The Excretory system

Functions of the kidneys

• Rid the body of wastes, especially nitrogenous wastes such as urea, uric acid, and creatinine.

• Maintain the constant concentration of materials in the body

• Regulate the balance of fluid, salt, and pH.

Structures of the urinary system

• Kidneys

• Urinary bladder

• Ureters

• Urethra

Nephron

The functional unit of the kidneys, they make urine.

Nephron structure

• Renal corpuscle

  • Bowman’s capsule

  • Glomerulus

• A renal tubule

  • Proximal convoluted tubule

  • Loop of Henle

  • Distal convoluted tubule

• Their associated blood supply

Renal corpuscle

Where filtration takes place, made of the glomerulus and Bowman’s capsule.

Glomerulus

Mass of blood capillaries

Afferent arteriole

Where blood enters the glomerulus from the renal artery.

Efferent arteriole

Where blood leaves the glomerulus, divides into network of peritubular capillaries.

Peritubular capillaries

Network of capillaries surrounding the renal tubule.

Podocytes

Specialised cells that line the Bowman’s capsule.

Have finger like projections that wrap around the capillaries of the glomerulus.

The spaces between these ‘fingers’ are filtration slits.

Filtration

Removal of waste substances occurring in the glomerulus, where fluid is forced out of the blood and into the bowman’s capsule.

Blood pressure

High within the the glomerulus, aiding the movement of fluid.

Diameter

Wider in the afferent arteriole than in the efferent. This increases the resistance to blood flow and increases blood pressure, forcing water and dissolved blood components out of the capillary.

Filtrate substances

Consists of all the materials in the blood, except red and white blood cells and plasma proteins, as they are too large.

Renal tubule

Where selective reabsorption takes place

Selective reabsorption

Materials from the filtrate required by the body being returned to the blood in the peritubular capillaries.

Reabsorption useful features

A large surface area for reabsorption to take place is provided through:

• Convolutions of the renal tubule

• High number of nephrons in the kidneys

• Microvilli line the proximal tubule and create a brush border.

Facultative reabsorption

The permeability of the cells of the renal tubule being altered depending on the body’s needs.

Tubular secretion

Adds materials from the blood to the filtrate. Aims to:

• Maintain blood pH by removing excess hydrogen and ammonium ions

• Maintain urine pH

Urine process

• Water and other substances not reabsorbed drain into the collecting ducts, then renal pelvis.

• Urine drains into the ureters and is pushed by waves of muscle contraction 25-30cm into the bladder.

• The urethra carries urine from the bladder to the exterior of the body.

Composition of urine

Varies according to diet/water intake/health, etc. Typically contains:

• Approx. 96% water

• Approx. 4% other solutes, organic molecules, ions, and other metabolic wastes:

  • Urea (2%)

  • Creatinine

  • Uric acid

  • Sodium, chloride, other ions (1.5%)

• Normally does not contain glucose or a significant amount of protein.

Uric acid

A component of the urine which is created from the metabolism of purines.

Purines

Come from the breakdown of nucleic acids when cells die, and also occur in many foods.

Creatinine

A component of urine, produced in muscle from the breakdown of creatinine phosphate.

Liver functions

• Detoxifies alcohol, antibiotics, and other drugs.

• Deactivates hormones and converts them into forms which can be excreted by the kidneys.

• Breaks down haemoglobin from dead erythrocytes to produce bile pigments, which are passed out of the body within the faeces.

Protein

• Comes from worn out/broken cells and are broken down into amino acids, which are used to make new proteins.

• Can be lost from the body via urine, skin, hair, and fingernails.

• Help form cell structures, enzymes, antibodies, and glandular secretions.

• Excess in the diet cannot be stored in the cells of the body. The body needs to break it down to remove it.

Carbohydrates

Initially used by the body for energy.

Fats

When carbohydrates are depleted, _ in food and stored _ are used to release energy.

Proteins for energy

Not utilised in energy-releasing reactions the majority of the time, however:

• In order to use proteins for energy, the body can metabolise large amounts of proteins.

Deamination

The stripping of nitrogen from amino acids and nitrogenous bases (RNA). Removes an amino group from an amino acid molecule.

Nitrogen

Occurs in the amino (NH₂) part of the amino acid. Toxic to the body and must be removed, so in deamination, it is stripped from amino acids and nitrogenous bases (RNA).

Deamination location

Occurs in the liver, with the aid of enzymes (deaminases).

1. Deamination

The removed amino acid group (NH₂) is converted by liver cells into ammonia (NH₃).

Amino acids —(Enzymes)→ Amino group

Amino acid + oxygen —(Enzymes)→ ammonia + organic compounds

2. Deamination

Ammonia is converted into urea and removed from the body in the urine.

Ammonia + CO₂ + energy —(liver cells)→ urea + water

3. Deamination

The remaining part of the amino acid is converted into a carbohydrate and broken down to release energy (plus carbon dioxide and water).

Amino acid → carbohydrate (glucose, which is broken down to release energy)

Ammonia

1. Produced by proteins

2. Excreted by the kidneys,

3. Eliminated from the body in the urine, & also lost in sweat.

Soluble in water and is highly toxic to cells, so converted into urea, which is much safer for the body.

Nitrogenous wates

• Urea (from amino acids - moderate toxicity)

• Creatinine (from muscle metabolism - high toxicity)

• Uric acid (from RNA - weak toxicity)

Organs involved in processing or excretion of wastes

• Lungs - Excrete carbon dioxide produced by all body cells during cellular respiration.

• Liver - Processes many substances so that they can be excreted.

• Sweat glands in the skin - Secrete sweat, which is largely water, for cooling.

• Alimentary canal (tube from mouth to anus) - Passes out bile pigments, which enter the small intestine with the bile.

• Kidneys - Principle excretory organs, responsible for maintaining the constant concentration of materials in the body fluids.

Liver functions concluded

• Deamination

• Detoxifies alcohol and other drugs

• Deactivates hormones & converts them into a form the liver is able to excrete.

• Breaks down haemoglobin from dead red blood cells to produce bile pigments, which are then passed out of the body with the faeces.

• Produces bile, which aids in the digestion of fat via emulsification, and the absorption of fats.

Skin function

Contains sweat glands in its lower layers, which are involved in excretion.

• A duct carries sweat secreted to a hair follicle or to the skin surface, where it opens at a pore.

• Cells surrounding the glands are able to contract and squeeze the sweat to the skin surface.

Sweat glands

Secrete about 500mL of water per day. Dissolved in this water and being excreted out of the skin are:

• Sodium chloride

• Lactic acid

• Urea

Renal corpuscle activities

• Filtration of blood from the glomerulus’ capillaries.

• Formation of filtrate in the glomerular capsule.

Proximal convoluted tubule and loop of Henle activities

• Passive reabsorption of potassium, chloride, and bicarbonate ions.

• Active reabsorption of glucose and sodium.

• Passive reabsorption of water by osmosis.

Distal convoluted tubule activities

• Active reabsorption of sodium ions.

• Active reabsorption of water, depending on the body’s water needs.

• Secretion of hydrogen and potassium ions, creatinine, and certain drugs such as penicillin.

Collecting duct activities

• Active reabsorption of water, depending on the body’s water needs.

Composition of urine

Solution of water with dissolved wastes such as:

• Urea

• Creatinine

• Various ions such as sodium, chloride, and potassium

• Low levels of other solutes