Human Respiratory and Urinary System Anatomy

Posted on Jun 25, 2026 in Biology

The Respiratory System

The respiratory system is a group of organs and tissues that help us in breathing or respiration. This group of organs and tissues is known as the respiratory tract, which includes the passage of air from the nose to the alveoli in the lungs.

  • Path: Air → O₂ (Oxygen)
  • Breathing: This is generally a physical and mechanical process of taking air (O₂) into the body/lungs and expelling it (CO₂).
  • Respiration: This is the complete biochemical process of exchanging gases (O₂/CO₂), which includes breathing, the transport of gases to the body’s cells, the production of energy (ATP), and the expelling of CO₂.

Process Flowcharts

Inhalation/Exhalation Track:
Nose (O₂ entry / CO₂ exit) → Lungs → Alveoli → Blood

Cellular Respiration Track:
Blood Cells (O₂ enters / CO₂ exits)

Chemical Reaction in Cells:
O₂ + C₆H₁₂O₆ → CO₂ + H₂O + ATP (energy)

Anatomy of the Respiratory System

The respiratory system consists of the following organs:

Upper Respiratory Tract

  • Nose / Nasal Cavity
  • Mouth / Oral Cavity
  • Pharynx
  • Larynx

Lower Respiratory Tract

  • Trachea
  • Bronchi
  • Bronchioles
  • Lungs & Alveoli

1. Nasal Cavity / Nose

The nose is the external organ of the respiratory system responsible for inhaling and exhaling air.

  • Anatomy: It includes the External Nose (visible part), Nasal Cavity (internal passage), Nostrils (the opening of the nose), Nasal Septum (thin wall separating two nasal passages), and Turbinate Bones (curved bones that warm and humidify air).
  • Physiology / Functions: Air enters through the nostrils where it is warmed, humidified, and filtered by hairs. It also contains olfactory receptors for the sense of smell.

2. Mouth

The mouth is not considered a primary part of the respiratory system, but it serves as an alternate entry pathway for air.

  • Anatomy: Includes the oral cavity, lips, tongue, and palate (roof of the mouth).
  • Physiology: Used for breathing when the nose is congested; also helps in chewing and contains taste buds.

3. Pharynx

A muscular tube that connects the nose and mouth to the larynx (voice box). It is a common pathway for both the respiratory and digestive systems.

  • Anatomy: Approximately 12 cm long. It includes the Nasopharynx (upper), Oropharynx (middle), and Laryngopharynx (lower).
  • Functions: Acts as a passage for air and food; contains lymphoid tissue (tonsils, adenoids) to fight infections.

4. Larynx (Voice Box)

A cartilaginous organ responsible for producing sound.

  • Anatomy: Located in the anterior neck at the level of the 3rd to 6th cervical vertebrae. It features the Adam’s apple, thyroid cartilage, cricoid cartilage, arytenoid cartilage, and vocal cords.

5. Trachea

Also known as the windpipe, it connects the larynx to the bronchi.

  • Anatomy: 10–11 cm long, composed of 16–20 C-shaped rings of hyaline cartilage. It is made of cricoid cartilage and tracheal rings.

6. Bronchi and Bronchioles

Bronchi are tubes that branch off from the trachea, acting as air passages. Bronchioles are smaller tubes branching from the bronchi that lead to the alveoli for gas exchange.

7. Alveoli

Tiny, hollow air sacs where gas exchange with the blood occurs.

  • Anatomy: Human lungs contain around 300 million alveoli covered by capillaries.
  • Type 1 Alveolar Cells (Squamous): Thin cells for gas exchange (approx. 95%).
  • Type 2 Alveolar Cells (Cuboidal): Thicker cells that produce surfactants.

The Lungs

The lungs are a pair of cone-shaped, spongy organs in the thoracic cavity responsible for exchanging oxygen and carbon dioxide.

  • Anatomy: Located on either side of the heart. The right lung is slightly larger (600–700g) than the left (500–600g). Total weight is approximately 1.1–1.3 kg.
  • Lobes: The right lung has three lobes (Superior, Middle, Inferior); the left lung has two (Superior, Inferior).
  • Parts: Apex (upper), Base (lower, resting on the diaphragm), Costal surface (facing ribs), and Medial surface (containing the hilum).
  • Mediastinum: The area between the lungs occupied by the heart.

Functions of the Lungs

  • Respiration: Exchanging O₂ (inhalation) and CO₂ (exhalation).
  • Filtration: Removing dust and bacteria from inhaled air.
  • pH Regulation: Helping regulate blood pH by removing excess H+ ions.

The Diaphragm

A dome-shaped muscle separating the chest and abdominal cavities. It facilitates breathing by contracting and relaxing to increase lung volume during inhalation.

Mechanism of Respiration

Respiration involves the coordination of the lungs, diaphragm, rib cage, and nervous system. The average adult breathing rate is 15–18 times per minute.

1. Inhalation (Inspiration)

An active process requiring energy to take oxygen into the lungs.

2. Exhalation (Expiration)

A generally passive process where CO₂ is removed. The diaphragm relaxes, the thoracic cavity volume decreases, and air leaves the alveoli through the nostrils or mouth.

3. Gaseous Exchange

  • External Exchange: Oxygen diffuses from alveoli into the blood, while CO₂ diffuses into the alveoli via pressure gradients.
  • Internal Exchange (Cellular Respiration): O₂ is transported to tissue cells, and metabolic CO₂ is brought back to be exhaled.

Lung Volumes and Capacities

  • Tidal Volume (TV): Air inhaled/exhaled during normal rest (~500 mL).
  • Inspiratory Reserve Volume (IRV): Extra air forcibly inhaled (~2500–3000 mL).
  • Expiratory Reserve Volume (ERV): Extra air forcibly exhaled (~1000–1200 mL).
  • Residual Volume (RV): Air remaining after forced exhalation (~1200 mL).
  • Total Lung Capacity (TLC): TV + IRV + ERV + RV (~6000 mL).
  • Vital Capacity (VC): TV + IRV + ERV (~4600 mL).
  • Functional Residual Capacity (FRC): Air remaining after normal exhalation (~2400 mL).
  • Inspiratory Capacity (IC): Total air inhaled after normal exhalation (~3600 mL).

Artificial Respiration and Resuscitation

Artificial Respiration assists or replaces natural breathing via mechanical or manual means (e.g., ventilators). Resuscitation (like CPR) aims to restore breathing and circulation to prevent brain damage or death.

The Urinary System

The urinary system (renal system) removes waste and excess fluid as urine to maintain homeostasis. It regulates electrolytes, acid-base balance, and produces hormones for blood pressure and RBC production.

Anatomy of the Urinary System

  • A pair of kidneys
  • A pair of ureters
  • A urinary bladder
  • A urethra

The Kidneys

Bean-shaped organs in the retroperitoneal space (lower back) that filter blood. They weigh approximately 150g and measure 12cm x 6cm x 3cm.

Nephrons

The structural and functional units of the kidney. There are approximately 1–1.25 million nephrons per kidney in adults.

  • Bowman’s Capsule: Contains the Glomerulus (capillary network). Together they form the Renal Corpuscle.
  • PCT (Proximal Convoluted Tubule): Reabsorbs water, glucose, and 65% of electrolytes.
  • Loop of Henle: Regulates electrolyte balance; the descending limb reabsorbs water, while the ascending limb reabsorbs sodium.
  • DCT (Distal Convoluted Tubule): Reabsorbs 5–10% of NaCl.
  • Collecting Duct: Collects urine from multiple nephrons for excretion.

Physiology of Urine Formation

Normal urine output is 1–1.5 liters per day. The process involves:

  1. Filtration (Glomerular Filtration): Occurs in the glomerulus. The normal GFR is 125 mL/min.
  2. Reabsorption: Essential nutrients and water are taken back into the bloodstream.
  3. Secretion: Waste products are secreted into the tubule.

Ureters, Bladder, and Urethra

  • Ureters: 25–30 cm tubes connecting kidneys to the bladder.
  • Urinary Bladder: A muscular organ storing 400–600 mL of urine.
  • Urethra: Carries urine out of the body (20 cm in males; 4–5 cm in females).

Micturition (Urination)

The process of emptying the bladder involves a reflex loop: Bladder filling → Stretch receptor stimulation → Afferent impulses to the spinal cord → Efferent impulses via the pelvic nerve → Detrusor muscle contraction and sphincter relaxation → Voiding.

Renal Role in Acid-Base Balance

The kidneys maintain pH by excreting excess H+ ions, reabsorbing bicarbonate (HCO₃⁻), and producing ammonia (NH₃) to buffer urine.

The RAAS Mechanism

The Renin-Angiotensin-Aldosterone System regulates blood pressure and fluid balance:

  1. Liver releases Angiotensinogen + Kidney releases Renin → Angiotensin-I.
  2. Angiotensin-I + ACE → Angiotensin-II.
  3. Angiotensin-II causes vasoconstriction and Na+ reabsorption, increasing blood pressure and fluid retention.