Blood Vessels

BSCI202: Anatomy & Physiology II

Objectives

• Describe the three types of blood vessels and their histology.

• Describe the tunics of blood vessel walls and the function of each layer.

• List the structural differences between artery, capillary, and vein.

• Identify specified blood vessels on models and figures.

• Discuss the unique features of special circulations:

  • Coronary circulation

  • Pulmonary circulation

  • Hepatic portal system

  • Cerebral circulation (circle of Willis)

  • Fetal circulation

The Circulatory System

• Defined as a closed transport system.

• Blood flow pathway:

  • Heart → large arteries → medium arteries → arterioles → capillary beds → venules → medium veins → large veins → heart

• Function of the capillary walls:

  • Oxygen (O2) and nutrients diffuse from blood to tissues.

  • Waste products and carbon dioxide (CO2) diffuse from tissues to blood.

Microscopic Structure of Blood Vessels

• Comprised of three tunics surrounding the lumen:

  1. Tunica intima/interna

  • Innermost layer.

  • Consists of a thin layer of endothelium with a connective tissue (CT) basement membrane.

  1. Tunica media

  • Middle layer.

  • Composed of circularly arranged smooth muscle and elastic connective tissue.

  • Functionality: Allows for changes in lumen diameter.

  1. Tunica adventitia/externa

  • Most superficial layer.

  • Provides protection and anchors vessels.

  • Made up of areolar or fibrous connective tissue.

Types of Blood Vessels

• Arteries: Transport blood away from the heart.

  • Expand during systole and recoil passively during diastole.

  • Types of arteries:

  • Elastic arteries:

    • Found near the heart; large and labeled as "conducting vessels"; highly expandable.

  • Muscular arteries:

    • Medium-sized and described as "distributing vessels"; include named arteries of the body.

  • Arterioles:

    • Smallest arteries; known as "resistance vessels".

• Capillaries:

  • Provide for the exchange of materials in tissues.

  • Narrowest vessels with walls composed solely of endothelium (one cell layer thick).

• Veins: Transport blood toward the heart.

  • Contain approximately 65% of total blood volume.

  • Low-pressure vessels due to thin walls and large lumens.

  • Adaptations for venous return:

  • Valves: Formed by folds of tunica intima, preventing backflow.

    • Highly concentrated in the limbs.

  • Skeletal muscle pump: Skeletal muscle contractions squeeze veins.

  • Respiratory pump: During inspiration, there is a decrease in intrathoracic pressure and an increase in intra-abdominal pressure, which promotes blood flow from abdominal veins to thoracic veins.

The Aorta

• Aorta: Largest artery in the body.

  • Originates from the left ventricle of the heart.

  • Regions of the aorta:

  • Ascending aorta: Leaves the left ventricle.

  • Aortic arch: Arches to the left.

  • Thoracic aorta: Descends through the thorax.

  • Abdominal aorta: Passes through the diaphragm into the abdominopelvic cavity.

Arterial Branches

Ascending Aorta and Aortic Arch

• Ascending Aorta:

  • Right and left coronary arteries serve the heart.

• Aortic Arch:

  • Brachiocephalic trunk: Splits into:

  • Right common carotid artery

  • Right subclavian artery

  • Left common carotid artery: Splits into left internal and external carotid arteries.

  • Left subclavian artery: Branches into the left vertebral artery and left internal thoracic artery.

  • Subclavian artery transitions into axillary artery → brachial artery → radial and ulnar arteries.

Arterial Branches of the Descending Thoracic Aorta

• Intercostal arteries: Supply the muscles of the thoracic wall.

• Bronchial arteries: Supply the lungs.

• Esophageal arteries: Serve the esophagus.

• Phrenic arteries: Serve the diaphragm.

Arterial Branches of the Abdominal Aorta

• Celiac trunk: First branch of the abdominal aorta with three components:

  • Left gastric artery: Serves the stomach.

  • Splenic artery: Serves the spleen.

  • Common hepatic artery: Serves the liver.

• Superior mesenteric artery: Supplies most of the small intestine and the proximal half of the large intestine.

• Left and right renal arteries: Serve the kidneys.

• Left and right gonadal arteries: Known as ovarian in females and testicular in males.

• Lumbar arteries: Supply the abdominal muscles and trunk.

• Inferior mesenteric artery: Supplies the distal half of the large intestine.

• Left and right common iliac arteries: Final branches of the aorta.

  • Internal iliac arteries: Serve the pelvic organs.

  • External iliac arteries: Enter the thigh → femoral artery → popliteal artery → anterior and posterior tibial arteries.

Special Circulations

Coronary Circulation

• Blood in heart chambers does not nourish the myocardium (heart muscle).

• The heart has its own circulatory system:

  • Coronary arteries: Branch from the aorta to supply the heart muscle with oxygenated blood.

  • Cardiac veins: Drain the myocardium of blood.

  • Coronary sinus: A large vein on the posterior side of the heart that receives blood from cardiac veins.

  • This blood empties into the right atrium via the coronary sinus.

Features of Coronary Circulation

• Arteries originate behind aortic semilunar valves.

• Interarterial anastomoses: Ensure a continued blood supply despite partial occlusions of individual arteries.

Pulmonary Circulation

• Capillary beds around alveoli facilitate gas exchange:

  • Exchange carbon dioxide (CO2) for oxygen (O2).

• Nourishing blood supply for lungs is provided by bronchial arteries, distinct from pulmonary circulation.

Hepatic Portal System

• Hepatic portal circulation drains from:

  • Digestive organs

  • Spleen

  • Pancreas

• Major vessels:

  • Inferior and superior mesenteric veins

  • Splenic vein

  • Left gastric vein

• Capillary beds in the digestive system absorb nutrients and toxins.

• Hepatic portal vein: Carries this blood to the liver.

  • The liver helps maintain normative concentrations of glucose, fat, and protein in blood.

  • The liver also acts as a filter for toxins from the blood.

• Blood drains into the hepatic vein as it exits the liver.

Cerebral Arterial Circle (Circle of Willis)

• Also called the circle of Willis.

• Internal carotid arteries divide into anterior and middle cerebral arteries, supplying most of the cerebrum.

• Vertebral arteries merge within the skull to form the basilar artery.

  • The basilar artery serves the brain stem and cerebellum.

• Posterior cerebral arteries arise from the division of the basilar artery, supplying the posterior cerebrum.

• Anterior and posterior blood supplies are interconnected via small communicating arterial branches.

Vena Cava

• Superior vena cava and inferior vena cava enter the right atrium of the heart.

  • Superior vena cava drains the head and arms.

  • Inferior vena cava drains the lower body.

Veins Draining into the Superior Vena Cava

• Arm drainage pathway:

  • Radial and ulnar veins → brachial vein → axillary vein.

• Cephalic vein: Drains the lateral aspect of the arm into the axillary vein.

• Basilic vein: Drains the medial aspect of the arm into the brachial vein.

• Median cubital vein: Connects basilic and cephalic veins at the elbow.

• Subclavian vein: Receives blood from the axillary vein; also drains skin and muscles of the head and neck via the external jugular vein.

  • Vertebral vein: Drains the posterior part of the head.

  • Internal jugular vein: Drains dural sinuses of the brain.

• Left and right brachiocephalic veins collect blood from:

  • Subclavian veins

  • Vertebral veins

  • Internal jugular veins

• Brachiocephalic veins join to form the superior vena cava.

• Azygous vein: Drains the thoracic cavity.

Veins Draining into the Inferior Vena Cava

• Tibial veins (anterior and posterior) drain the legs.

• Posterior tibial vein → popliteal vein → femoral vein → external iliac vein.

• Great saphenous vein: Longest vein in the body; receives superficial drainage from the legs.

• Each common iliac vein (left and right) results from the merger of internal and external iliac veins.

• Right gonadal vein: Drains the right ovary in females and right testicle in males.

  • Left gonadal vein: Empties into the left renal vein.

• Left and right renal veins: Drain the kidneys.

• Hepatic portal vein: Drains digestive organs and travels through the liver before entering systemic circulation.

• Left and right hepatic veins: Drain the liver.

Special Circulations: Fetal Circulation

• Fetal circulation: Fetus exchanges gases, nutrients, and wastes through the placenta.

• Umbilical cord: Contains three vessels.

  • Umbilical vein: Carries nutrient- and oxygen-rich blood to the fetus.

  • Umbilical arteries: Two vessels carrying carbon dioxide and debris-laden blood from the fetus to the placenta.

• Blood flow bypass:

  • Liver: Through the ductus venosus and enters the inferior vena cava.

  • Lungs: Blood entering the right atrium is shunted directly into the left atrium through the foramen ovale.

  • Ductus arteriosus: Connects the aorta and pulmonary trunk (becomes ligamentum arteriosum at birth).

Development of the Circulatory System in the Fetus

• A simple "tube heart" (without chambers) develops in the embryo, starting to pump by the fourth week post-fertilization.

• The heart develops into a four-chambered organ by the end of the seventh week post-fertilization.

• Few structural changes occur after the seventh week post-fertilization.

Key Vasculature in Fetal Circulation Illustration

• Key structures in the fetal circulatory system include:

  • Aortic arch

  • Superior vena cava

  • Ductus arteriosus → Ligamentum arteriosum

  • Pulmonary artery

  • Pulmonary veins

  • Heart

  • Liver

  • Ductus venosus → Ligamentum venosum

  • Hepatic portal vein

  • Umbilical vein

  • Inferior vena cava

  • Umbilicus

  • Common iliac artery

  • Umbilical arteries

• Each vessel plays a crucial role in efficiently delivering oxygen and nutrients, as well as removing waste, ensuring fetal health pre-birth.