Introduction to the Circulatory System

Definitions

• Circulatory system: made up of the cardiovascular system and the lymphatic system

• Cardiovascular system: made up of the arterial system, the heart, and the venous system

The heart

• Pulmonary circulation: right side of heart → lungs → left side of heart

• Systemic circulation: left side of heart → capillary beds of organs and tissues → right side of heart

Layers of the heart

• Epicardium (external): visceral serous pericardium

• Myocardium (middle): cardiac muscle layer

• Endocardium (internal): continuous with endothelium of blood vessels connecting with the heart

Cardiac valves

Conduction system of the heart

1. Electrical impulse starts at SA node - atria contract

2. Impulse travels to AV node at AV septum

3. Impulse travels down left and right bundles in the IV septum

4. Impulse spreads out to myocardium through conducting fibers - ventricles contract

The arterial system

• Each artery (and its branches) carries oxygenated blood to a territory

Structure of blood vessels

• Tunica intima (internal): endothelium

• Tunica media (middle): smooth muscles and elastic fibres

• Tunica adventitia (external): connective tissue

Sympathetic tone

• Sympathetic tone: background, low level contraction of smooth muscle in arterioles

• Due to tonic production of action potentials to arterioles by sympathetic nerves

Anastomoses, end arteries and infarction

• Anastomoses: where arteries connect to each other without an intervening capillary network
• Provides alternative routes for blood flow to supply cells distal to an arterial occlusion
• Each alternate route given by an anastomosis is called a collateral

• End artery: the only arterial blood supply to a given area of the body

• Infarction: irreversible cell death due to hypoxia caused by loss of arterial bloody supply
• Result of untreated occlusion of an end artery

The aorta and its branches

• Left and right coronary arteries supply blood to heart muscle

• Branches of the arch of the aorta supply blood to the upper limbs and head

• Descending aorta supplies thorax and abdomen

• Within the abdomen, the abdominal aorta branches into the two common iliac arteries that provide blood to the pelvis and lower limbs

The venous system

• Veins drain deoxygenated blood from a territory

Venous return

• Venous valves

• Skeletal muscle pump

• Accompanying veins

Structure of the venous system

• Superficial veins: smaller, run within the superficial fascia, drain into deep veins

• Deep veins: larger, run deep to the deep fascia in cavities, often in neurovascular bundles

• Hepatic portal venous system: drains venous blood from absorptive parts of the GI tract and associated organs for cleaning

• Systemic venous system: drains venous blood from all of the organs and tissues into the superior or inferior vena cavae

The lymphatic system

• Lymphatic capillaries collect tissue fluid (lymph)

Lymphatic circulation

1. Lymphatics carry lymph through lymph nodes

2. Eventually lymph is returned into the central veins in the root of the neck
1. Lymph from superficial lymphatics → deep lymphatics
2. Lymph from right lymphatic duct → right venous angle
3. Lymph from thoracic duct → left venous angle