Excretion of Nitrogenous Waste

WALT

• To recap types of nitrogenous waste

• To identify the ultrastructure of the kidney

• To explain ultrafiltration

• Excretion is the process by which waste products of the organism’s metabolism are eliminated from the body.

• Secretion is the production by the cells substances useful to the body.

• Egestion is the removal of undigested food & other substances which have never been involved in metabolism.

Some definitions..

There are three main waste products of nitrogenous metabolism:

• Ammonia

• Urea

• Uric acid

Excretory Products

Ammonia

• Ammonia is derived from the breakdown of proteins and nucleic acids in the body.

• It is very toxic and is never allowed to accumulate within the body tissues or fluids.

• It is extremely soluble and diffuses readily across cell membranes.

• It is the main excretory product of marine invertebrates & all fresh water animals.

• The synthesis of uric acid spends more energy than urea, but it is advantageous to many animals.

• It is insoluble, non-toxic and requires very little water for its removal from the body -- a suitable product for animals in desert places.

Uric Acid

Carbon dioxide + ammonia urea + water

CO2 + 2NH3 (NH2) 2CO + H2O

• Urea is produced by the ornithine cycle with expenditure of some energy.

• It is much less toxic than ammonia, less water is needed for its elimination because the tissues can tolerate higher concentrations of it.

Urea

The Mammalian Kidney

• The kidney has two main functions:

• It removes metabolic waste from the body through the process of excretion as urine

• It regulates the water and ion content of the blood. This keeps the water potential of the blood constant.

Position of the urinary organs

Kidney Structure

Kidney Cross Section

cortex

medulla

pelvis

Collecting duct

Renal medulla

Renalcortex

Nephrontubule

The functions of the kidney

• Ultrafiltration – filtering blood unbder pressure

• Selective reabsorption – reabsorbing the useful substances

• Production of an ion gradient in the medulla – to allow production of hypertonic urine if necessary

• Adjustment of the water and ion content of the blood – to maintain homestasis

• Each kidney is made of millions of tiny tubules called nephrons, each of which consists of a Bowman's capsule and a uriniferous tubule joining to a collecting duct.

• A branch of the renal artery enters the Bowman's capsule as the glomerulus, then splits up into capillaries which spread out and wrap around the tubule. Finally, these capillaries join up to the renal vein.

The Structure of the kidney

Fig. 49.17(TE Art)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Glomerulus

Renal cortex

Renal medulla

Bowman'scapsule Proximal

convoluted tubule

Descending limb of loop of Henle

Loop of Henle

Distalconvolutedtubule

Ascending limb of loop of Henle

Collecting duct

To ureter

Vasa recta

Peritubularcapillaries

Ultrafiltration

• Blood reaching the glomerulus is under high pressure because:

• The afferent arteriole is wider than the efferent arteriole

• The heart has a high pumping force

Ultrafiltration

• This pressure results in small molecules from the blood being squeezed out through the capillary walls and into the capsular space of the Bowmans Capsule

Ultrafiltration

Fig. 49.18(TE Art)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Renal tubuleExcretion

Filtration

Reabsorption to blood

Secretion from blood

Glomerulus

Bowman'scapsule

• There are inner podocytes and outer squamous epithelium

• The glomerular filtrate contains water, amino acids, glucose, minerals and urea; blood cells & proteins are too large to pass through

• Ultrafiltration is a passive process and selection of substances passing from the blood into the glomerular filtrate is made entirely according to the relative molecular mass

Cells of Bowman's capsule

Cells of Bowman's capsule

• The walls of the glomerulus have 3 layers:

• The inner layer of flat endothelial cells that make the capillary walls.

• The basement membrane of the capillaries

• The podocytes of the Bowmans capsule:

Podocyte

Excretion of Nitrogenous Waste

Have we met our objectives?

• To recap types of nitrogenous waste

• To identify the ultrastructure of the kidney

• To explain ultrafiltration