The human urinary system: structure and functions of the kidney

What you'll learn

This revision guide covers the structure and functions of the human urinary system, with specific focus on kidney anatomy and physiology. You'll learn how the kidneys maintain homeostasis through excretion, filtration, and osmoregulation—all essential concepts for the CXC CSEC Human and Social Biology examination.

Key terms and definitions

Excretion — the removal of metabolic waste products from the body, including urea, carbon dioxide, and excess salts

Urea — a nitrogenous waste product formed in the liver from the breakdown of excess amino acids; less toxic than ammonia and excreted in urine

Ultrafiltration — the process in the Bowman's capsule where blood pressure forces small molecules from the glomerulus into the nephron, while blood cells and proteins remain in the bloodstream

Selective reabsorption — the active and passive transport of useful substances (glucose, water, amino acids, mineral salts) from the nephron tubules back into the blood

Osmoregulation — the control of water and salt concentration in body fluids to maintain constant internal environment

Nephron — the functional unit of the kidney; approximately one million nephrons per kidney filter blood and produce urine

Antidiuretic hormone (ADH) — a hormone released by the pituitary gland that increases water reabsorption in the collecting ducts when the body is dehydrated

Glomerular filtrate — the fluid that passes from the blood in the glomerulus into the Bowman's capsule, containing water, glucose, amino acids, salts, and urea but no blood cells or proteins

Core concepts

Components of the urinary system

The human urinary system consists of four main organs working together to remove waste and regulate water balance:

The kidneys are two bean-shaped organs located in the lower back region (lumbar area), one on each side of the vertebral column. Each kidney measures approximately 10-12 cm in length and receives blood through the renal artery. The kidneys filter blood, remove waste products, and regulate water and salt balance.

The ureters are two muscular tubes, approximately 25-30 cm long, that transport urine from the kidneys to the bladder using peristaltic contractions. These tubes have thick muscular walls that prevent backflow of urine.

The urinary bladder is a muscular sac that stores urine temporarily. The bladder wall contains smooth muscle and can stretch to hold approximately 400-600 cm³ of urine. When full, stretch receptors in the bladder wall send signals to the brain triggering the urge to urinate.

The urethra is a tube that carries urine from the bladder to outside the body during urination (micturition). The urethra contains a sphincter muscle that controls the release of urine.

Structure of the kidney

Understanding kidney structure is essential for explaining its functions in the CSEC examination.

External structure:

• Dark red-brown colour due to rich blood supply
• Outer convex surface and inner concave surface with the hilum (where blood vessels and ureter connect)
• Protected by a tough fibrous capsule
• Surrounded by adipose (fat) tissue for protection and insulation

Internal structure (longitudinal section):

The cortex is the outer region of the kidney, darker in appearance, containing the Bowman's capsules and blood capillaries. This is where ultrafiltration occurs.

The medulla is the inner region, lighter in colour, containing the collecting ducts and loops of Henle arranged in cone-shaped structures called renal pyramids.

The renal pelvis is a funnel-shaped cavity that collects urine from the collecting ducts and channels it into the ureter.

Blood supply:

• Renal artery (from aorta) brings oxygenated blood containing waste products
• Renal vein (to vena cava) removes deoxygenated blood with waste removed
• The kidney receives approximately 25% of cardiac output, demonstrating its importance

Structure and function of the nephron

The nephron is the microscopic functional unit of the kidney. Understanding its structure is crucial for explaining urine formation.

Components of the nephron:

The Bowman's capsule is a cup-shaped structure in the cortex that surrounds a knot of blood capillaries called the glomerulus. The capsule has a specialized inner layer with podocytes (cells with finger-like projections) that help in filtration.

The glomerulus is a tight ball of capillaries with thin walls and a large surface area. Blood enters through the afferent arteriole (wider) and leaves through the efferent arteriole (narrower), creating high blood pressure necessary for filtration.

The proximal convoluted tubule is a coiled section immediately after the Bowman's capsule, located in the cortex. Its walls are one cell thick with microvilli to increase surface area for reabsorption. This is where most selective reabsorption occurs.

The loop of Henle is a U-shaped section that extends from the cortex down into the medulla and back up. It plays a key role in water reabsorption and creating concentration gradients.

The distal convoluted tubule is another coiled section in the cortex where fine-tuning of salt and water balance occurs under hormone control.

The collecting duct receives filtrate from several nephrons and passes through the medulla to the renal pelvis. ADH acts here to control water reabsorption.

The process of urine formation

Urine formation involves three main processes that you must be able to describe and explain for the examination.

1. Ultrafiltration (in the Bowman's capsule)

High blood pressure in the glomerulus forces small molecules through the capillary walls and the capsule membrane. This pressure is created because:

• The afferent arteriole is wider than the efferent arteriole
• The heart pumps blood at high pressure
• The glomerular capillaries have thin walls

What passes through (glomerular filtrate):

• Water
• Glucose
• Amino acids
• Mineral salts (sodium, chloride, potassium)
• Urea
• Other small molecules

What remains in the blood:

• Blood cells (too large)
• Platelets (too large)
• Plasma proteins (too large)

Approximately 180 litres of filtrate are produced daily, but only 1.5 litres leave as urine.

2. Selective reabsorption (mainly in the proximal convoluted tubule)

Useful substances are reabsorbed from the filtrate back into the blood capillaries surrounding the tubules. This process ensures the body retains necessary materials.

Substances reabsorbed:

• All glucose (by active transport) — glucose is never present in normal urine
• All amino acids (by active transport)
• Most water (about 85% by osmosis)
• Mineral salts (amount varies depending on body needs)

Substances not reabsorbed:

• Urea (waste product to be excreted)
• Excess salts
• Excess water

The tubule cells have adaptations for efficient reabsorption:

• Numerous mitochondria providing energy for active transport
• Microvilli increasing surface area
• One-cell-thick walls reducing diffusion distance

3. Formation of urine (in collecting ducts)

As filtrate moves through the loop of Henle and collecting ducts, more water is reabsorbed. The final composition of urine depends on:

• Body hydration levels
• Salt intake
• ADH concentration

Normal urine composition:

• Water (95%)
• Urea (2%)
• Mineral salts (2%)
• Other substances including creatinine, uric acid (1%)

The yellow colour comes from urochrome, a pigment from broken-down hemoglobin.

Functions of the kidney

The kidneys perform multiple homeostatic functions beyond simple waste removal.

Excretion of nitrogenous waste: The liver produces urea from deamination of excess amino acids. The kidneys filter this urea from blood and excrete it in urine, preventing toxic buildup. In the Caribbean, where diets may include high-protein foods like saltfish, red kidney beans, and chicken, efficient protein metabolism and urea excretion are particularly important.

Osmoregulation: Kidneys regulate water and salt concentration in body fluids. In hot Caribbean climates, people lose significant water through sweating. The kidneys compensate by:

• Producing concentrated urine when dehydrated (less water, darker colour)
• Producing dilute urine when excess water is consumed (more water, lighter colour)

ADH controls this process: when blood water concentration drops (detected by hypothalamus), the pituitary gland releases more ADH, making collecting duct walls more permeable to water, increasing reabsorption.

pH regulation: Kidneys help maintain blood pH around 7.4 by controlling hydrogen ion and bicarbonate ion excretion.

Blood pressure regulation: Kidneys help regulate blood volume and therefore blood pressure by controlling water reabsorption.

Maintaining salt balance: Kidneys excrete excess salts while retaining necessary amounts. This is important in Caribbean populations where salt intake from preserved foods (saltfish, corned beef) may be high.

Kidney disorders and their management

Understanding common kidney problems helps appreciate kidney function.

Kidney stones: Hard deposits of minerals and salts that form in kidneys. More common in hot climates where dehydration concentrates urine. Prevention includes drinking adequate water—especially important for Caribbean students and workers in outdoor industries like agriculture and construction.

Kidney infection (pyelonephritis): Bacterial infection usually spreading from the bladder. Symptoms include fever, back pain, and cloudy urine. Requires antibiotic treatment.

Kidney failure: When kidneys cannot adequately filter blood, toxic waste accumulates. Causes include:

• Diabetes (leading cause globally and in the Caribbean)
• Hypertension (high blood pressure)
• Chronic kidney disease
• Acute injury

Treatment options:

Dialysis — artificial filtering of blood. In haemodialysis, blood passes through a machine containing a semi-permeable membrane bathed in dialysis fluid. Small molecules (urea, salts) diffuse out while blood cells and proteins remain. Patients typically require dialysis 3 times weekly, each session lasting 3-4 hours. Access to dialysis varies across Caribbean nations, with larger territories having better facilities.

Kidney transplant — surgical replacement with a healthy kidney from a donor. This offers better quality of life than dialysis but requires lifelong immunosuppressant drugs and compatible donors.

Worked examples

Example 1: Structure and filtration (6 marks)

Question: (a) Name the structure in the kidney where ultrafiltration occurs. (1 mark) (b) Explain why blood cells and proteins are not present in the glomerular filtrate. (2 marks) (c) Describe the role of the afferent and efferent arterioles in ultrafiltration. (3 marks)

Model answer: (a) Bowman's capsule (1 mark)

(b) Blood cells and proteins are too large to pass through the pores in the capillary walls/basement membrane (1 mark), so they remain in the blood (1 mark).

(c) The afferent arteriole is wider than the efferent arteriole (1 mark). This creates high blood pressure in the glomerulus (1 mark), which forces small molecules through the capillary walls into the Bowman's capsule (1 mark).

Example 2: Composition comparison (4 marks)

Question: The table shows the percentage composition of glucose and urea in three fluids in the kidney.

Identify fluids A, B, and C. Explain your answers. (4 marks)

Model answer: Fluid A = Blood plasma (in renal artery) — contains glucose and low concentration of urea (1 mark)

Fluid B = Glomerular filtrate — has same glucose and urea concentration as blood because small molecules pass through during ultrafiltration (1 mark)

Fluid C = Urine — no glucose because all reabsorbed/high urea concentration because urea not reabsorbed and water is reabsorbed concentrating the urea (1 mark for no glucose, 1 mark for high urea with explanation)

Example 3: Application question (5 marks)

Question: A sugarcane worker in Jamaica drinks very little water during an 8-hour shift in the hot sun. Explain how the kidneys will respond to maintain water balance in his body. (5 marks)

Model answer: The worker loses water through sweating (1 mark). Blood becomes more concentrated/less water in blood (1 mark). This is detected by the hypothalamus (1 mark). The pituitary gland releases more ADH (1 mark). ADH increases permeability of collecting duct walls to water/more water is reabsorbed from filtrate back into blood (1 mark). Less urine is produced and it is more concentrated/darker in colour (1 mark). [Any 5 marks]

Common mistakes and how to avoid them

• Confusing filtration and reabsorption: Filtration occurs only in the Bowman's capsule where substances leave the blood. Reabsorption occurs mainly in the proximal convoluted tubule where useful substances return to the blood. Use the terms precisely and in the correct context.

• Stating that glucose is reabsorbed "if needed": Glucose is always completely reabsorbed in healthy kidneys. It should never appear in urine. Any glucose in urine indicates diabetes or kidney damage—this is not selective reabsorption.

• Forgetting that urea is formed in the liver, not the kidney: The kidney only filters and excretes urea; it does not produce it. Deamination of amino acids occurs in the liver, producing ammonia which is converted to less toxic urea.

• Confusing the ureter with the urethra: The ureter connects kidney to bladder (two of them); the urethra connects bladder to outside (one only). Remember: ureTER = kidneT to bladder; ureTHRA = bladder to THRough/outside.

• Not explaining mechanisms fully: When describing processes like selective reabsorption, mention the mechanism (active transport for glucose and amino acids, osmosis for water) and the adaptations that make it efficient (microvilli, mitochondria).

• Vague statements about ADH: Be specific: ADH increases the permeability of collecting duct walls to water, allowing more water to be reabsorbed by osmosis. Don't just say "ADH saves water."

Exam technique for "The human urinary system: structure and functions of the kidney"

• "Describe" versus "Explain": When asked to describe a process (e.g., ultrafiltration), state what happens in sequence. When asked to explain, give reasons why it happens. "Describe" typically earns 1 mark per point stated; "explain" requires linked cause-and-effect statements.

• Use proper anatomical terms: Instead of "tube," specify whether you mean ureter, urethra, nephron tubule, or collecting duct. Replace "filter" with "glomerulus" or "nephron" where appropriate. Examiners reward precise terminology.

• Link structure to function: When describing kidney/nephron structures, connect features to their roles. For example: "The proximal convoluted tubule has microvilli which increase surface area for reabsorption" earns more marks than just listing the structures.

• Draw and label carefully: If asked to draw a nephron or kidney structure, use a pencil, make it large enough to label clearly, and include only the structures specified in the question. Label lines should touch the structure and not cross each other. Typical diagrams earn 1 mark per correct structure and 1 mark per correct label—accuracy matters.

Quick revision summary

The urinary system comprises kidneys, ureters, bladder, and urethra. Each kidney contains approximately one million nephrons—the functional units responsible for blood filtration. Urine formation involves three processes: ultrafiltration in the Bowman's capsule (high pressure forces small molecules from blood), selective reabsorption mainly in the proximal convoluted tubule (glucose, amino acids, and most water return to blood), and formation of concentrated urine in collecting ducts (controlled by ADH). Kidneys perform vital functions: excretion of urea, osmoregulation, pH balance, and blood pressure regulation—maintaining homeostasis essential for survival.