Circulation: The Heart

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The Animal Heart

The Animal Heart

We've already examined how nutrients are broken down and transported across membranes in the digestive system, and how gasses cross the membranes of the respiratory system into the blood stream. Now we look at how these nutrients are transported from the digestive system organs like the small intestine to other cells in the body.

Comparison of circulation systems

The variation in circulation systems among vertebrates is quite large. The number of chambers in the heart differs, with more chambers meaning higher blood pressure and faster transport for nutrients and oxygen throughout the organism. The chart below sets out some of the differences between invertebrates and the different vertebrates.

Animal type	Circulation type	Advantages/Disadvantages	Diagram of system
Invertebrates	1-2 chambered hearts, open system (no capillary connection between arteries and veins)	No separate interstitial fluid Direct diffusion of nutrients from blood to body cells Low blood pressure Small body size
Fish, juvenile amphibians	2-chambered heart, closed. Blood passes from heart through gill capillaries before dispersal to body capillaries.	Direct diffusion of gases into the blood stream via gills. Low blood pressure from heart. Circulation pressure supported by water pressure from aquatic environment. (Amphibians) Conversion of system during metamorphosis
Adult amphibians, Reptiles	3-chambered hearts	Higher blood pressure supports terrestrial activity.
Birds and Mammals	4-chambered hearts, closed systems	Higher blood pressure supports greater activity (in both) and larger size (in mammals).

Mammalian Circulatory Systems

Courtesy Wikipedia

The circulatory system consists of the blood vessels themselves and the heart which provides the motive force to move the blood through the vessels. When blood leaves the heart, it passes through the blood vessels in the following order:

aorta → arteries → arterioles → met-arterioles/capillaries → venules → veins → vena cava

The aorta and vena cava are chambers of the heart. The arteries and arterioles carry oxygenated blood outbound from the heart. The met-arterioles and capillaries allow gases, nutrients, and leucocytes to escape from the blood vessels into the body cells to perform their functions, and collect dead or dying leucocytes and cell metabolism waste products for removal. The venules and veins carry the oxygen-depleted blood and wastes back to through the kidneys and liver for processing and then back to the heart for the next cycle.

In the heart, the flow of blood goes through the heart chambers and the lungs in the following order.

vena cava → right auricle (intake) → right ventricle → pulmonary artery → lungs (for oxygenation) → pulmonary vein → left auricle → left ventricle → aorta → arteries

Notice that the heart and lung system is intimately connected: blood flows into one side of the heart, out of that side to the lungs, then back into the heart to be pumped again to the body.

The Human Heart

Courtesy Wikipedia

The human heart is comprised of cardiac muscle, an involuntary muscle which you cannot control directly. It has four chambers connected by valves which control blood flow, ensuring that it occurs at the right times and in the right direction. Congenital (inherited) heart disease usually involves a defect in one or more valves which allows blood to flow backwards into a heart chamber, reducing heart efficiency.

The heart contracts at regular intervals which are controlled by nerve tissue at the sinoatrial node in the heart. Under certain circumstances, the medulla in the brain may take over, releasing or reducing the amount of norepinephrine and acetylcholine in the blood stream. When these hit the heart muscles, they respond with rapid heartbeat (during flight from predators, planned exercise, and panic situations such as taking an exam) or slow down the heart rate (during sleep).

The cardiac output rate is a measure of the volume of blood moved in a given amount of time; in a healthy individual, this is about 5 liters or 1.25 gallons per minute. In extreme conditions such as heavy exercise, the rate can increase to four times the normal output.

Blood Pressure

As blood is pumped out of the heart, it exerts pressure on the walls of the blood vessel system. The highest pressure occurs as the blood enters the aorta, and gets less as the blood moves further and further from the heart. By the time the blood is starting to move back to the heart through the veins, blood pressure is so low that the impulse from the heart can no longer move the blood. That's why the walls of the blood vessels are actually smooth muscle tissue, which can contract slightly to squeeze the blood and move it along. Veins also have valves which prevent backflow, so that the blood is always forced toward the heart, even uphill against gravity as the blood flows from your feet up your legs.

Blood pressure in humans is expressed as a ratio between the pressure during a heart contraction (systol) and the pressure when the heart is relaxed (diastole). Normal, healthy adult humans have a ratio of about 120/70. Blood pressures higher than 140/90 or lower that 100/70 are considered abnormal. High blood pressures indicate that the heart is doing too much work to pump the blood through the body, and is a source of concern. Low blood pressure means that the heart is operating efficiently; in some cases, however, it may mean that during heavy exercise, the heart will not be able to pump quickly enough to meet the demand for oxygen, and the person will faint.

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