The cardiovascular system consists of two main circuits: the pulmonary circulation (right heart to lungs and back) and the systemic circulation (left heart to body and back). These circuits are connected in series.
Overview of the Circulatory Pathways
The heart is a dual pump with four chambers that drives blood through two circuits:
Right heart (pulmonary pump):
- Receives deoxygenated blood from the body
- Pumps blood to the lungs for oxygenation
- Low-pressure system (25-30 mmHg systolic)
Left heart (systemic pump):
- Receives oxygenated blood from the lungs
- Pumps blood to the body
- High-pressure system (120 mmHg systolic)
The Complete Blood Flow Pathway
Step-by-step Flow
- Superior and inferior vena cavae deliver deoxygenated blood to the right atrium
- Right atrium contracts, pushing blood through the tricuspid valve
- Right ventricle contracts, pushing blood through the pulmonary valve
- Pulmonary trunk divides into left and right pulmonary arteries
- Pulmonary arteries carry deoxygenated blood to the lungs
- Pulmonary capillaries surround alveoli for gas exchange
- Pulmonary veins carry oxygenated blood from the lungs
- Left atrium receives blood from the pulmonary veins
- Left atrium contracts, pushing blood through the mitral valve
- Left ventricle contracts, pushing blood through the aortic valve
- Aorta distributes oxygenated blood to systemic circulation
- Arteries, arterioles, capillaries deliver oxygen to tissues
- Venules, veins collect deoxygenated blood
- Vena cavae return blood to the right atrium
Pulmonary Circulation
The pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium.
Components
| Structure | Function |
|---|---|
| Right ventricle | Pump for pulmonary circulation |
| Pulmonary trunk | Main artery from right ventricle |
| Right pulmonary artery | To right lung |
| Left pulmonary artery | To left lung |
| Pulmonary capillaries | Gas exchange with alveoli |
| Right pulmonary veins (2) | Return from right lung |
| Left pulmonary veins (2) | Return from left lung |
| Left atrium | Receiving chamber |
Pressure and Resistance
| Parameter | Value |
|---|---|
| Pulmonary artery systolic pressure | 15-30 mmHg |
| Pulmonary artery diastolic pressure | 5-10 mmHg |
| Mean pulmonary artery pressure | 10-15 mmHg |
| Pulmonary capillary wedge pressure | 4-12 mmHg |
| Pulmonary vascular resistance | 1-3 Wood units |
| Left atrial pressure | 5-10 mmHg |
Unique Features
Low-pressure system:
- Pulmonary systolic pressure is 1/6 of systemic pressure
- Right ventricular wall is thin (3-5 mm)
- Pulmonary arteries have thinner walls than systemic arteries
Hypoxic pulmonary vasoconstriction:
- Alveolar hypoxia causes local vasoconstriction
- Diverts blood to better-ventilated regions
- Opposite to systemic circulation (where hypoxia causes vasodilation)
Bronchial circulation:
- Part of the systemic circulation
- Supplies oxygenated blood to lung tissue
- Drains into pulmonary veins (anatomic shunt)
Systemic Circulation
The systemic circulation carries oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium.
Components
| Structure | Function |
|---|---|
| Left ventricle | Pump for systemic circulation |
| Aorta | Main distributing artery |
| Arterial system | Distribution to organs |
| Capillary beds | Gas and nutrient exchange |
| Venous system | Collection and return |
| Superior vena cava | Return from upper body |
| Inferior vena cava | Return from lower body |
| Right atrium | Receiving chamber |
Major Arterial Branches
Aortic arch branches:
- Brachiocephalic trunk (right subclavian + right common carotid)
- Left common carotid artery
- Left subclavian artery
Descending aorta branches:
| Region | Branches |
|---|---|
| Thoracic | Bronchial, esophageal, pericardial, intercostal, subcostal |
| Abdominal | Celiac trunk, superior mesenteric, renal, gonadal, inferior mesenteric |
Major Venous Tributaries
Superior vena cava drainage:
- Head and neck: Internal jugular veins
- Upper extremities: Subclavian veins
- Chest and upper back: Azygos system
Inferior vena cava drainage:
- Lower extremities: Femoral, iliac veins
- Abdominal viscera: Via hepatic veins (after portal circulation)
- Kidneys: Renal veins
- Pelvis: Internal iliac veins
The Portal Circulation
The portal circulation is a specialized venous system that carries blood from the gastrointestinal tract to the liver before returning to the heart.
Portal Vein Formation
The portal vein is formed by the confluence of:
- Superior mesenteric vein: Drains small intestine and right colon
- Splenic vein: Drains spleen, pancreas, and left colon
- Inferior mesenteric vein: Drains left colon and rectum (joins splenic vein)
Portal Circulation Pathway
- Capillaries in intestinal villi absorb nutrients
- Blood flows into mesenteric veins
- Portal vein carries blood to the liver
- Liver sinusoids process the blood
- Hepatic veins drain into the inferior vena cava
Physiologic Importance
- The liver processes nutrients, toxins, drugs before they reach systemic circulation
- The liver removes bacteria from portal blood
- The liver regulates blood glucose, lipid, and protein metabolism
The Fetal Circulation (Overview)
The fetal circulation differs from the adult circulation due to the placenta (not the lungs) being the organ of gas exchange.
Key Shunts
| Shunt | Structure | Function |
|---|---|---|
| Foramen ovale | Opening between atria | Bypasses pulmonary circulation |
| Ductus arteriosus | Connection between pulmonary trunk and aorta | Bypasses pulmonary circulation |
| Ductus venosus | Bypass through the liver | Connects umbilical vein to IVC |
Changes at Birth
- Closure of foramen ovale: Increased left atrial pressure seals the flap
- Closure of ductus arteriosus: Increased O2 causes smooth muscle contraction
- Closure of ductus venosus: Loss of umbilical flow
The Lymphatic System and Circulation
The lymphatic system returns interstitial fluid to the circulation:
| Component | Structure | Volume |
|---|---|---|
| Lymph capillaries | Blind-ended vessels | 2-4 L/day |
| Lymphatic vessels | Valved conduits | Collecting |
| Lymph nodes | Filter stations | Immune surveillance |
| Thoracic duct | Main collecting duct | Drains lower body, left upper body |
| Right lymphatic duct | Drains right upper body | Smaller volume |
Functions
- Return of filtered plasma proteins to circulation
- Removal of interstitial fluid
- Immune surveillance
- Fat absorption (lacteals in intestinal villi)
Physiologic Shunts
Anatomic Shunts
| Shunt | Location | Direction |
|---|---|---|
| Thebesian veins | Myocardium to cardiac chambers | Left-to-right |
| Bronchial veins | Lungs to pulmonary veins | Left-to-right (anatomic) |
| Pleural vessels | Pleura to pulmonary veins | Small |
Pathologic Shunts
| Shunt | Condition | Direction |
|---|---|---|
| Ventricular septal defect | Congenital | Usually left-to-right |
| Atrial septal defect | Congenital | Usually left-to-right |
| Patent ductus arteriosus | Congenital | Left-to-right |
| Tetralogy of Fallot | Congenital | Right-to-left |
Circulatory Volumes
| Compartment | Volume (mL) | Percentage |
|---|---|---|
| Heart | 300-400 | 7-8% |
| Pulmonary circulation | 500-600 | 10-12% |
| Systemic arteries | 800-1000 | 15-20% |
| Systemic capillaries | 200-300 | 4-5% |
| Systemic veins | 3000-3500 | 60-65% |
| Total blood volume | 5000-6000 | 100% |