Human Kidney & Respiratory Systems: Structure & Function

Posted on Sep 15, 2025 in Biology

The Human Urinary System: Kidneys & Associated Organs

The kidneys are reddish-brown, bean-shaped organs that play a vital role in filtering blood and producing urine.

Kidney Characteristics & Location

Key Parameters:

  • Length: 11 cm
  • Width: 6-7 cm
  • Thickness: 5 cm

Position:

The kidneys are located on each side of the spinal cord, with one behind the stomach and the other behind the liver. The right kidney lies slightly lower than the left due to the liver’s position, making it also slightly smaller.

The Nephron: Kidney’s Functional Unit

The nephron is the small, independent functional unit of the kidney. Each nephron has its own blood supply and a collecting tubule that leads to the urethra. Each kidney contains approximately one million nephrons.

Nephron Components:

  • Renal Corpuscle:
    • Glomerulus: A network of capillaries responsible for filtration.
    • Bowman’s Capsule: Encloses the glomerulus.
  • Renal Tubule:
    • Proximal Convoluted Tubule
    • Loop of Henle
    • Distal Convoluted Tubule

Key Functions of the Kidneys

  • Formation of urine
  • Regulation of blood pressure
  • Regulation of blood pH
  • Regulation of ions (e.g., Na+, Ca2+)
  • Regulation of water concentration

Urine Formation Process

Urine formation involves several steps:

  • Filtration: Occurs in the glomerulus.
  • Reabsorption: After initial filtration, approximately 1.5-2 liters of fluid are filtered, but most of it is reabsorbed back into the body.
  • The final product is secondary urine.

The Human Respiratory System: Breathing & Gas Exchange

The primary purpose of the respiratory system is to obtain oxygen for the body and remove carbon dioxide.

Divisions of the Respiratory System

Upper Respiratory Tract:

  • Nose
  • Nasal Cavity
  • Pharynx
  • Nasopharynx

Lower Respiratory Tract:

  • Larynx
  • Trachea
  • Bronchial Tree

Respiratory Zones

Conducting Zone:

Includes all cavities and structures that transport gas into the respiratory division.

Respiratory Zone:

Includes structures involved in gas exchange. Air enters the respiratory system through the nose and mouth.

Anatomy of Respiratory Organs

The Nose:

The nose has two main sections:

  • Olfactory Section: Responsible for the sense of smell.
  • Respiratory Section: Lined with epithelium, supported by bone and cartilage. It serves as the entrance for air, is richly supplied with arteries, veins, and capillaries, and contains a mucous lining that produces mucus and tiny hairs called cilia.
Functions of the Nose:
  • Warms inhaled air
  • Moistens and filters inhaled air
  • Conducts inhaled air into the pharynx

The Pharynx:

A funnel-shaped passageway that connects the nasal and oral cavities to the larynx through the nasopharynx (the upper portion of the pharynx).

The Larynx (Voice Box):

Shaped like a triangular box, composed of a framework of muscles (true vocal cords and false vocal cords, which support the true vocal cords) and cartilages bound by elastic tissue. The largest cartilage is the thyroid cartilage, which forms the “Adam’s apple” in men.

Functions of the Larynx:
  • Passageway for air
  • Produces sound through vibrations of the true vocal cords

The epiglottis is a cartilage that prevents food or fluid from entering the trachea.

The Trachea (Windpipe):

A passageway about 9-15 cm long and 2.5 cm in diameter, connecting the larynx with the bronchi. It is composed of 16-20 C-shaped rings of hyaline cartilage. Medial to the lungs, it splits to form the right and left bronchi.

Function:

Passageway for air.

The Bronchi & Bronchial Tree:

The right and left bronchi each enter the lungs on their respective sides and branch into smaller tubes called bronchioles, forming the bronchial tree. They are covered by ciliated epithelium and many tiny glands that produce mucus. Bronchioles continue to subdivide until they end in tiny air sacs called alveoli, which are the primary sites of gas exchange between air and blood.

The Lungs:

Spongy, paired organs situated within the thoracic cavity, separated from each other by the mediastinum (the area between the lungs).

Lung Lobes:
  • Right Lung: Has three lobes (upper, middle, lower).
  • Left Lung: Has two lobes (upper, lower).

The lungs are surrounded by a serous membrane called the pleura:

  • Visceral Pleura: Covers the outer surface of the lung.
  • Parietal Pleura: Lines the thoracic wall.

Lungs consist of nearly 300 million tiny alveoli, which contain respiratory epithelium and many blood capillaries.

Mechanism of Breathing

Levels of Respiration:

Respiration occurs at two different levels:

  • External Respiration: Gas exchange between the lungs and the external environment.
  • Internal Respiration: Gas exchange in the cells and tissues of the body.
  • Cellular Respiration: An aerobic process at the cellular level that requires oxygen to break down glucose, releases CO2, and produces a large amount of ATP.

External Breathing Phases:

External breathing consists of two phases that involve lung ventilation, distribution, and diffusion of gases.

1. Inhalation (Inspiration):

Air is pulled into the lungs. The diaphragm and external intercostal muscles contract. The diaphragm moves down, and the external intercostal muscles pull the ribs up, expanding the rib cage. The volume of the thoracic cavity increases, causing the intrathoracic pressure to become lower than atmospheric pressure, creating negative breathing pressure. Gases move from higher pressure to lower pressure.

2. Exhalation (Expiration):

Air is pushed out of the lungs. The diaphragm moves back up. The volume of the thoracic cavity decreases, and pressure increases.

Ventilation & Lung Capacities:

Ventilation: The movement of air, alternating between exhalation and inhalation.

Vital Capacity of the Lungs: The maximum amount of air that can be moved in and out of the respiratory system.

  • Women: 2.5-4 L
  • Men: 3.5-5 L

Normal Respiration Rate: 16-18 breaths per minute.

Lung Mechanics & Control:

The lungs are not muscles; they are passive organs of breathing and cannot expand or contract by themselves. Inhalation and exhalation are produced by the movements of the large, flat muscle located along the bottom of the rib cage (the diaphragm) and the muscles between the ribs (the external intercostal muscles). The main respiratory centers are located in the medulla oblongata and pons of the brainstem.

Internal Breathing & Gas Transport

Internal breathing involves the exchange of gases between the cells of the body and the blood.

Oxygen Transport:

The oxygen-carrying molecule in the blood is a red-colored protein found in red blood cells called hemoglobin. Each milliliter of blood contains approximately 1.25 x 1015 hemoglobin molecules. Over 98% of oxygen is carried in the blood bound to hemoglobin within red blood cells, forming oxyhemoglobin.

Carbon Dioxide Transport:

Carbon dioxide (CO2) is transported in the blood in several ways:

  • Dissolved directly in the blood plasma.
  • Bound to the surface of red blood cells.
  • In the form of bicarbonate ions.

Common Respiratory System Diseases

  • Pneumonia: An infection in one or both lungs, often caused by bacteria like Streptococcus.
  • Asthma: Inflammation and muscle tightening around the airways, making breathing difficult.
  • Bronchitis: Inflammation of the bronchi, leading to mucus accumulation.
  • Lung Cancer: Uncontrolled cell growth in the lungs.

Urinary System Quick Facts

  • Kidneys: Primarily responsible for blood filtration.
  • Ureter: The tube that carries urine from the kidneys to the urinary bladder.
  • Urinary Bladder: Where urine is stored before its release.
  • Sphincter: The muscle that controls the release of urine from the urinary bladder.
  • Urinary Incontinence: The medical term for involuntary urine leakage.
  • Common Urinary System Symptoms: Fever, frequent urination, abdominal pain.