Excretion in Humans: Kidneys and Skin

What you'll learn

This revision guide covers excretion in humans, focusing on the kidneys and skin as the two main excretory organs. You will learn how these organs remove metabolic waste products from the body, maintain water balance, and regulate body temperature. The content aligns directly with the CXC CSEC Integrated Science syllabus requirements for human excretion.

Key terms and definitions

Excretion — the removal of toxic waste products of metabolism from the body, including carbon dioxide, urea, and excess water and salts.

Urea — a nitrogen-containing waste product formed in the liver from the breakdown of excess amino acids; transported in the blood to the kidneys for removal.

Nephron — the functional unit of the kidney; each kidney contains approximately one million nephrons that filter blood and produce urine.

Ultrafiltration — the process in the Bowman's capsule where blood pressure forces small molecules (water, glucose, salts, urea) from the blood capillaries into the kidney tubule.

Selective reabsorption — the process in the kidney tubule where useful substances (all glucose, most water, some salts) are reabsorbed back into the blood.

Osmoregulation — the control of water content and salt concentration in the body to maintain constant internal conditions.

Homeostasis — the maintenance of constant internal conditions in the body, including temperature, water content, and blood glucose levels.

Deamination — the removal of the amino group from excess amino acids in the liver, producing ammonia which is then converted to urea.

Core concepts

The need for excretion

The body constantly produces waste products from metabolic reactions that must be removed to prevent poisoning:

• Carbon dioxide is produced during aerobic respiration in all cells. It is removed via the lungs during breathing (though this is not technically excretion, it removes a metabolic waste).
• Urea is produced in the liver from the breakdown of excess amino acids that cannot be stored. Amino acids come from dietary protein or worn-out body proteins.
• Excess water and salts must be removed to maintain the correct concentration of body fluids.

The accumulation of these wastes would alter blood pH, damage cells, and disrupt enzyme activity. In the Caribbean context, workers in outdoor industries such as sugar cane harvesting or construction in Trinidad, Jamaica, or Barbados must maintain proper excretion and hydration despite high temperatures and physical exertion.

Structure and function of the kidney

The kidneys are two bean-shaped organs located at the back of the abdomen, one on each side of the spine. Each kidney is supplied with blood through the renal artery and drained by the renal vein.

Main functions of the kidneys:

• Remove urea from the blood
• Control water content of the blood (osmoregulation)
• Control salt concentration in the blood
• Maintain blood pH

Structure of a nephron:

Each nephron consists of:

1. Bowman's capsule — a cup-shaped structure that surrounds a knot of blood capillaries called the glomerulus
2. Glomerulus — a tight ball of capillaries where blood pressure is high, forcing filtration
3. Kidney tubule — a long, twisted tube surrounded by blood capillaries where reabsorption occurs
4. Collecting duct — where urine from many nephrons collects before passing to the ureter

Blood supply to the nephron:

• The renal artery branches into many arterioles, each supplying a glomerulus
• The arteriole leaving the glomerulus is narrower than the one entering, creating high blood pressure in the glomerulus
• Capillaries surround the tubule to reabsorb useful substances

Urine formation: ultrafiltration and selective reabsorption

Stage 1: Ultrafiltration in the Bowman's capsule

High blood pressure in the glomerulus forces small molecules through the capillary walls into the Bowman's capsule. This filtrate contains:

• Water
• Glucose
• Urea
• Salts (ions such as sodium, chloride, potassium)

Large molecules cannot pass through and remain in the blood:

• Red blood cells
• White blood cells
• Platelets
• Plasma proteins

Stage 2: Selective reabsorption in the kidney tubule

As the filtrate flows along the tubule, useful substances are reabsorbed back into the blood:

• All glucose is reabsorbed by active transport (requires energy from respiration). Healthy individuals have no glucose in their urine. The presence of glucose in urine indicates diabetes mellitus, which is increasingly common in Caribbean populations due to dietary changes and lifestyle factors.

• Most water is reabsorbed by osmosis. The amount reabsorbed varies depending on the body's hydration state.

• Some salts are reabsorbed as needed to maintain the correct blood concentration.

• Urea is not reabsorbed (though some may diffuse back passively). Most remains in the tubule.

Stage 3: Formation of urine

The liquid remaining in the tubule after selective reabsorption is urine, which contains:

• Water (approximately 95%)
• Urea (approximately 2%)
• Salts (approximately 2%)
• Other waste substances (approximately 1%)

Urine passes from the collecting duct into the ureter, then to the bladder for storage, and is eventually expelled through the urethra during urination.

Control of water content (osmoregulation)

The kidneys adjust the amount of water in urine according to the body's needs:

When you drink a lot of water:

• Blood becomes dilute (too much water)
• Brain detects this change
• Pituitary gland releases less antidiuretic hormone (ADH)
• Kidney tubules reabsorb less water
• Large volume of dilute urine is produced
• Blood concentration returns to normal

When you are dehydrated (e.g., after exercise in hot Caribbean weather):

• Blood becomes concentrated (too little water)
• Brain detects this change
• Pituitary gland releases more ADH
• Kidney tubules reabsorb more water
• Small volume of concentrated (darker) urine is produced
• Blood concentration returns to normal

This is an example of homeostasis through negative feedback. A cane cutter in Guyana working in the hot sun loses significant water through sweating and produces concentrated urine to conserve water.

The skin as an excretory organ

While the skin's primary function is protection, it also plays a role in excretion and temperature regulation through sweat production.

Structure of the skin relevant to excretion:

The skin has two main layers:

1. Epidermis — the outer layer made of dead cells that are constantly replaced
2. Dermis — the inner layer containing:
   • Sweat glands
   • Hair follicles
   • Blood capillaries
   • Nerve endings
   • Fat cells (in deeper layer)

Sweat glands are coiled tubes in the dermis that produce sweat. The sweat duct carries sweat to the surface where it is released through pores.

Sweat production and temperature regulation

Composition of sweat:

• Water (approximately 99%)
• Salts (mainly sodium chloride, approximately 0.9%)
• Small amounts of urea (approximately 0.1%)
• Traces of other waste substances

Sweat is produced continuously, but the rate increases during exercise or in hot conditions.

Temperature regulation through sweating:

When body temperature rises (during exercise or in hot weather):

• Temperature receptors in the skin and brain detect the increase
• The hypothalamus in the brain sends nerve signals to sweat glands
• Sweat glands produce more sweat
• Sweat evaporates from the skin surface
• Evaporation requires heat energy, which is taken from the body
• Body temperature decreases

In Caribbean countries where temperatures regularly exceed 30°C, such as Barbados, Trinidad, or Jamaica, efficient sweating is essential for maintaining body temperature during outdoor activities including cricket matches, Carnival celebrations, or agricultural work.

Additional temperature regulation mechanisms:

• Vasodilation — blood vessels near the skin surface widen (dilate), allowing more blood to flow near the surface. More heat is lost by radiation. The skin appears flushed or red.

• Vasoconstriction — blood vessels near the skin surface narrow (constrict), reducing blood flow near the surface. Less heat is lost. The skin appears pale.

Comparison of excretion by kidneys and skin

Both organs work together to maintain homeostasis. On a hot day in Port of Spain or Kingston, a person may produce less urine (kidneys conserve water) while producing more sweat (skin loses water for cooling).

Worked examples

Example 1: Explaining ultrafiltration (4 marks)

Question: Describe the process of ultrafiltration in the kidney. [4 marks]

Model answer:

• High blood pressure in the glomerulus (1 mark)
• Forces small molecules through capillary walls (1 mark)
• Into the Bowman's capsule (1 mark)
• Small molecules include water, glucose, urea, and salts, while large molecules like blood cells and proteins remain in the blood (1 mark)

Examiner note: Use precise terminology. "High pressure" alone is insufficient; specify "high blood pressure in the glomerulus." Name the Bowman's capsule specifically rather than just saying "kidney."

Example 2: Understanding selective reabsorption (5 marks)

Question: After ultrafiltration, glucose is present in the filtrate but not in urine. Explain why. [5 marks]

Model answer:

• Glucose is a useful substance needed by cells for respiration (1 mark)
• All glucose is reabsorbed from the kidney tubule (1 mark)
• By active transport (1 mark)
• Back into the blood capillaries surrounding the tubule (1 mark)
• Therefore no glucose remains in the urine of healthy individuals (1 mark)

Examiner note: The question word "explain" requires reasons, not just description. Link glucose reabsorption to its usefulness and specify the mechanism (active transport).

Example 3: Applying knowledge to Caribbean context (6 marks)

Question: A cane cutter in Guyana works for 8 hours in hot, humid conditions. Describe and explain changes in his urine production during the working day. [6 marks]

Model answer:

• The worker sweats heavily to cool down (1 mark)
• Loses large amounts of water through the skin (1 mark)
• Blood becomes more concentrated/less water in blood (1 mark)
• Brain detects this and pituitary gland releases more ADH (1 mark)
• Kidney tubules reabsorb more water back into blood (1 mark)
• Produces small volume of concentrated (dark yellow) urine (1 mark)

Examiner note: This question tests application of knowledge to a real situation. Show the sequence: water loss → blood concentration change → hormonal response → kidney response → urine change.

Common mistakes and how to avoid them

• Confusing excretion with egestion. Excretion removes metabolic waste products made by cells (urea, carbon dioxide). Egestion removes undigested food (faeces) that was never absorbed into body cells. Faeces are not excretory products.

• Saying the kidney filters blood. Be precise: ultrafiltration occurs in the Bowman's capsule where the glomerulus filters blood under high pressure. The whole kidney doesn't filter; specific structures do.

• Forgetting that ALL glucose is reabsorbed. Healthy urine contains no glucose. Any glucose in urine indicates diabetes or kidney disease. This is a common exam trap question.

• Mixing up vasodilation and vasoconstriction. Vasodilation (widening of blood vessels) occurs when hot to increase heat loss. Vasoconstriction (narrowing) occurs when cold to reduce heat loss. Remember: dilation = wider, like diluting a drink makes it more spread out.

• Calling sweat "an excretory product." Sweat is produced primarily for temperature regulation, not excretion. Though it contains small amounts of urea, this is a secondary function. The kidney is the main excretory organ for urea.

• Not explaining HOW evaporation cools the body. State that evaporation requires heat energy, which is taken from the body surface, lowering body temperature.

Exam technique for "Excretion in Humans: Kidneys and Skin"

• "Describe" questions require you to state what happens, naming structures and processes. For 4 marks, give four separate points. Example: "Describe how urine is formed" needs ultrafiltration, selective reabsorption, and specific details.

• "Explain" questions need reasons or mechanisms. Use linking words like "because," "therefore," "this causes." For example: "The blood becomes concentrated, therefore ADH is released, which causes more water reabsorption."

• Diagram questions are common for nephron structure. Practice labeling: glomerulus, Bowman's capsule, kidney tubule, collecting duct, blood vessels (renal artery and vein). Show direction of blood flow with arrows.

• Application questions may use Caribbean scenarios (outdoor workers, athletes in hot climates, effects of dehydration). Apply your knowledge to the specific situation described, mentioning sweating, concentrated urine, or ADH release as relevant.

Quick revision summary

Excretion removes metabolic waste products. Kidneys filter blood through ultrafiltration in the Bowman's capsule, then selectively reabsorb useful substances (all glucose, most water, some salts) in the tubule, producing urine containing urea, water, and salts. ADH controls water reabsorption for osmoregulation. Skin excretes small amounts of urea in sweat but primarily regulates temperature through sweat evaporation. Both organs maintain homeostasis by removing waste and controlling water content.