The Human Excretory System

Module One: Life Processes in Plants and Animals

Paper One

Excretion & Homeostasis• Homeostasis is the process of maintaining the ideal internal

conditions (i.e. correct temperature, right amount of water and glucose & other solutes) for the body to work at it’s optimum.

• Excretion is the process of removing metabolic waste products and other toxins.

• Osmoregulation is maintaining the correct balance between water and solutes.

• Excretion, which includes osmoregulation, is thus extremely important in maintaining homeostasis.

• Secretion is the release of useful substances, e.g. hormones, from the body. Thus, it is not excretion. Egestion, i.e. defecation, is also not excretion.

Removal of Waste Products

• Certain waste products would become highly toxic if they were to accumulate. This could damage tissues.

• An excess of water could also lead to a number of complications.

• Thus, the waste products must be removed – they continually move into the bloodstream, which carries them to the excretory organs.

Raw Materials (Food and 02)

Useful Materials Useless Materials

Egested (faeces) Metabolised

Metabolic Waste Products (urea and CO2)

Excreted

Useful Products

Excretory Organs• There are four main excretory organs:

– Lungs– Colon– Skin– Kidneys

• The liver is not an excretory organ, but produces many products which are excreted elsewhere. Toxins and drugs as well as alcohol, is broken down in the liver. Hence, an excess of smoking, medication and alcohol is extremely harmful to the liver.

The Lungs• The carbon dioxide released from cellular respiration is

carried to the lungs in the blood. It then diffuses across the respiratory membrane and is exhaled.

• A small amount of heat and water is excreted this way.

The Colon• Bile pigments, from the break down of haemoglobin, and

cholesterol are synthesised in the liver. • They pass into the small intestine as bile and are finally

excreted in the faeces as bile salts, from the colon.• Mucus and bacteria are too excreted through the colon.

The Skin• Sweat, which is excreted through the skin, contains water,

salts and some urea. • As the water in the sweat is excreted, heat is lost and the

body is cooled. • Sweat is a form of excretion as it rids the body of waste, as

well as a form of secretion as it maintains the body temperature.

The Kidneys• When amino acids and nucleic acids are broken down,

nitrogenous wastes are released as ammonia, urea, uric acid and creatinine. Ammonia is toxic if it accumulates and is therefore converted to less toxic urea in the liver.

• The following substances are made in the liver and excreted by the kidneys:– Urea, the main nitrogenous waste compound secreted. It is formed by

the breakdown of excess amino acids in the process of deamination.– Uric acid, the nitrogenous end product of nucleic acid metabolism.– Creatinine is formed from creatinine phosphate, found in the muscle

cells.– Non-nitrogenous waste, e.g. CO2, excess water, ions, hormones,

poisons and drugs.

The Urinary System• The main function of the urinary system is to maintain

homeostasis by regulating the volume and concentration of body fluids. It filters and reabsorbs certain materials from the blood.

• The urinary system is made up of the following parts:A) Two kidneysB) A bladderC) An urethra

Urinary System: Kidneys• The kidneys aid in:

– excretion as they filter waste out of the blood– regulate the water and salt balance in the body.– One kidney – the right kidney – is slightly lower than the left as it is

pushed down by the liver, which is larger on the right.

• Each kidney contains:– A renal artery (a branch of the abdominal aorta), which carries waste

products to the kidney and supplies the kidneys with oxygen and nutrients.

– A renal vein that contains the purest blood in the body. It carries CO2 to the inferior vena cava.

– Ureter that carries urine from the kidney to the bladder.

Urinary System: Bladder• Stores urine• Is stimulated by impulses from a motor nerve, to contract to expel

the urine.• Has sphincter muscles at the base to control the flow of urine

Urinary System: Urethra

• Carries urine from the bladder to outside the body

AortaCarries oxygenated blood, food and waste from the heart

Inferior vena cavaCarries deoxygenated blood and other substances back to the heart

Renal ArteryCarries blood from body to kidneys

KidneyExcretory and osmoregulatory organ.

Motor NerveStimulates bladder nerve

Renal VeinCarries blood from kidneys to inferior vena cava

UreterCarries urine from kidneys to bladder

BladderStores urine

UrethraCarries urine from bladder to outside the body

Adrenal GlandRegulates Salt

The Kidneys• Found in abdominal cavity below the diaphragm, near the

posterior, on either side of the vertebral column.• Kept in position with connective tissue, the peritoneum, as

well as renal blood vessels. They are wedged in with other organs.

• Externally they are bean-shaped, dark red and the size of a large bar of soap.

• The inner, concave border is called the hilum.• Surrounded by three layers of protective tissue:

– A tough, fibrous renal capsule on the surface protects them from disease.

– A middle layer of adipose tissue cushions them against blows.– An outer layer of fibrous connective tissue, the renal fascia, anchors the

kidneys to surrounding structures.

Renal CapsuleProtects kidney

Nephron

MedullaMade up of pyramids

PyramidMade up of collecting ducts

CalyxCollects urine from collecting ducts

PapillaTips of each pyramid, fits into calyx

Renal arteryCarries blood to kidneys

Renal veinCarries blood fromkidneys

UreterCarries urine from pelvis to bladder for storage

Blood Supply of Kidney

The kidney needs to have a constant supply of blood in order to control the composition of body fluids.• The renal artery, a branch from the aorta, enters the kidney

at the hilum. It supplies blood rich in nitrogenous waste, oxygen and nutrients.

• The renal vein carries purified, deoxygenated blood to the inferior vena cava, and then to the heart.

• Glomerular Filtration– The fluid part of the blood is filtered from the glomerulus into the

cavity of the Bowman’s capsule.

• Tubular Reabsorption– As the fluid flows along the renal tubule, useful substances are

reabsorbed back into the bloodstream according to the body’s needs.

• Tubular Excretion– In addition, certain unwanted substances in the blood are actively

excreted into the tubules.

How the Kidneys Function

• A passive, non selective process.• Fluids and solutes are forced through the glomerular

membrane by hydrostatic pressure.• The glomerular filtrate has the same composition as blood,

without the blood cells and plasma proteins. These are too large to fit through the glomerular membrane.

• Substances in glomerular filtration: Blood plasma without protein molecules which includes:– Useful substances – water, glucose, amino acids, vitamins, hormones,

and ions.– Waste substances – e.g. nitrogenous waste such as urea, uric acid and

creatinine.

Glomerular Filtration

• Occurs in the loop of Henle.• Substances are reabsorbed in the following ways:

– Active reabsorption– Passive reabsorption

Tubular Reabsorption

This is the process by which substances are reabsorbed.• Carrier molecules on the microvilli join up with certain

molecules from the filtrate and actively transport them through the epithelial cells to the blood.

• Energy from ATP is used to join the molecule to the carrier molecule. The following are actively reabsorbed:– All organic nutrients such as glucose, amino acids and water

soluble vitamins are completely reabsorbed.– Sodium ions and fat soluble vitamins are selectively reabsorbed,

according to the needs of the body.

Active Reabsorption

• Passively = no energy needed.• About 65% of the water is passively reabsorbed from the

filtrate in the proximal convoluted tubule by osmosis.• Chloride ions passively follow the path of sodium ions.• Urea, uric acid and creatinine is not reabsorbed.

Passive Reabsorption

• Takes place in proximal and distal tubules and is reabsorption in reverse.

• The following molecules and ions are taken from the blood and deposited into the filtrate:– Hydrogen and potassium ions (secreted directly)– Creatinine and uric acid– Drugs, preserves and colourants (actively excreted)

Tubular Excretion

• About 96% of urine is water.• 1.5% is salts, mainly sodium chloride.• Urea makes up 2%• Small quantities of drugs, colourants, hormones and

preservatives.• About 1.5 litres of urine is produced daily.

Composition of Urine

Maintain homeostasis in the following ways:• Excreting nitrogenous waste• Osmoregulation – water and salts• Maintain pH of body fluids• Maintain electrolytic (salt) balance of body fluids by absorbing

and/or secreting ions.

Key Functions of Kidneys