Lesson 19: Circulatory & Respiratory Systems

Learning Objectives

Compare and contrast the structure and functions of the major classes of circulatory systems in animals.
List the components of mammalian blood and their functions in exchange, transport, and defense.
Summarize the bicarbonate buffer system and how it maintains pH homeostasis in blood.
Describe the role of the major structures of the heart during contraction and relaxation.
Identify the types of blood vessels and their functions in maintaining and regulating blood flow.
Explain the role of the lymphatic system and why it is essential for a functioning circulatory system.
Compare and contrast the respiratory structures and mechanisms in different animals.
Describe the mechanisms for ventilating human lungs.

Circulatory System

Animals must exchange nutrients and substances with their environment.
Exchange occurs as dissolved substances move between an aqueous environment and the plasma membrane of cells.
Unicellular organisms: Obtain nutrients through diffusion.
Large, multicellular organisms: Require a body plan that allows all cells to have access to liquid.

Simple Animals and Circulatory Systems

Many smaller, simple animals (e.g., cnidarians, flatworms) use a gastrovascular cavity for nutrient distribution.
- Gastrovascular cavity: A central cavity with a single opening used for digestion and nutrient distribution.
More complex multicellular animals need a proper circulatory system.
- Circulatory system: A network of organs and vessels that transport molecules throughout the body of an animal.
- Enables liquid transport of nutrients, oxygen, hormones, and other substances to internal cells.

Basic Components of the Circulatory System

The circulatory system has three basic parts:
  1. Circulatory fluid.
  2. Network of interconnected vessels.
  3. Heart.
Two major types of animal circulatory systems:
- Open circulatory system: Fluid exits blood vessels and bathes tissues and organs directly.
- Closed circulatory system: Blood is confined to vessels, kept separate from interstitial fluid.

Blood Composition

Vertebrate blood consists of various cells suspended in plasma:
  - Plasma: The liquid matrix in which blood cells are suspended, transporting nutrients, waste, respiratory gases, and hormones.
  - Erythrocytes (Red blood cells): Carry O2. Mature erythrocytes lack a nucleus, allowing more room for hemoglobin.
    - Hemoglobin: An iron-containing protein that binds to O2, with a single erythrocyte containing about 250 million hemoglobin molecules and the capacity to carry a billion O2 molecules.

Types of Blood Cells

Leukocytes (White blood cells): Five varieties, all involved in fighting infections.
- Found in interstitial fluid and lymphatic system.
Platelets: Pinched-off cytoplasmic fragments of specialized bone marrow cells; important in blood clotting.

CO2 in Blood

Blood transports CO2, a byproduct of cellular respiration.
When CO2 reacts with H2O in the bloodstream, it forms carbonic acid (H2CO3), aiding efficient CO2 transport in blood.
Carbonic acid quickly dissociates into H+ ions and bicarbonate (HCO3-).
- Increased H+ concentration leads to a decrease in blood pH (increased acidity).

Blood pH Regulation

To maintain proper blood pH, bicarbonate and carbonic acid act as a buffer system:
  - Buffer: A solution that minimizes changes to pH.
  - If blood becomes too basic, carbonic acid converts to bicarbonate or carbonate to bind to excess H+ ions.
  - If blood becomes too acidic, bicarbonate binds to excess H+ ions to form carbonic acid.
This is known as the Bicarbonate buffer system.
In the lungs, carbonic acid is converted back to CO2, expelled via exhalation, increasing blood pH.

Cardiovascular System

Vertebrates, such as humans, have a closed circulatory system known as the cardiovascular system:
  - Cardiovascular system: Closed circulatory system with a heart and a branching network of blood vessels: arteries, capillaries, and veins.
  - Arteries: Carry blood away from the heart to organs.
  - Capillaries: Microscopic vessels that allow exchange between blood and interstitial fluid.
  - Veins: Carry blood back to the heart.

Circulation Type

Mammals exhibit double circulation:
  - Double circulation: Comprises two separate circuits through which blood passes through the heart after each circuit:
    1. Pulmonary circuit: Right side pumps oxygen-poor blood to gas exchange tissues (lungs/gills).
    2. Systemic circuit: Left side pumps oxygen-rich blood to capillary beds throughout the body.

Structure of the Mammalian Heart

The structure of the heart varies among vertebrates, focusing on the mammalian heart, which has:
  - Four chambers: Two atria and two ventricles.
    - Atria (singular: atrium): Chambers that receive blood.
    - Ventricles: Chambers that pump blood out of the heart.

Cardiac Cycle

Consists mostly of cardiac muscle, facilitating the cardiac cycle:
- Cardiac cycle: Alternating contractions and relaxations of the heart.
- Heart rate: Frequency of heart contractions.

Blood Vessel Dynamics

Blood exits the heart via arteries, rushing into capillaries.
Blood pressure: The pressure exerted by blood on vessel walls, assisting in blood flow.
Hormones and smooth muscle control vasodilation and vasoconstriction in arteries, regulating blood flow into specific capillaries.
Small molecules exit capillaries through diffusion, vesicles, or pores.
Blood pressure generally exceeds osmotic pressure, causing a net loss of liquid in capillaries.

Lymphatic System

The human body loses 4-8 liters of fluid from capillaries daily due to osmotic pressure.
Lost fluid is returned to the circulatory system by the lymphatic system:
  - Lymphatic system: A network of vessels and nodes that return fluids, proteins, and cells to the blood.
  - Lymph: Colorless fluid derived from interstitial fluid in the lymphatic system.
  - Lymph circulates before draining into large veins leading back to the circulatory system at the neck.
  - Lymph nodes: Located along lymph vessels filtering lymph, containing cells that attack viruses and bacteria, and can swell during infections.
The lymphatic system may also play a role in disorders like asthma, an area of active research.

Cardiovascular Disease

Cardiovascular disease: Disorders affecting the heart or blood vessels.
Over 750,000 individuals in the USA die annually from cardiovascular diseases.
Atherosclerosis: A cardiovascular disease characterized by fatty deposits (plaques) developing in artery walls, obstructing and hardening arteries.
Heart attack: Damage or death of cardiac muscle tissue due to prolonged blockage of coronary arteries.
Stroke: Death of nervous tissue in the brain due to ruptured or blocked arteries in the neck or head.
Genetics play a role in the propensity for cardiovascular diseases, influenced by lifestyle factors (exercise, diet – including trans fats, and smoking).

Gas Exchange

Gas exchange: Uptake of molecular O2 from the environment and discharge of CO2.
Gas exchange organs differ among animals:
  - Gills: Outfoldings of body surface suspended in water for extracting O2.
  - Tracheal system: Branching air-filled tubes in insects transporting oxygen directly to cells.
  - Lungs: Infolded respiratory surfaces in many terrestrial animals for extracting O2 from air.

Human Ventilation

Ventilation: Flow of air or water over respiratory surfaces.
Air enters the trachea (windpipe), which branches into two lungs via smaller tubes known as bronchioles.
At the tips of the bronchioles are alveoli, where O2 diffuses into capillaries, and CO2 diffuses out.
Human lungs contain millions of alveoli, providing 50 times more surface area than skin.

Breathing Mechanisms

Breathing: Ventilation of lungs through alternating inhalation and exhalation.
Various mechanisms for breathing exist across species:
  - Frogs use positive pressure breathing, forcing air into lungs.
  - Birds have special air sacs that require multiple inhalations and exhalations for a complete breathing cycle.
  - Mammals utilize negative pressure breathing:
    - Diaphragm: A muscle forming the bottom wall of the thoracic cavity.
      - Inhalation: Diaphragm contracts, expanding thoracic cavity volume, creating lower pressure than air and pulling air into lungs.
      - Exhalation: Diaphragm relaxes, decreasing thoracic cavity volume, increasing air pressure, and forcing air out.

Breathing Regulation

Breathing is regulated by the medulla oblongata:
  - Medulla oblongata: Part of the hindbrain that controls autonomic functions like breathing, heart activity, and digestion.
  - It senses changes in blood pH due to CO2 concentration.
  - An increase in CO2 lowers blood pH, and the medulla responds by increasing diaphragm rate until blood pH returns to 7.4.