Excretion and Homeostasis - Human and Social Biology

Kidneys Kidneys Lungs Skin Liver

Figure 1: Major excretory organs in the human body

Introduction to Excretion and Homeostasis

Excretion is the process of removing metabolic waste products from the body, while homeostasis is the maintenance of a stable internal environment despite external changes. These processes are essential for proper cellular function and overall health.

Key Concepts:

Excretory Organs and Their Functions

1. Kidneys

2. Liver

3. Lungs

4. Skin

Structure and Function of the Urinary System

Kidney Kidney Ureter Ureter Bladder Urethra

Figure 2: The human urinary system

Nephron Structure and Function

The nephron is the functional unit of the kidney, with approximately 1 million per kidney.

Afferent Arteriole Bowman's Capsule Proximal Tubule Loop of Henle Distal Tubule Collecting Duct Peritubular Capillaries

Figure 3: Structure of a nephron showing blood filtration process

Urine Formation Process:

  1. Ultrafiltration:
  2. Selective Reabsorption:
  3. Water Reabsorption:
  4. Tubular Secretion:

Homeostatic Mechanisms

1. Osmoregulation (Water Balance)

High Water Concentration Low Water Concentration Hypothalamus Less ADH More ADH Dilute Urine Produced Concentrated Urine Produced

Figure 4: Osmoregulation through ADH control

2. Thermoregulation

When Body is Too Hot When Body is Too Cold
Vasodilation of skin capillaries Vasoconstriction of skin capillaries
Sweating increases Sweating decreases
Hairs lie flat Hairs stand up (goosebumps)
Metabolic rate decreases Shivering increases heat

Common Disorders

Disorder Causes Symptoms Treatment/Prevention
Kidney Failure Diabetes, hypertension, infections Reduced urine, swelling, fatigue Dialysis, kidney transplant
Diabetes Insipidus ADH deficiency Excessive urination, thirst ADH replacement
Heat Stroke Overheating, dehydration High body temp, confusion Cooling, hydration
Gout Uric acid crystal buildup Joint pain, swelling Diet change, medication

Glossary of Terms

ADH (Antidiuretic Hormone)
Hormone that increases water reabsorption in kidneys.
Dialysis
Artificial filtering of blood when kidneys fail.
Excretion
Removal of metabolic waste from the body.
Glomerulus
Network of capillaries where blood filtration begins.
Homeostasis
Maintenance of stable internal conditions.
Nephron
Functional unit of the kidney that filters blood.
Osmoregulation
Control of water and solute concentrations.
Thermoregulation
Maintenance of stable body temperature.
Urea
Nitrogenous waste product from protein metabolism.
Ultrafiltration
High-pressure filtration in Bowman's capsule.

Self-Assessment Questions

1. List four excretory organs and their main waste products.
Kidneys: Urea, water, salts
Lungs: Carbon dioxide, water vapor
Liver: Bile pigments (in feces)
Skin: Water, salts, small amounts of urea (in sweat)
2. Describe the process of ultrafiltration in the nephron.
High blood pressure in the glomerulus forces small molecules (water, glucose, urea, salts) through the capillary walls into Bowman's capsule, forming glomerular filtrate. Large molecules like proteins and blood cells remain in the blood.
3. How does ADH regulate water content in the body?
When blood is too concentrated, the pituitary gland releases ADH, which increases the permeability of collecting ducts to water, allowing more water reabsorption into blood, producing concentrated urine. When blood is dilute, less ADH is released, resulting in dilute urine.
4. Compare excretion and egestion.
Excretion: Removal of metabolic waste products (urea, CO₂) from cells
Egestion: Elimination of undigested food material (feces) from digestive tract
5. Explain two ways the skin helps maintain homeostasis.
1) Temperature regulation: Sweat evaporates to cool body; vasodilation/constriction controls heat loss
2) Excretion: Sweat glands remove water, salts, and small amounts of urea
6. What would happen if the kidneys stopped functioning?
Toxic urea would accumulate in blood (uremia), electrolyte imbalance would occur, blood pH would become unstable, and excess water would lead to tissue swelling. Without treatment (dialysis or transplant), this would be fatal.
7. How does the liver contribute to excretion?
The liver converts toxic ammonia (from protein breakdown) into less toxic urea (ornithine cycle), breaks down hemoglobin into bile pigments (excreted in feces), and detoxifies substances like alcohol and drugs.
8. Describe the body's response to overheating.
Skin blood vessels dilate (vasodilation) to increase heat loss, sweat production increases (evaporative cooling), hairs lie flat to reduce insulation, and metabolic rate may decrease.
9. Why is selective reabsorption important in the nephron?
It reclaims useful substances (glucose, amino acids, needed salts and water) from the filtrate back into blood, preventing their loss in urine while still allowing waste removal.
10. How does dialysis treatment mimic kidney function?
Blood flows past a semi-permeable membrane where waste products diffuse out (like glomerular filtration), while essential substances remain. The dialysis fluid maintains proper concentration gradients, similar to kidney's regulatory function.

Summary of Key Points