Lecture 14

The Circulatory System – Blood Vessels

General Anatomy of Blood Vessels

  • The Vessel Wall consists of multiple layers
  • The tunica interna lines the inside of the vessel and is exposed to blood
  • It consists of simple squamous epithelium called endothelium overlying a basement membrane
  • The endothelium acts as a selectively permeable barrier to materials entering or leaving the bloodstream
  • It secretes chemicals that stimulate the muscle of the vessel wall to contract or relax
  • The tunica media is the middle layer, and it’s usually the thickest
  • It consists of smooth muscle, collagen, and sometimes elastic tissue
  • It strengthens the vessel and provides for changes in vessel diameter
  • The tunica externa is the outermost layer
  • It consists of loose connective tissue
  • It anchors the vessel and provides passages for small nerves, lymphatic vessels, and smaller blood vessels
  • Arteries
  • These are blood vessels that carry blood away from the heart
  • They have relatively strong, resilient tissue to withstand high blood pressure surges
  • Categories
  • Conducting arteries
  • These are the largest arteries
  • The tunica media consists of layers of elastic sheets alternating with layers of smooth muscle, collagen, and elastic fibers
  • These arteries expand during ventricular systole to receive blood and recoil during diastole
  • This expansion takes pressure off the blood, so that there is less stress on arteries downstream
  • Distributing arteries
  • These are branches that distribute blood to specific organs
  • The smooth muscle is more conspicuous than the elastic tissue
  • Resistance arteries
  • These are small arteries that are too variable in number and location to have individual names
  • The smallest of these are called arterioles
  • Arterial Sense Organs
  • Carotid sinuses
  • These are baroreceoptors that respond to changes in blood pressure
  • They are located in the wall of the internal carotid artery, just above the branch point
  • Carotid bodies
  • These are chemoreceptors that monitor changes in blood composition
  • They monitor pH, CO2, and O2
  • Aortic bodies
  • These are similar to carotid bodies but found in the aortic arch
  • Capillaries
  • For materials such as nutrients, wastes, and hormones to pass between blood and tissue fluids, they must pass through the walls of capillaries (or venules, which are less common and less permeable)
  • They are composed of only an endothelium and basement membrane
  • Most cells of the body are within a few cell widths of a capillary
  • Types of capillaries
  • Continuous capillaries occur in most tissues
  • The endothelial cells are held together by tight junctions to form an uninterrupted tube
  • Endothelial cells are separated by narrow intercellular clefts that allow small solutes to pass through, while holding back plasma proteins and formed elements of the blood
  • Some continuous capillaries have pericytes that contain contractile proteins which contract to regulate blood flow
  • Fenestrated capillaries are found in organs where rapid absorption or filtration occurs, such as the kidneys or small intestine
  • The contain filtration pores that allow for rapid passage of small molecules but still retain larger proteins and larger particles in the bloodstream
  • Sinusoids are irregular blood-filled spaces in the liver, bone marrow, spleen, and some other organs.
  • The endothelial cells are separated by wide gaps, and the cells also have large fenestrations through them
  • Even proteins and blood cells can pass through these pores.
  • Sinusoids may contain macrophages or other specialized cells
  • Capillary beds
  • Capillaries are organized into groups called capillary beds
  • When precapillary sphincters are open, the capillaries are filled with blood and engage in exchanges with tissue fluid
  • When the sphincters are closed, the blood bypasses the capillaries
  • In skeletal muscles, 90% of the capillaries have little to no flow during periods of rest
  • During exercise, blood flow to the skin and intestines is reduced
  • Veins
  • Veins are relatively thin walled
  • They are subjected to relatively little pressure
  • An arteries, pressure may surge to 120 millimeters of mercury
  • In veins, it averages about 10 mmHg
  • Many medium sized veins have venous valves
  • Venous valves are infoldings of the tunica interna that meet in the middle of the lumen
  • They allow blood to flow in only one direction
  • The pressure of blood in the veins is not high enough to push up towards the heart in a standing or sitting person, so the upward flow of blood depends on the actions of skeletal muscles
  • When the skeletal muscles that surround a vein contract, they force blood through the valves
  • Circulatory Routes
  • The simplest and most common route of blood flow is:
  • Heart  arteries  capillaries  veins  heart
  • In a portal system, blood flows through two consecutive capillary networks before returning to the heart
  • These systems occur in the kidneys, between hypothalamus and pituitary, and between the intestines and the liver

Anatomy of the Pulmonary Circuit

  • The system begins with the pulmonary trunk, which ascends from the right ventricle
  • The pulmonary trunk branches into right and left pulmonary arteries
  • The pulmonary arteries branch and connect to the lungs, where blood unloads CO2 and loads O2
  • Blood then leaves the lungs to return to the heart through pulmonary veins

Anatomy of the Systemic Arteries

  • The Aorta and its Major Branches
  • The ascending aorta rises from the left ventricle
  • The aortic arch curves like an inverted U
  • There are three branches from the aortic arch
  • The brachiocephalic trunk
  • The left common carotid artery
  • The left subclavian artery
  • The descending aorta proceeds down behind the heart in two parts
  • The thoracic aorta is above the diaphragm
  • The abdominal aorta is below the diaphragm
  • Arterial Supply to the Head and Neck
  • Origins of head-neck arteries
  • The common carotid arteries branch from the brachiocephalic trunk on the right side and from the aortic arch on the left side
  • The vertebral arteries arise from the right and left subclavian arteries and run through the transverse foramina of the cervical vertebrae
  • Continuation of the head-neck arteries
  • The external carotid arteries ascend to the side of the head external to the cranium
  • The internal carotid arteries enter through the carotid foramina and supplies 80% of the blood to the crebrum
  • Arterial Supply to the Upper Limb
  • The subclavian artery travels under the clavicle and above the first rib
  • The axillary artery is a continuation of the subclavian artery
  • The brachial artery is a continuation of the axillary artery and continues down the brachium
  • Two arteries branch from the brachial artery
  • The radial artery follows the radius
  • The ulnar artery follows the ulna
  • Arterial Supply to the Thorax
  • Bronchial arteries extend from the thoracic aorta to the lungs (systemic circulation only)
  • Esophageal arteries extend from the thoracic aorta to the esophagus
  • (Posterior) intercostals arteries run from the thoracic aorta to the posterior portion of the thoracic cavity between the ribs
  • The superior phrenic arteries run from the thoracic aorta to the superior aspect of the diaphragm
  • Arterial Supply to the Abdomen
  • Inferior phrenic arteries supply blood to the inferior surface of the diaphragm
  • Celiac trunk branches to upper abdominal viscera
  • Splenic artery goes to the spleen
  • Left gastric artery goes to the stomach
  • Hepatic artery goes to the liver
  • Superior mesenteric artery supplies the intestines
  • Suprarenal arteries supply the adrenal glands
  • Renal arteries supply the kidneys
  • Gonadal arteries supply the gonads
  • Inferior mesenteric artery supplies the distal end of the large intestine
  • Common iliac arteries supply the urinary and reproductive organs and lower limbs
  • Arterial Supply to the Pelvic Region and Lower Limb
  • The internal iliac artery supplies the pelvic wall and viscera
  • The external iliac artery supplies mainly the lower limb
  • The femoral artery passes through the femoral triangle to through the thigh

Anatomy of the Systemic Veins

  • Venous Drainage of the Head and Neck
  • The internal jugular receives blood from most of the brain
  • The external jugular vein drains superficial structures
  • The vertebral vein drains the cervical vertebrae and spinal cord
  • Veinous Drainage of the Upper Limb
  • The cephalic vein winds around the radius as it travels up the forearm and continues into the brachium
  • The basilic vein travels up the posterior aspect of the forearm and continues into the brachium
  • The axillary vein continues both the cephalic vein and basilic vein
  • The Azygos System
  • The right ascending lumbar vein becomes the azygos vein in the thorax
  • The azygos vein receives blood from the right posterior intercostal veins which drain the chest muscles
  • The left ascending lumbar continues into the thorax as the hemiazygous vein
  • The hemiazygous vein receives blood from posterior intercostals veins and some esophageal veins
  • Major Tributaries of the Inferior Vena Cava
  • The inferior vena cava is formed by the union of the right and left common iliac veins
  • The gonadal veins empty into the inferior vena cava
  • The renal veins drain the kidneys into the inferior vena cava
  • The suprarenal veins drain the adrenal glands
  • The hepatic veins drain the liver
  • The inferior phrenic veins drain the inferior aspect of the diaphragm
  • The Hepatic Portal System
  • The hepatic portal system connects capillaries of digestive organs to the hepatic sinusoids of the liver
  • The hepatic portal vein carries blood to the liver to provide it with nutrients
  • The system also allows the blood to be cleansed of bacteria and toxins within the toxin prior to sending it to the rest of the body
  • Venous Drainage of the Lower Limbs and Pelvic Organs
  • The external iliac vein is formed by the union of the femoral vein and great saphenous vein
  • The internal iliac vein drains the gluteal muscles, the medial aspect of the urinary bladder, the and some reproductive organs

Fetal Circulation and changes after birth

  • The fetus has certain shunts by which most blood bypasses the nonfunctional lungs
  • The foramen ovaleis an opening between the two atria
  • The ductus arteriosus is a short vessel from the left pulmonary artery to the aorta
  • Most of the blood that the right ventricle pumps into the pulmonary trunk takes this bypass directly to into the aorta instead of following the usual path to the lungs
  • After birth, when th;.e lungs are functional, these shunts close, leaving a fossa ovalis in the interatrial septum and ligamenumt artriosum between the aortic arch and left pulmonary artery
  • In the fetus, a shunt called the ductus venosus bypasses the liver which is not very functional before birth
  • This vein caries blood returning from the placenta through the umbilical vein
  • After birth, the ductus venosus constricts and blood is forced to flow through the liver