The Circulatory System Blood Vessels

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