The excretory system

The Excretory System is made up of the urinary system , the respiratory system (removes CO2 from the blood), sweat glands (they expel water and salt) and the liver which eliminates certain toxic substances, a small amount of cholesterol and the products resulting from the breakdown of haemoglobin. When haemoglobin is destroyed bilirubin is obtained, enters the liver and becomes one of the components of bile. Bilirubin also accounts for the yellow colour of urine or even bruises.

Linking things together: Where can you find haemoglobin? what is its function? What cells contain haemoglobin? Where are these cells destroyed? Where is the bile produced? Where is it released into and what is its function?

You see? Everything is connected. This is a good activity to help you remember.

The Urinary System.

The Kidneys.
     
Each kidney has an external part (cortex), an internal part (medulla) and a hollow chamber (the renal pelvis).



Each kidney is made up of over a million nephrons, which filter the blood to produce urine.


Urine is a liquid obtained from the blood and made up of water, electrolytes (iones such as Sodium, Calcium, Potassium or Chloride that regulate or affect many metabolic processes), and waste products (mainly urea and uric acid).

Urine is produced in the nephrones in a process that consists of three phases: Filtration, Reabsorption and Secretion..

1.Filtration: Most of the water and solutes in the blood plasma filter out of the glomerulus (blood capillary) and into the Bowman's capsule (first part of the nephrone). This liquid, similar to blood plasma but without proteins, is called primary urine, and contains waste products but also many useful substances.

So, what do we need now? Can we afford losing such amount of water, nutrients, hormones, minerals?

2. Reabsorption: Useful substances must be reabsorbed (go back into the blood) while waste products will continue to the ureters.
The proximal convoluted tubule reabsorbs most of the nutrients, while the Loop of Henle reabsorbs water and ions. The distal convoluted tubule regulates the concentration of Sodium, Potassium and Hydrogen ions. Finally, the concentration or dilution of urine is adjusted depending of the body needs.

As you see, the urinary system eliminates waste products of our metabolism, but also ensures the needed balance of many ions in our internal medium and mantains blood volume and pressure.

Parts of a nephron: 

1. Bowman's capsule: it is a round sac which contains a curled-up capillary called glomerulus.

2. Proximal and distal convoluted tubules.

3. Loop of Henle. U-shaped tube which is between the proximal and distal tubules.

The nephrons lead to the collecting ducts, which drain into the renal pelvis. Then, the ureters connect the renal pelvis to the bladder, an elastic sac where urine collects.

When the bladder fills up, the micturition reflex makes the bladder contract and push the urine out via the urethra. At the beginning of the urethra there is a sphincter (ring of smooth muscle) which stays closed as long as the micturition reflex does not occur.