Excretion

Excretion of Water
Excretion is the removal of waste products of metabolism from the body. Many unicellular and simple multicellular animals have no special organs for excretion. Nitrogenous waste products are simply excreted across the surface membrane into the surrounding water. Some protozoa, however, have a contractile vacuole, which excretes excess water from the organism.
Excretion in Flatworms
Flat worms such as planarians, flukes and tapeworms have the beginnings of an excretory system, consisting of two or more longitudinal branching tubules running the length of the body. This type of excretory system is called a flame cell system.
Excretion in Earthworms
Each segment in an earthworm’s body has its own pair of excretory organs called the nephridia that open independently to the outside. Each nephridium consists of an open ciliated funnel that functionally corresponds to the bulb in the flame-cell system, a coiled tubule running from the nephrostome, an enlarged bladder into which the tubule empties and a nephridiopore through which wastes are expelled to the outside.
A network of capillaries is closely associated with each tubule. Materials move from the body fluids into the nephridium through the open nephrostome but some materials are also picked up by the coiled tubule directly from the blood in the capillaries.

Excretion in Insects
The excretory organs of insects are called malpighian tubules. These originate as outpocketings (diverticular) of the insect digestive system, located at the junction of the midgut and hindgut. These blind sacs are directly bathed in the blood in the animal’s body, where they absorb fluid from the blood in the distal end of the malpighian tubules.

As the absorbed fluid flows through the proximal end of the tubules, nitrogenous wastes precipitate as uric acid and much of the water and various salts are reabsorbed. The concentrated urine, mainly uric acid, passes into the hindgut and then into the rectum. More fluid is reabsorbed here, leaving a mixture of urine and faeces to be expelled from the body.
Excretion in the Vertebrate
Marine mammals such as whales possess kidneys that can concentrate urine and excrete hypertonic urine. A bird’s kidney can conserve water but it is unable to concentrate urine to the level of mammals because they have short loops of Henle.
Reptiles, amphibians and fish have no discernible loops of Henle and therefore cannot concentrate urine. Amphibians and freshwater bony fish produce large quantities of dilute urine. Marine bony fish produce very little hypotonic urine.
In general, vertebrates living in fresh water produce copious amounts of urine. Marine dwellers, on the other hand, produce little quantities of hypotonic urine and eliminate excess salt from their bodies by use of accessory glands like the gills, rectal glands, head and nasal glands.
The human kidney
The human urinary system consists of a pair of pea-shaped kidneys, two urinary ureters and a bladder. Each kidney is made up of a renal capsule, cortex and medulla. The basic functional unit of the kidney is the nephron. The nephron is a tube that starts from the Bowman’s capsule in the cortex, then forms a loop that extends into the medulla and back into the cortex and finally back through the medulla into the pelvis. The glomerulus and the Bowman’s capsule make up the malphigian body.

The human kidney contains about 1 million nephrons or about 80 km of tubules. The nephrons control the composition of blood by three processes: filtration, re-absorption and concentration (see below).
The kidneys remove urea and other toxic wastes from the blood, forming a dilute solution called urine in the process. The two kidneys have a very extensive blood supply and the whole blood supply passes through the kidneys every 5 minutes, ensuring that waste materials do not build up. The renal artery carries blood to the kidneys while the renal vein carries blood, now with far lower concentrations of urea and minerals, away from the kidneys. The urine formed passes down the ureter to the bladder.

The important part of the kidney is a folded tube called a nephron. There are thousands of nephrons in each kidney. Urine formation by the nephron can be divided into five stages.
1. Utrafiltration
The renal artery bringing blood to the kidney splits into numerous arterioles each feeding a nephron. The arteriole splits into numerous capillaries that form a knot called a glomerulus. The glomerulus is enclosed by the renal capsule or Bowman’s capsule, the first part of the nephron. The arteriole leading into the glomerulus, called the afferent arteriole, is wider than the one leading out, called the efferent arteriole, so there is high blood pressure in the capillaries of the glomerulus. This pressure forces plasma out of the blood by utrafiltration. Both the capillary walls and the capsule walls are formed from a single layer of flattened cells with gaps between them, so that all molecules with a molecular mass of less than 70k are squeezed out of the blood to form a filtrate in the renal capsule. Only blood cells and large proteins remain in the blood.
2. Reabsorption
The proximal convoluted tubule is the longest and widest part of the nephron. It is lined with epithelial cells containing microvilli and numerous mitochondria. In this part of the nephron, over 80% of the filtrate is reabsorbed into the tissue fluid and then to the blood. This ensures that all the useful materials that were filtered out of the blood are now returned to the blood. All glucose, all amino acids and 85% of mineral ions are reabsorbed by active transport from the filtrate to the tissue fluid. They then diffuse into the blood capillaries. Small proteins are reabsorbed by pinocytosis, digested, and the amino acids diffuse into the blood. 80% of the water is reabsorbed to the blood by osmosis and some urea is reabsorbed to the blood by diffusion. Urea is a small, uncharged molecule that can pass through membranes by lipid diffusion passively down its concentration gradient.

3. Loop of Henle
The function of the loop of Henle is to make tissue fluid in the medulla hypertonic compared to the filtrate in the nephron. The loop of Henle does this by pumping sodium and chloride ions out of the filtrate into the tissue fluid. The first part of the loop (the descending limb) is impermeable to ions, but some water leaves by osmosis. This makes the filtrate more concentrated as it descends. The second part of the loop (the ascending limb) contains a Na+ and a Cl- pump, which actively transport these ions out of the filtrate into the surrounding tissue fluid. The ascending limb is impermeable to water. So the tissue fluid becomes more salty (hypertonic) and the filtrate becomes less salty (hypotonic). Since the filtrate is most concentrated at the base of the loop, the tissue fluid is also more concentrated at the base of the medulla, where it is three times more concentrated than seawater.
4. Homeostasis and Secretion
The distal convoluted tubule is the homeostatic part of the kidney. Here substances are actively transported into the filtrate from the blood. This region of the nephron has a brush border of microvilli with pumps that pump substances by active transport. The substances secreted include H+, K+, ethanol, toxins, drugs and other substances.
5. Concentration
The concentration of urine takes place in the collecting ducts. The amount of urine and water conservation are under the control of the hormone ADH produced by the hypothalamus. As the collecting duct passes through hypertonic salt region in the medulla, water leaves the filtrate by osmosis, concentrating the urine. An increase in ADH increases the permeability of the cells of the collecting tubule; more water is reabsorbed from the filtrate, making the urine more concentrated. A reduction in the concentration of ADH causes the cells of the collecting tubule to be less permeable so that less water is absorbed and the urine becomes more dilute. Alcohol and caffeine can inhibit the release of ADH, allowing people to pass a lot of dilute urine.
The bladder
The collecting ducts all join in the pelvis of the kidney to form the ureter, which leads to the bladder. The filtrate, now called urine, is produced continually by each kidney and drips into the bladder for storage. The bladder is an expandable bag, and when it is full, stretch receptors in the elastic walls send impulses to the medulla, which causes the sphincter muscles to relax, causing urination (or micturition). This is an involuntary reflex action that we can learn to control to some extent when we are young.
In addition to its role in osmoregulation, the kidney helps maintain the pH of the blood constant
Artificial Kidney
Kidneys may be severely damaged by disease or an accident and if the damage involves both kidneys, it may result in a renal failure. A person suffering from renal failure is unable to remove toxic waste products from the body, which then accumulate in the blood, and the individual will die if not treated with an artificial kidney or given a kidney transplant.

In an artificial kidney, blood circulates from an artery in the patient’s arm to the kidney machine and returns to a vein. In the process, the patient’s blood is passed through a semi-permeable membrane in contact with a balanced salt solution, or dialysis fluid. As the blood flows through the dialysis machine, waste products, including urea, diffuse out of the blood and into the dialysis fluid. At the same time, salts diffuse from the dialysis machine into the blood. In this way, the blood is purified of poisonous waste substances.
In a six hour dialysis, between 50 to 200 g of urea can be removed from a patient’s blood. This far exceeds the urea clearance of normal kidneys and allows the patient to undergo treatment only about twice a week.