The Excretory System

The kidney

The main organs concerned with excretion in the body are the kidneys.   They are situated just above the hips at the back of the abdomen and are then connected via the ureter to the bladder which is then connected to the urethra.   The kidneys are supplied with blood from the renal artery.   The most important structure in the kidneys are the microscopic nephrons, which are the units that filter the blood to produce urine.

Structure of the nephron

Bowman's capsule

The nephron consists of Bowman's capsule which filters everything, except for the blood cells and plasma protein,  out of the blood into the proximal (close) tubule.   This is accomplished by a difference in size and therefore pressure of the blood vessels entering and leaving the glomerulus (the network of blood vessels in the Bowman's capsule).  The increased pressure forces the filtrate through the pores in Bowman's capsule into the proximal tubule.  The filtration is also aided by the specialised permeable epithelium on the walls of the Bowman's capsule.

Proximal and distal convoluted tubule

Now that all the potentially dangerous substances from the blood have been removed,   the useful substances are ready to be reabsorbed back into the blood stream.   The main substances that are re-absorbed in the proximal convoluted tubule are glucose, water and salt.    The reabsorbtion of salts and glucose takes place as a result of active transport (against a concentration gradient), this process requires energy.   Some water and salts are also reabsorbed in the distal tubule. Dangerous substances in the blood are also secreted into the distal tubule. From the proximal tubule the filtrate, now containing urea, sodium and chloride ions and water, moves into the loop of Henle.

Loop of Henle

The loop of  Henle is a U - shaped loop that extends from the cortex to the medulla of the kidney.  The effect of this  loop is to concentrate salts in the medulla. With the concentrated salts in the medulla it is then possible to secrete a solution that is hypertonic ( has a higher salt concentration than blood.)   At all places in the loop of Henle salt is actively removed, but in the ascending loop it is not allowed to diffuse back in.  An increasing salt concentration is then created in the descending limb with the highest concentration deepest in the medulla.   There is then a decreasing salt concentration in the ascending limb.

After the loop of Henle the filtrate passes through the distal (far) tubule. It then continues on to the collecting duct where water is reabsorbed.  The water is reabsorbed by diffusion because of the higher salt concentration on the outside  in the medulla of the kidney. The filtrate which now consist mainly of urea and other harmful substances, as well as a varying amount of water depending on the need for it in the body, is excreted via the urethra.

The Role of the Kidney in Homeostasis

The kidney is essential in maintaining the correct amount of water in the blood.    Osmoreceptors in the brain determine the concentration of the water in the blood, and secrete a hormone ADH accordingly.  The presence or lack of ADH controls the permeability of the walls of the collecting duct.  ADH also controls the feelings of thirst.