The circulatory path of the blood is:
Beginning with the left ventricle
, blood is pumped through the aorta
. From the aorta, branch many smaller arteries
, which themselves branch into still smaller arterioles
, which branch into smaller capillaries
. Blood from the capillaries is collected into venules
, which themselves collect into larger veins
, which collect again into the superior and inferior vena cava
. The vena cava empty into the right atrium of the heart
. This first half of the circulation as just described is called the systemic circulation
From the right atrium, blood is squeezed into the right ventricle
. The right ventricle pumps blood through the pulmonary arteries
, to arterioles, to the capillaries of the lungs. From the capillaries of the lungs, blood collects in venules, then in veins, and finally in the pulmonary veins
leading to the heart. The pulmonary veins empty into the left atrium
, which fills the left ventricle. This second half of the circulation is called the pulmonary circulation
Since there are no opening for the blood to leave the vessels, the entire system is said to be a closed circulatory system
The left ventricle contracts with the most force to propel the blood through the systemic circulation.
occurs when the ventricles contract; distole
occurs during relaxation of the entire heart and then contraction of the atria.
The blood is propelled by hydrostatic pressure created by the contraction of the heart. The rate of these contractions is controlled by the autonomic nervous system, but the autonomic nervous system does not initiate the contractions. The heart contracts automatically, paced by a group of specialized cardiac muscle cells called the sinoatrial node (SA node)
located in the right atrium. The SA node contrats, spreading its contractions to the surrounding cardiac muscles via electrical synapses
made from gap junctions
The vagus nerve
innervates the SA node, slowing the contractions while also increasing digestive activity in the intestines.
The action potential branches out through the ventricular walls via conductive fibers called Purkinje fibers
. The Purkinje fibers in the ventricles allow for a more unified, stronger contraction.
- Arteries are elastic, and stretch as they fill with blood. Arteries are wrapped in smooth muscle. Epinephrine is a powerful vasopressin causing arteries to narrow.
- Arterioles are very small. Constriction and dilation of arterioles can be used to regulate blood pressure as well as rerouting blood.
are microscopic blood vessels. Capillary walls are only one cell thick. Nurtient and gas exchange with any tissue other than a vascular tissue takes place only across capillary walls, and not across arterioles or venules.
There are four ways for materials to cross capillary walls: 1) pinocytosis 2) diffusion or transport through capillary cell membranes 3) movement through pores in the cells called fenestrations 4) movement through the space between the cells.
As blood flows into a capillary, hydrostatic pressure is greater than osmotic pressure, and net fluid flow is out of hte capillary. Osmotic pressure will overcome hydrostatic pressure near the venule end of a capillary, and net fluid flow is into the capillary and out of the interstitum.
- Blood velocity is inversely proportional to the cross-sectional area. The blood moves the slowest through the capillaries. To compensate for low blood pressure, veins have a valve system that prevents the back flow of blood. Pulmonary arteries contain the most deoxygenated blood in the body.
- Blood pressure increases near the heart and decreases to its lowest in capillaries.