Life Processes · Lesson 12 of 13
Excretion
“Kidneys rescue useful materials while sending wastes on their way.”
• Define excretion and distinguish it from egestion. • Describe the human excretory system. • Explain filtration and selective reabsorption in nephrons. • Explain how urine volume is regulated. • Compare natural kidney function with haemodialysis. • Describe excretory strategies in plants.
Cellular reactions produce useful energy and new substances, but they also produce wastes. Carbon dioxide and nitrogenous compounds can become harmful if allowed to accumulate. Excretion removes these metabolic wastes and helps maintain a stable internal environment.
Excretion is the biological removal of harmful metabolic waste products from an organism.
Unicellular organisms can release many wastes directly across their surface by diffusion. Complex multicellular organisms require specialised organs because most cells are far from the external environment and wastes must first be transported away from them.
Excretion In Human Beings
The human urinary system consists of two kidneys, two ureters, a urinary bladder and a urethra. Kidneys filter the blood and form urine. Ureters carry urine to the muscular bladder, which stores it. When bladder pressure produces the urge and control permits, urine leaves through the urethra.
| Part | Main function |
|---|---|
| Kidneys | Filter blood, selectively reabsorb useful substances and form urine |
| Ureters | Carry urine from kidneys to bladder |
| Urinary bladder | Stores urine temporarily |
| Urethra | Carries urine out of the body |
Structure And Functioning Of Nephrons
A nephron is the microscopic structural and functional unit of a kidney that filters blood and modifies the filtrate to form urine.
At the beginning of a nephron, a cluster of thin-walled capillaries lies within the cup-shaped Bowman’s capsule. Blood pressure filters water and many small dissolved substances into the capsule, while blood cells and most large proteins remain in blood. The filtrate then passes along a coiled tubule surrounded by capillaries.
Useful substances such as glucose, amino acids, required salts and most water are selectively reabsorbed from the tubule into blood. Additional secretion and adjustment of water and ions help form urine. The final urine enters collecting passages and then the ureter.
| Stage | What happens | Importance |
|---|---|---|
| Filtration | Water and small solutes enter Bowman’s capsule from capillaries | Separates many wastes from blood |
| Selective reabsorption | Useful solutes and much water return to blood | Prevents loss of valuable substances |
| Final adjustment | Water and dissolved wastes are balanced | Regulates urine concentration and internal conditions |
| Urine flow | Urine enters ureter and bladder | Allows storage and controlled removal |
Regulation Of Urine
The amount of water reabsorbed depends on the body’s water balance and the amount of dissolved waste that must be removed. When water is scarce, more is reabsorbed and urine becomes more concentrated. When excess water is present, less is reabsorbed and a larger volume of dilute urine is produced. This adjustment prevents both unnecessary water loss and waste accumulation.
Artificial Kidney — Haemodialysis
Haemodialysis is a treatment in which a patient’s blood passes alongside dialysis fluid across a semipermeable membrane so nitrogenous wastes diffuse out before purified blood returns to the body.
Dialysis fluid has an osmotic composition similar to blood but lacks nitrogenous waste. Waste therefore diffuses from blood into the fluid. Healthy kidneys, however, do more than filter: nephron tubules selectively reabsorb useful substances and regulate water and salts. Dialysis lacks this complete biological selectivity and must be performed repeatedly when kidneys fail.
| Feature | Healthy kidney | Haemodialysis |
|---|---|---|
| Filtration surface | Nephrons | Semipermeable tubing |
| Waste removal | Filtration plus tubular processing | Diffusion into dialysis fluid |
| Selective reabsorption | Extensive | Not performed like nephron reabsorption |
| Operation | Continuous in the body | Repeated treatment sessions |
A donated kidney can restore kidney function for a suitable recipient. Donation requires medical, ethical and legal safeguards and informed consent. Some organs and tissues can be donated by living people, while many donations occur after death.
Excretion In Plants
Plants remove oxygen from photosynthesis and carbon dioxide from respiration mainly by diffusion. Excess water leaves through transpiration. Other wastes may be stored in vacuoles, accumulated in leaves that later fall, deposited as resins and gums in old xylem, or released into surrounding soil. The presence of many dead tissues makes storage and shedding practical strategies.
Problem
Glucose enters the initial filtrate of a healthy person. Why is it normally absent from final urine?
- 1.Glucose is small enough to enter the initial filtrate.
- 2.It is valuable to the body and should not be lost.
- 3.Nephron tubules selectively reabsorb it into surrounding blood.
- 4.Therefore, little or no glucose remains in normal final urine.
Problem
Predict the urine produced when a person is dehydrated.
- 1.The body must conserve water.
- 2.Nephrons reabsorb a greater fraction of filtered water.
- 3.Less water remains to carry wastes out.
- 4.A smaller volume of more concentrated urine is produced.
Problem
Compare an alveolus and the filtration region of a nephron.
- 1.Both have thin surfaces closely associated with capillaries.
- 2.The thin barrier and large collective area support rapid exchange.
- 3.An alveolus exchanges gases by diffusion between air and blood.
- 4.A nephron begins by filtering water and small solutes from blood, then selectively reabsorbs substances along its tubule.
- 5.Thus, both specialise in exchange, but their materials and later processing differ.
Urine is not simply “filtered blood.” The initial filtrate is extensively modified by selective reabsorption and other tubular processes before it becomes urine.
Quiz
What is the functional unit of a kidney?
Where is the initial nephron filtrate collected?
Why is glucose normally not lost in urine?
What happens during dehydration?
How may a plant remove wastes?
Practice Problems
- Distinguish excretion from egestion with one example of each.
- Trace urea from body cells until it leaves through the urethra.
- Explain filtration and selective reabsorption in a nephron.
- Compare the expected urine of a well-hydrated person and a dehydrated person.
- Compare alveoli and nephrons in structure, exchanged materials and function.
Key Takeaways
• Excretion removes metabolic wastes. • Kidneys form urine, ureters carry it, the bladder stores it and the urethra releases it. • Nephrons filter blood and then selectively reabsorb useful substances. • Water reabsorption changes according to body water balance and waste load. • Haemodialysis removes wastes but does not reproduce all nephron functions. • Plants use diffusion, transpiration, storage, shedding and secretion into soil. • Alveoli and nephrons both use thin capillary-associated exchange surfaces but perform different tasks.