circulatory system

Circulatory System – Blood (Part 1)

Main Functions of the Circulatory System

• Carries oxygen, nutrients, and hormones to all cells
  
  

• Removes waste (like CO₂ and urea) from cells
  
  

• Works with other systems (digestive, respiratory, endocrine, etc.)
  
  

• Protects against blood loss and harmful substances
  
  

• Regulates body temperature
  
  

Why Circulation is Needed

• Unicellular organisms: no circulatory system — gas exchange happens directly with the environment
  
  

• Simple multicellular organisms: thin membranes allow gas exchange
  
  

• Complex multicellular organisms: need a circulatory system to deliver oxygen/nutrients and remove wastes
  
  

Basic Parts of Any Circulatory System

1. Pump (Heart) – pushes blood through the body
   
   

2. Blood Vessels – tubes that transport blood
   
   

3. Blood – the fluid that carries materials
   
   

Types of Circulatory Systems

• Open System: blood flows freely in body cavities; cells are bathed in blood
  
  

  • Ex: insects, snails, crustaceans
    
    

• Closed System: blood stays inside vessels; separated from cell fluid
  
  

  • Ex: worms, squids, vertebrates (including humans)
    
    

Blood Overview

• Blood = specialized connective tissue
  
  

• Made of:
  
  

  1. Plasma (55%) – liquid part
     
     

  2. Formed Elements (45%) – solid parts (RBCs, WBCs, platelets)
     
     

1. Plasma

• ~90% water, ~10% proteins, nutrients, and ions
  
  

• Proteins include:
  
  

  • Albumin: controls water movement (prevents dehydration/swelling)
    
    

  • Fibrinogen: helps clot blood
    
    

  • Globulins: transport fats/lipids
    
    

• Contains: glucose, minerals, vitamins, gases, and waste
  
  

• Too much sodium (Na⁺) → more water in blood → higher blood pressure
  
  

Plasma vs Serum

2a. Red Blood Cells (RBCs / Erythrocytes)

• Made in bone marrow
  
  

• No nucleus or mitochondria (more room for oxygen)
  
  

• Biconcave shape → large surface area for gas exchange
  
  

• Contain hemoglobin (protein with iron) that binds to oxygen
  
  

• Main job: carry oxygen to cells and remove CO₂
  
  

2b. White Blood Cells (WBCs / Leukocytes)

• Made in bone marrow
  
  

• Have nuclei, appear white/clear, move like amoebas
  
  

• Main job: defend the body from infections
  
  

Types:

• Neutrophils: most common; destroy pathogens by phagocytosis (engulfing and digesting them)
  
  

• Lymphocytes: control immune responses and make antibodies
  
  

• Pus = dead WBCs + bacteria from infections
  
  

2c. Platelets (Thrombocytes)

• Not actual cells – pieces of larger cells from bone marrow
  
  

• No nucleus
  
  

• Main job: help form blood clots
  
  

Blood Clotting Process

1. Platelets stick to torn vessel edges
   
   

2. Release chemicals → start clotting
   
   

3. Chemicals trigger fibrinogen → fibrin strands
   
   

4. Fibrin forms a mesh that traps RBCs
   
   

5. A scab forms to stop bleeding
   
   

Blood Cell Summary

🩸 Blood Vessels – Simplified Notes

Learning Goal:

Understand and explain the structure and function of the different blood vessels.

Main Types of Blood Vessels (5 Total)

1. Arteries
   
   

2. Arterioles
   
   

3. Capillaries
   
   

4. Venules
   
   

5. Veins
   
   

Each type has a unique structure and function.

Circulatory System Overview

The human circulatory system has two circuits (called double circulation):

1⃣ Pulmonary Circuit (Heart ↔ Lungs)

• Deoxygenated blood → lungs via pulmonary arteries
  
  

• Oxygenated blood → back to heart via pulmonary veins
  
  

• Low-pressure system (short distance)
  
  

2⃣ Systemic Circuit (Heart ↔ Body)

• Oxygenated blood → body via systemic arteries
  
  

• Deoxygenated blood → returns via systemic veins
  
  

• High-pressure system (must push blood long distances)
  
  

🔴 Arteries & Arterioles

Structure

• Small inner diameter
  
  

• 3 layers:
  
  

  1. Outer: connective tissue + elastic fibers
     
     

  2. Middle: smooth muscle
     
     

  3. Inner: thin layer of epithelial cells
     
     

• Thick, strong, and elastic walls
  
  

Function

• Carry oxygenated blood away from heart to body
  
  

  • Exception: pulmonary arteries carry deoxygenated blood to lungs
    
    

• Elastic walls stretch when heart pumps, then recoil → creates pulse
  
  

Special Arteries

• Aorta: largest artery (from heart)
  
  

• Coronary arteries: supply heart muscle itself with oxygen
  
  

Arterioles

• Small branches of arteries
  
  

• Controlled by the nervous system
  
  

• Can change diameter:
  
  

  • Vasodilation: widen → more blood flow, cools body
    
    

  • Vasoconstriction: narrow → less blood flow, keeps heat
    
    

🔵 Capillaries

Structure

• Smallest blood vessels (RBCs pass through single file)
  
  

• No muscle, just thin walls → easy for substances to pass through
  
  

• Connect arterioles ↔ venules
  
  

• Form networks (capillary beds) that slow blood flow
  
  

Function

• Gas and nutrient exchange between blood and tissues
  
  

• Oxygen & nutrients move out of blood, waste moves in
  
  

• Slow flow = more time for exchange
  
  

Capillary Control

• Pre-capillary sphincters (tiny muscles) open or close capillary beds:
  
  

  • Dilate = more blood to that area
    
    

  • Constrict = less blood when not needed
    
    

🔵 Venules & Veins

Venules

• Small veins that collect blood from capillaries and carry it to larger veins
  
  

Veins – Structure

• Thin walls, less elastic than arteries
  
  

• Larger inner diameter
  
  

• Have one-way valves to stop blood from flowing backward
  
  

Function

• Carry deoxygenated blood back to the heart
  
  

  • Exception: pulmonary veins carry oxygenated blood from lungs to heart
    
    

• Transport CO₂ and waste away from cells
  
  

How Blood Moves in Veins

• Skeletal muscles squeeze veins when you move → push blood upward
  
  

• Valves open and close to keep blood flowing toward the heart
  
  

Vein Problems

• Blood pooling: when muscles don’t contract enough (less movement)
  
  

• Varicose veins:
  
  

  • Valves weaken, veins stretch and bulge
    
    

  • Common with aging or standing too long
    
    

⚡ Quick Comparison Chart

❤ The Heart – Simplified Notes

Learning Goals

Understand and explain:

• Structure of the heart
  
  

• Pathway of blood
  
  

• Cardiac cycle
  
  

• Heart sounds
  
  

• Cardiac contractions
  
  

• Blood pressure
  
  

1⃣ Structure of the Heart

Basic Facts

• Muscular pump made of:
  
  

  • Endocardium → inner lining
    
    

  • Myocardium → heart muscle (cardiac muscle)
    
    

• Located in the middle of the chest, protected by ribs, spine, and sternum
  
  

• Has a built-in pacemaker (controls heartbeat)
  
  

Chambers of the Heart (4 Total)

Valves of the Heart

Valves prevent blood from flowing backward and are held by heart strings (tendons).

Septum: wall that separates right (deoxygenated) and left (oxygenated) sides.

2⃣ Pathway of Blood (Double Circulation)

🩶 Deoxygenated Blood (Body → Lungs)

Think: Right = Lungs

1. Body → Vena cavae
   
   

2. → Right atrium
   
   

3. → Tricuspid valve
   
   

4. → Right ventricle
   
   

5. → Semilunar valve
   
   

6. → Pulmonary arteries → Lungs
   
   

   • CO₂ released, O₂ picked up
     
     

❤ Oxygenated Blood (Lungs → Body)

Think: Left = Leaves lungs

1. Lungs → Pulmonary veins
   
   

2. → Left atrium
   
   

3. → Bicuspid (Mitral) valve
   
   

4. → Left ventricle
   
   

5. → Semilunar valve
   
   

6. → Aorta → Body
   
   

3⃣ Cardiac Cycle (Heartbeat)

A complete heartbeat = all 4 chambers contract and relax once

Two Main Phases

4⃣ Heart Sounds

• Heartbeat sound = “lubb-DUBB”
  
  

• Caused by valves closing
  
  

5⃣ Cardiac Contractions (Electrical Control of Heartbeat)

The heart is myogenic → beats on its own (without brain signals).

Electrical Pathway

1. SA Node (Sinoatrial Node) – natural pacemaker, sets rhythm
   
   

2. Sends electrical signal → Atria contract
   
   

3. Signal goes to AV Node (Atrioventricular Node)
   
   

4. Travels down Bundle of His → Purkinje Fibres
   
   

5. Ventricles contract → blood pumped out
   
   

6⃣ Blood Pressure (B.P.)

Definition

• Pressure of blood pushing on artery walls
  
  

• Increases when arteries experience too much force
  
  

Two Readings

Normal BP = 120 / 80 mmHg

How It’s Measured

• Tool: Sphygmomanometer (inflatable cuff)
  
  

• Placed on upper arm, measures pressure in brachial artery
  
  

• Steps:
  
  

  1. Cuff inflates → blocks artery
     
     

  2. When blood first starts to flow = systolic pressure (heard as “whoosh”)
     
     

  3. When sound stops = diastolic pressure
     
     

🩸 Blood pressure decreases as blood moves farther from the heart.

🧠 Quick Summary Table