WASTES and EXCRETORY SYSTEMS Unicellular organisms get rid of wastes by the use of a contractile vacuole. It expels excess water that enters the cell through osmosis since the organism is hypertonic to the surrounding solution. More complex organisms need to store wastes and use special cells that are in the excretory system to remove wastes from the body. The excretory system also helps regulate body water.

EXCRETING WASTES The body must eliminate wastes as some products of cellular processes can be toxic if they accumulate. Lungs: CO 2 Colon: Toxic wastes from the digestive system We eat and absorb much more protein than required in order to maintain tissues and promote cell growth. Liver: transforms ingested toxins such as alcohol, heavy metals and the waste from protein metabolism into soluble compounds that can be removed by the kidney. Kidneys are about the size of a fist and have a mass of 0.5 kg. They are important in removing waste, balancing blood pH and maintaining water balance.

WASTES EXCRETED BY THE KIDNEYS Excess protein is converted into carbohydrates, leaving the amino group (NH 2 ). NH 2 must be removed by the liver in a process called deamination. Deamination creates ammonia (NH 3 ) a water soluble gas. – It is quite toxic (0.005 mg can kill a human). Fish are able to pass ammonia out through their gills, but land animals cannot do this, they must store ammonia. The liver takes two ammonia molecules and joins them with carbon dioxide to form urea. Urea is times less toxic than ammonia. The blood dissolves 33 mg of urea per 100 ml.

The UREA Cycle

Uric acid is another waste product. It is produced when nucleic acids are broken down. This occurs in the perioxisomes. Uric acid is only slightly soluble in water and easily precipitates out of solution forming crystals. These contribute to the formation of kidney stones; produce the excruciating pain of gout when deposited in the joints. Most mammals have an enzyme — uricase — for breaking uric acid down into a soluble product. However, during the evolution of great apes and humans, the gene encoding uricase became inactive. A predisposition to gout is our legacy.

Uric acid does offer our bodies some advantages – it is a potent antioxidant and thus can protect cells from damage by reactive oxygen species (created in cellular respiration) and when released from damaged cells can stimulate a stronger immune response. Uric acid is the chief nitrogenous waste of  Insects  lizards and snakes  birds (It is the whitish material that birds leave on statues.) Ammonia, urea and uric acid are all removed from the body by kidneys.

(EXTRA! – listen, don’t write) There are several inherited diseases of the urea cycle caused by mutations in genes encoding one or another of the necessary enzymes. The most common of these is an inherited deficiency of ornithine transcarbamylase, an enzyme needed for the conversion of ornithine to citrulline. It results in elevated levels of ammonia that may be so high as to be life-threatening. It is an X-linked disorder; therefore most commonly seen in males. It can be cured by a liver transplant. It can also be caused by a liver transplant! In 1998, an Austrian woman was given a new liver from a male cadaver who — unknown to the surgeons — had a mutation in his single ornithine transcarbamylase gene. The woman's blood level of ammonia shot up, and she died a few days later.

THE URINARY SYSTEM

Renal arteries pass blood to the kidneys. The kidneys can have as much as 25% of the body's blood at any given time. Wastes are filtered from blood into the kidneys and sent to the bladder by the ureters. The bladder has a urinary sphincter which acts as a valve and causes urine to stay put. When the bladder has 200 mL of urine accumulated it expands and stretch receptors send a signal to the brain that it is time to urinate. At 400 mL the messages become more frequent. At 600 mL voluntary control is lost and the urinary sphincter opens, urine passes out of the body through the urethra and the bladder is voided.

KIDNEY STRUCTURE Kidneys have: – an outer layer of connective tissue called the cortex, which encircles the kidney, – an inner layer under the cortex called the medulla, – a hollow chamber called the renal pelvis which connects the kidney with the ureter. The kidney has around one million small functional units inside of it called nephrons, they span the area of the cortex and the medulla. d/cybercas/images/kim2b.jpg

NEPHRON The afferent arterioles supply blood to the kidney from the renal artery. They end in a capillary network called the glomerulus, which is the site of filtration. The glomerulus is not like a normal capillary bed, it does not transfer blood to a venule; instead blood exits the glomerulus into the efferent arterioles. The efferent arterioles lead to a network of peritubular capillaries which surround the tubules of the nephron. The glomerulus is surrounded by the Bowman's capsule. It, along with the afferent and efferent arterioles and the glomerulus are in the cortex of the kidney.

FLUID FLOW Fluids that are to be processed as urine enter the Bowman's capsule from the blood, so they are drawn out of the afferent arterioles into the nephron at the Bowman's capsule. The Bowman's capsule leads to a thin tubule called the proximal tubule. Urine goes from the proximal tubule around the loop of Henle, which descends into the medulla of the kidney. Urine then moves through the distal tubule and then into the collecting ducts, which collect urine from many nephrons and lead into the renal pelvis of the kidney. doc/Pilsinl/188.gif Homework p 345 #1-6 and p 348 #1,3,5