The Human Excretory

System

Human Excretory System functions to remove waste from the human body; this system consists of specialized structures and

capillary networks that assist in the excretory process; this system includes the kidney and its functional unit, the nephron.

The

KIDNEY

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.

Kidney

Parts

and Its

Function

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

The Nephron(Parts and Structure)

3 STAGES

OF

URINE FORMATION

1. FILTRATION

Urine formation begins with the process of filtration, which goes on continually in the renal corpuscles. As blood courses through the glomeruli, much of it is fluid, containing both useful chemicals and dissolved waste materials, soaks out of the blood through the membranes (by osmosis and diffusion) where it is filtered and then flows into the Bowman's capsule. This process is called glomerular filtration.

The water, waste products, salt, glucose, and other chemicals that have been filtered out of the blood are known collectively as glomerular filtrate. The glomerular filtrate consists primarily of water, excess salts (primarily Na+ and K+), glucose, and a waste product of the body called urea. Urea is formed in the body to eliminate the very toxic ammonia products that are formed in the liver from amino acids.

Since humans cannot excrete ammonia, it is converted to the less dangerous urea and then filtered out of the blood. Urea is the most abundant of the waste products that must be excreted by the kidneys.

The total rate of glomerular filtration (glomerular filtration rate or GFR) for the whole body (i.e., for all of the nephrons in both kidneys) is normally about 125 ml per minute. That is, about 125 ml of water and dissolved substances are filtered out of the blood per minute.

2. REABSORPTION

It is the movement of substances out of the renal tubules back into the blood capillaries located around the tubules (called the peritubular copillaries). Substances reabsorbed are water, glucose and other nutrients, and sodium (Na+) and other ions. Reabsorption begins in the proximal convoluted tubules and continues in the loop of Henle, distal convoluted tubules, and collecting tubules.

Large amounts of water - more than 178 liters per day - are reabsorbed back into the bloodstream from the proximal tubules because the physical forces acting on the water in these tubules actually push most of the water back into the blood capillaries. In other words, about 99% of the 180 liters of water that leave the blood each day by glomerular filtration returns to the blood from the proximal tubule through the process of passive reabsorption.

The nutrient glucose (blood sugar) is entirely reabsorbed back into the blood from the proximal tubules. In fact, it is actively transported out of the tubules and into the peritubular capillary blood. None of this valuable nutrient is wasted by being lost in the urine. However, even when the kidneys are operating at peak efficiency, the nephrons can reabsorb only so much sugar and water. Their limitations are dramatically illustrated in cases of diabetes mellitus, a disease which causes the amount of sugar in the blood to rise far above normal.

3. SECRETION

Secretion is the process by which substances move into the distal and collecting tubules from blood in the capillaries around these tubules. In this respect, secretion is reabsorption in reverse. Whereas reabsorption moves substances out of the tubules and into the blood, secretion moves substances out of the blood and into the tubules where they mix with the water and other wastes and are converted into urine.

These substances are secreted through either an active transport mechanism or as a result of diffusion across the membrane. Substances secreted are hydrogen ions (H+), potassium ions (K+), ammonia (NH3), and certain drugs. Kidney tubule secretion plays a crucial role in maintaining the body's acid-base balance, another example of an important body function that the kidney participates in.

The URINARY System

Functions of the

Parts of the

Urinary System

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.

KIDNEYS

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.

URETERS

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.

URETHRAThe 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.