HUMAN EXCRETORY SYSTEM

Human Excretory System

The 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. The human excretory system includes the kidneys and their functional unit, the nephron. The excretory activity of the kidneys is modulated by specialized hormones that regulate the amount of absorption within the nephron.

Kidneys

The human kidneys are the major organs of bodily excretion (see Figure 26-1). They are bean-shaped organs located on either side of the backbone at about the level of the stomach and liver. Blood enters the kidneys through renal arteries and leaves through renal veins. Tubes called ureters carry waste products from the kidneys to the urinary bladder for storage or for release.

The product of the kidneys is urine, a watery solution of waste products, salts, organic compounds, and two important nitrogen compounds: uric acid and urea. Uric acid results from nucleic acid decomposition, and urea results from amino acid breakdown in the liver. Both of these nitrogen products can be poisonous to the body and must be removed in the urine.

 

Nephron

The functional and structural unit of the kidney is the nephron. The nephron produces urine and is the primary unit of homeostasis in the body. It is essentially a long tubule with a series of associated blood vessels. The upper end of the tubule is an enlarged cuplike structure called the Bowman’s capsule. Below the Bowman’s capsule, the tubule coils to form the proximal tubule, and then it follows a hairpin turn called the loop of Henle. After the loop of Henle, the tubule coils once more as the distal tubule. It then enters a collecting duct, which also receives urine from other distal tubules.

Within the Bowman’s capsule is a coiled ball of capillaries known as a glomerulus. Blood from the renal artery enters the glomerulus. The force of the blood pressure induces plasma to pass through the walls of the glomerulus, pass through the walls of the Bowman’s capsule, and flow into the proximal tubule. Red blood cells and large proteins remain in the blood.

After plasma enters the proximal tubule, it passes through the coils, where usable materials and water are reclaimed. Salts, glucose, amino acids, and other useful compounds flow back through tubular cells into the blood by active transport. Osmosis and the activity of hormones assist the movement. The blood fluid then flows through the loop of Henle into the distal tubule. Once more, salts, water, and other useful materials flow back into the bloodstream. Homeostasis is achieved by this process: A selected amount of hydrogen, ammonium, sodium, chloride, and other ions maintain the delicate salt balance in the body.

The fluid moving from the distal tubules into the collecting duct contains urine. The urine flows through the ureters toward the urinary bladder. When the bladder is full, the urine flows through the urethra to the exterior.

Control of kidney function

The activity of the nephron in the kidney is controlled by a person’s choices and environment as well as hormones. For example, if a person consumes large amounts of protein, much urea will be in the blood from the digestion of the protein. Also, on a hot day, a body will retain water for sweating and cooling, so the amount of urine is reduced.

Some individuals suffer from a condition in which they secrete very low levels of ADH. The result is excessive urination and a disease called diabetes insipidus. Another unrelated form of diabetes, diabetes mellitus, is more widespread. People with this disease produce insufficient levels of insulin. Insulin normally transports glucose molecules into the cells. But when insulin is not available, the glucose remains in the bloodstream. The glucose is removed from the bloodstream in the nephron; to dilute the glucose, the nephron removes large amounts of water from the blood. Thus, the urine tends to be plentiful.

Hormones from the cortex of the adrenal glands also control the content of urine. These hormones promote reabsorption of sodium and chloride ions in the tubules. Thus, they affect the water balance in the body because water flows in the direction of high sodium and chloride content.

EXCRETORY SYSTEM OF ORGANISMS

Excretory Systems

The excretion process is concerned with the removal of waste products from the animal body. The relatively stable internal environment of the organism is dependent upon the process of excretion, a concept known as homeostasis. Homeostasis means “staying the same.” It refers to a steady-state equilibrium that exists internally in a healthy organism or cells, despite changes in the external environment. The excretory system plays a major role in homeostasis.

Because one-celled organisms are in constant contact with their environment, they do not need excretory organs. However, multicellular organisms need a mechanism to carry waste products from cells to the external environment. Flatworms, such as planaria, have a series of excretory cells called flame cells. Flame cells contain cilia that direct water and metabolic wastes to enter the cells and pass into excretory canals. The excretory canals join with other canals to form excretory tubules. Fluid from the excretory tubules leaves the body through pores. 

In earthworms, members of the phylum Annelida, the excretory system consists of structural units called nephridia (the singular is nephridium). Each nephridium contains a ciliated tunnel that leads to a long, coiled tubule, which leads to a bladderlike sac (a primitive bladder). Fluid moves from the internal environment into the funnel. As fluid passes through the tubule, cells in the tubular lining absorb useful compounds such as glucose, amino acids, and salts. The remaining materials constitute metabolic waste, and they are passed into the bladderlike sac. The sac later opens through a pore in the earthworm’s skin, from which the waste products are discharged.

Insects have a series of tubules for excretion called Malpighian tubules. Fluid enters at the upper end of the tubules and passes down their entire length. The cells in the tubular walls reabsorb precise amounts of water, salts, and other materials to maintain a delicate balance within the insect tissues. The tubules eventually lead to an insect’s intestine, where waste products are removed.