Excretion

EXCRETION

Excretion is the removal of unwanted substances such as toxic materials and wastes products of metabolism from organisms. These unwanted substances can be :-
1. Waste products from body metabolism (chemical reactions in the body).
2. Excess water and salts taken in with the diet.
3. Spent hormones.
4. Drugs and other foreign substances.

Excretion occurs in specialized organs of the body called the excretory organs. There are two excretory organs in human body :-
a) Lungs
b) Kidneys

The lungs are the excretory organ for removing carbon dioxide and excess water. The kidneys are responsible for removing several unwanted substances which include nitrogenous wastes, excess water, salts, toxins, hormones and drugs.

The Lungs

The lungs are where gaseous exchange occurs. In the alveoli of the lungs are where oxygen is absorbed by the blood and carbon dioxide is release and breathe out into the atmosphere. Similarly, excess water is excreted to the atmosphere when breathing out. The following diagram summaries the process :-
[Draw Fig 13.9 pg 126 Mackean]

The Kidneys
The kidneys are like a filter system that cleans the blood that flows through them by removing unwanted substances such as waste products, toxic substances, used hormones and drugs. There is a pair of kidneys inside the body.

Kidney Structure The kidney consists of three regions : Cortex, Medulla and Pelvis

[Draw Fig 14.2 pg 132 Mackean]

The main function of the kidneys is to remove urea (nitrogenous waste) and excessive water from the blood. The basic unit of the kidney is called the nephrons.

How the kidneys function
The kidneys have three basic mechanisms for separating the various components of the blood: filtration, reabsorption, and secretion. These three processes occur in the nephron (Figure 2), which is the most basic functional unit of the kidney. Each kidney contains approximately one million of these functional units.
Extra Notes :
The nephron contains a cluster of blood vessels known as the glomerulus, surrounded by the hollow Bowman's capsule. The glomerulus and Bowman's capsule together are known as the renal corpuscle. Bowman's capsule leads into a membrane-enclosed, U-shaped tubule that empties into a collecting duct. The collecting ducts from the various nephrons merge together, and ultimately empty into the bladder.

Nephron Segment
Function

Renal Corpuscle:
Glomerulus
Bowman's Capsule
Filtration:
Glomerulus filters proteins and cells from the blood.
All other blood components pass into Bowman's capsule, then into the tubule.

U-Shaped Tubule
Reabsorption and Secretion:
Semipermeable membranes surrounding the tubule allow selective passage of particles back into the blood (reabsorption), or from the blood into the tubule (secretion).

Collecting Duct
Collection:
Collects all material that has not returned to the blood through the tubular membranes. This material exits the kidney as urine.
Renal Corpuscle
Blood first enters the kidney through the renal artery (see Figure 1), which branches into a network of tiny blood vessels called arterioles. These arterioles then carry the blood into the tiny blood vessels of the glomerulus. It is here, in the renal corpuscle, where filtration occurs. The glomerulus filters proteins and cells, which are too large to pass through the membrane channels of this specialized component, from the blood. These large particles remain in the blood vessels of the glomerulus, which join with other blood vessels so that the proteins remain circulating in the blood throughout the body. The small particles (e.g., ions, sugars, and ammonia) pass through the membranes of the glomerulus into Bowman's capsule. These smaller components then enter the membrane-enclosed tubule in essentially the same concentrations as they have in the blood. Hence, the fluid entering the tubule is identical to the blood, except that it contains no proteins or cells.
Tubule
The tubule functions as a dialysis unit, in which the fluid inside the tubule is the internal solution and the blood (in capillaries surrounding the tubule) acts as the external solution. Particles may pass through the membrane and return to the blood stream in the process known as reabsorption, which is analogous to the movement of particles from the internal to the external solution in the dialysis experiment you performed in lab. The reabsorption of many blood components is regulated physiologically, as discussed below. Alternatively, particles may pass through the membrane from the blood into this tubule in the process known as secretion, which is analogous to the movement of particles from the external solution into the dialysis bag in the experiment you performed in lab. The most important particles that are secreted from the blood back into the tubules are H+ and K+ ions, as well as organic ions from foreign chemicals or the natural by-products of the body's metabolism.
Collecting Duct
The blood components that remain in the nephron when the fluid reaches the collecting duct are excreted from the body.The collecting duct from one nephron meets up with many others to feed into the ureter. The ureters (one from each kidney) enter the bladder, which leads to the urethra, where the liquid waste is excreted from the body. Hence, the material that is filtered and secreted from the blood into the tubule, less the amount that is reabsorbed into the blood, is ultimately excreted from the body.