

During haemodialysis, blood is channelled from a vein in the patient’s arm to the dialyser of a dialysis machine. Blood flows into the tubing of the dialyser. The tubing is bathed in a specially controlled dialysis fluid. The walls of the tubing are partially permeable. Small molecules such as urea and other metabolic waste products diffuse out of the tubing into the dialysis fluid. Blood cells, platelets and large molecules remain in the tubing. The filtered blood is then returned to a vein in the patient’s arm

Question 39

Features of a Dialysis Machine

Answer 39

* Tubing in the dialyser is long, narrow and coiled * Composition of dialysis fluid: contains no metabolic waste products; has the same concentration of essential nutrients (such as glucose, amino acids and essential salts) as that of healthy blood * Direction of blood flow in the tubing is opposite to the flow of the dialysis fluid

Question 40

# Explain the advantage that this feature provides. Tubing in the dialyser is long, narrow and coiled

Answer 40

Increases surface area to volume ratio to speed up the exchange of substances between the blood and the dialysis fluid

Question 41

# Explain the advantage that this feature provides. Composition of dialysis fluid: * contains no metabolic waste products * has the same concentration of essential nutrients (such as glucose, amino acids and essential salts) as that of healthy blood

Answer 41

* A steep concentration gradient exists which allows nitrogenous waste products (e.g. urea, uric acid and creatinine), and excess water and mineral salts, to diffuse out of the tubing into the dialysis fluid. The waste products are removed from the blood. This maintains the correct solute composition and water potential of the blood. * This ensures that such substances do not diffuse out of the patient’s blood into the dialysis fluid. Furthermore, if patient’s blood lacks any of these essential nutrients, it will diffuse from the dialysis fluid into the patient’s blood in the tubing.

Question 42

# Explain the advantage that this feature provides. Direction of blood flow in the tubing is opposite to the flow of the dialysis fluid

Answer 42

This maintains a concentration gradient between the patient’s blood and the dialysis fluid for the removal of metabolic waste products.