1. The Loop of Henle - maximum excretion with minimum water loss All land animals need to conserve water. This is because the external environment is usually drier than the internal environment of the animal’s body. Thus, water will tend to diffuse away from the body. There is also a need to excrete waste products such as urea, which is dissolved in water. This creates a problem! In mammals, the kidneys are responsible for both excretion of urea and osmoregulation (the control of body fluid concentration). So, the kidney must balance the need to excrete with the need to conserve water, in other words it must produce a small volume of highly concentrated urine.

2. The Loop of Henle - maximum excretion with minimum water loss The operating unit of the kidney is called the nephron. There are about 1 million nephrons in each of the 2 kidneys. Each nephron comprises 5 segments: Glomerulus Proximal (first) convoluted tubule Loop of Henle Distal (second) convoluted tubule Collecting duct The Loop of Henle allows mammals to produce urine which is more concentrated than body fluids…... The operating unit of the kidney is called the nephron. There are about 1 million nephrons in each of the 2 kidneys. Each nephron comprises 5 segments: Glomerulus Proximal (first) convoluted tubule Loop of Henle Distal (second) convoluted tubule Collecting duct The Loop of Henle allows mammals to produce urine which is more concentrated than body fluids…...

3. The Loop of Henle - maximum excretion with minimum water loss Cortex Medulla The nephrons are packed into the kidney: the glomeruli and convoluted tubules in the cortex, the loops and collecting ducts extending into the medulla. All the collecting ducts eventually join up to form the ureter, emptying urine into the bladder and away! Ureter

4. The Loop of Henle - maximum excretion with minimum water loss Cortex Water leaves - ion concentration in filtrate increases Filtrate reaches maximum concentration Chloride ions out (sodium follows) - ion concentration in filtrate decreases Medulla

5. The Loop of Henle - maximum excretion with minimum water loss To ureter Collecting duct Several nephrons empty into one collecting duct. The collecting duct passes through the progressively more concentrated medulla, losing water by osmosis. This water is reabsorbed by the capillaries. This water is conserved, and a highly concentrated urine is produced. Water reabsorbed into vasa recta, urine becomes more concentrated Cortex Medulla

6. The Loop of Henle - maximum excretion with minimum water loss Beaver Human Desert Rat The length of the loop of Henle is related to the environment that the animal lives in. A longer loop will conserve more water, so animals in drier environments have longer loops.

7. The Loop of Henle - maximum excretion with minimum water loss Now test yourselftest yourself Be warned - this test requires you to understand everything that goes on in the nephron Not for the faint-hearted! (if the test doesn’t appear straight away, click on ‘Reload’)