For an in-depth review of the circulatory system visit The Heart: An Online Exploration brought to you by the Franklin Institute.
A great way to explore this site is through its table of contents.
Circulation through the heart
As you can tell from the arrow in the diagram above, blood circulates in sequence from the body through the inferior or superior vena cava, right atrium , right ventricle, pulmonary arteries, lungs, pulmonary veins, left atrium, left ventricle, aorta, and back to the body.
Control of the heartHeart muscle or Cardiac muscle has special characteristics.
- It is striated like skeletal muscle but is entirely under autonomic control.
- Heart muscle can beat on its own tirelessly at about 45 contractions per minute.
- The branching fibers and interlocking membranes (intercalating disks) are unique to heart muscle
- It is able to respond rapidly to extrinsic or intrinsic stimuli.
Extrinsic Controls affecting heart rate involve the autonomic nervous system. If you need to dodge an incoming arrow or two the sympathetic nerves increase heart rate, and when conditions return to normal the parasympathetic nerves will slow it down.
Hormones such as adrenaline (epinephrine) can increase the rate and force of contractions while other hormones produced by pressure sensors can quiet a pounding heart.
Intrinsic controls give heart muscle the ability to contract in a rhythmic and productive fashion. A wave of nervous impulse originating from a modified muscle in a region of the right atrium called the sinoatrial (SA) node or pacemaker sweeps over the atria causing them to contract. A slight delay allows the impulse to reach the atrioventricular (AV) node immediately after blood enters the ventricles, initiating the simultaneous contraction of the ventricles.
Your pulse or the rate at which the heart beats is controlled by nerve cells in your hypothalamus. These cells set the pace of heart contraction depending on the concentration of dissolved gases in the blood and carried by hemoglobin. For instance it has been recently discovered that nitric oxide builds up when you exercise and it is this molecule that triggers heavy breathing during and after exercise.
Carbon dioxide is also an important regulator of breathing. "Vigorous exercise accelerates heartbeat in two ways;
* As cellular respiration increases, so does the carbon dioxide level in the blood. This stimulates receptors in the carotid arteries and aorta, and these transmit impulses to the medulla for relay - by the accelerator nerves - to the heart.
* As muscular activity increases, the muscle pump drives more blood back to the right atrium. The atrium becomes distended with blood, thus stimulating stretch receptors in its wall. These, too, send impulses to the medulla for relay to the heart." See Kimball's Biology Pages for more details
Circuits in the Human Circulatory System
Role of Capillaries
The BloodRed blood cells
White blood cells
How hemoglobin is involved in oxygen transport
Thanks to William Harvey, we know that the heart pumps blood; but what is responsible for lymph circulation?
When the lymph nodes become swollen with fluid, they appear--at least in some animals--to contract, according to recent research.
The lymphatic system picks up fluid and proteins that have been lost under pressure from the tips of the capillaries. Most of the fluid lost to the tissues in this way returns to the blood by osmosis; but the rest, 10 percent, and the proteins are picked up by the lymphatic vessels and dumped back into the blood in the neck region. This process maintains the fluid level of the circulatory system.
The mechanics of lymphatic circulation, however, have long been a mystery. Pumping was thought to depend on contraction of the lymph vessels, thanks especially to muscle contraction by the body's large masses of smooth muscle. Flap valves in the lymphatic vessels were known to prevent backflow.
Now, however, A. Noyan and colleagues at Hacettepe University, Ankara, have found that lymph nodes removed from calves and goats contract spontaneously in a regular pattern; those from sheep beat in a less regular pattern.
Lymph nodes, concentrated in the human body in the armpits, neck, and groin, were previously thought only to filter the lymph in order to remove dead cells, bacteria, and other unwanted materials.
(New Scientist, April 21, 1983.)
Modified April. 19, 2003