Human Excretory System functions to remove wastefrom the human body; this system consists ofspecialized structures and capillary networks thatassist in the excretory process; this system includes the
The kidneys are two small organs located near the vertebral column at the small of the back. The left kidney lies a little higher than the right kidney. They are bean-shaped, about 4 in. (10 cm) long and about 21/2 in. (6.4 cm) wide.
Renal Vein This has a large diameter and a thin wall. It carries blood away from the kidney and back to the right hand side of the heart. Renal Artery This blood vessel supplies blood to the kidney from the left hand side of the heart. This blood must contain glucose and oxygen because the kidney has to work hard producing urine. Blood in the renal artery must have sufficient pressure or the kidney will not be able to filter the blood.
Medulla The medulla is the inside part of the kidney. This is where the amount of salt and water in your urine is controlled. It consists of billions of loops of Henlé. These work very hard pumping sodium ions. Ureter The ureter carries the urine down to the bladder.
Cortex The cortex is the outer part of the kidney. This is where blood is filtered. We call this process “ultra-filtration” or “high pressure filtration” because it only works if the blood entering the kidney in the renal artery is at high pressure. Billions of glomeruli are found in the cortex. A glomerulus is a tiny ball of capillaries.
Nephron The basic structural and functional unit of the kidney. Its chief function is to regulate the concentration of water and soluble substances like sodium salts by filtering the blood, reabsorbing what is needed and excreting the rest as urine. A nephron eliminates wastes from the body, regulates blood volume and blood pressure, controls levels of electrolytes and metabolites, and regulates blood pH. a normal kidney contains 800,000 to 1.5 million nephrons
T h e Ne p h r o n(P a r t s a n d S t r u c t u r e )
1. FILTRATION Urine formation begins with the processof filtration, which goes on continually in therenal corpuscles. As blood courses through theglomeruli, much of it is fluid, containing bothuseful chemicals and dissolved wastematerials, soaks out of the blood through themembranes (by osmosis and diffusion) where itis filtered and then flows into the Bowmanscapsule. This process is called glomerularfiltration.
The water, waste products, salt, glucose,and other chemicals that have been filteredout of the blood are known collectively asglomerular filtrate. The glomerular filtrateconsists primarily of water, excess salts(primarily Na+ and K+), glucose, and a wasteproduct of the body called urea. Urea isformed in the body to eliminate the very toxicammonia products that are formed in the liverfrom amino acids.
Since humans cannot excrete ammonia,it is converted to the less dangerous urea andthen filtered out of the blood. Urea is themost abundant of the waste products thatmust be excreted by the kidneys. The total rate of glomerular filtration(glomerular filtration rate or GFR) for thewhole body (i.e., for all of the nephrons inboth kidneys) is normally about 125 ml perminute. That is, about 125 ml of water anddissolved substances are filtered out of theblood per minute.
2. REABSORPTION It is the movement of substances out ofthe renal tubules back into the bloodcapillaries located around the tubules (calledthe peritubular copillaries). Substancesreabsorbed are water, glucose and othernutrients, and sodium (Na+) and other ions.Reabsorption begins in the proximalconvoluted tubules and continues in the loopof Henle, distal convoluted tubules, andcollecting tubules.
Large amounts of water - more than178 liters per day - are reabsorbed back intothe bloodstream from the proximal tubulesbecause the physical forces acting on thewater in these tubules actually push most ofthe water back into the blood capillaries. Inother words, about 99% of the 180 liters ofwater that leave the blood each day byglomerular filtration returns to the bloodfrom the proximal tubule through theprocess of passive reabsorption.
The nutrient glucose (blood sugar) isentirely reabsorbed back into the blood fromthe proximal tubules. In fact, it is activelytransported out of the tubules and into theperitubular capillary blood. None of thisvaluable nutrient is wasted by being lost in theurine. However, even when the kidneys areoperating at peak efficiency, the nephrons canreabsorb only so much sugar and water. Theirlimitations are dramatically illustrated in cases ofdiabetes mellitus, a disease which causes theamount of sugar in the blood to rise far abovenormal.
3. SECRETION Secretion is the process by whichsubstances move into the distal and collectingtubules from blood in the capillaries aroundthese tubules. In this respect, secretion isreabsorption in reverse. Whereasreabsorption moves substances out of thetubules and into the blood, secretion movessubstances out of the blood and into thetubules where they mix with the water andother wastes and are converted into urine.
These substances are secreted througheither an active transport mechanism oras a result of diffusion across themembrane. Substances secreted arehydrogen ions (H+), potassium ions (K+),ammonia (NH3), and certain drugs.Kidney tubule secretion plays a crucialrole in maintaining the bodys acid-basebalance, another example of animportant body function that the kidneyparticipates in.
KIDNEYS The kidneys are two brownish, bean shaped organs about the size of a fist, they weigh about 5 ounces. They are located in the upper right and left back part of the abdominal cavity. Each kidney contains about 1,200,000 microscopic filters called nephrons. The main function or the kidneys are to maintain the water balance and to eliminate waste materials from the blood.
URETERS The left and the right ureters are long muscular tubes. They are about 12 inches long with a diameter 2 to 3 millimeters. The ureters connect pelvis of each kidney to urinary bladder. They carry urine from each kidney to the urinary bladder.
URINARY BLADDER The urinary bladder is a muscular sac that holds urine. It is located in front the pelvis and behind the pubis. As the bladder fills walls stretch signaling the desire to urinate.
URETHRA The urethra is a muscular tube which carries urine from the bladder to the outside part of the body. In the female, it is a one inch long from the bladder to the cleft of the labia. In the male, it is several inches long from the prostate gland to the penis. When one is about to urinate, a value in the urethra relaxes to allow the urine to flow out.