P r a c t i c e
1. Which of the following is NOT one of the cham-
bers in the four-chambered vertebrate heart?
a. right atrium b. right ventricle c. left alveolar d. left ventricle
2. Which of the following is true about blood flow
in the four-chambered vertebrate heart circula- tory system?
a. Blood in the pulmonary vein is oxygenated. b. Blood in the pulmonary artery is oxygenated. c. Blood in the aorta is not oxygenated.
d. Blood in the vena cava is oxygenated. 3. Which of the following two items are the major
components of blood?
a. proteins and lipids b. plasma and cells c. proteins and platelets d. cells and lipids
4. Platelets perform which of the following
functions?
a. blood clotting b. carrying oxygen
c. carrying carbon dioxide d. disease protection
5. Capillary beds occur between a. arteries and veins.
b. aortas and vena cavas. c. arterioles and venules. d. atria and ventricles.
6. Red blood cells perform which of the following
functions? a. blood clotting b. carrying oxygen c. disease protection d. wound healing –C I R C U L AT I O N : T H E C A R D I O VA S C U A L R S Y S T E M–
A n s w e r s
1. c. The four-chambered vertebrate heart is made
up of the right and left ventricles and the right and left atria. The atria receive blood from the body (the vena cava) and the lungs (the pulmonary vein). The ventricles push blood out of the heart to the body (the aorta) or the lungs (the pulmonary artery).
2. a. Remember that all veins carry blood toward
the heart and all arteries carry blood away from the heart. Thus, usually arteries (like the aorta) will be carrying oxygenated blood, and veins (like the vena cava) will be carrying blood that is not oxygenated. However, in the heart-to-lung part of the system, the vessel carrying blood away from the heart and toward the lungs is an artery (the pulmonary artery), but it is carrying blood that is not oxygenated because that’s why it’s going to the lung (to become oxygenated). Likewise, the vessel carrying blood from the lungs to the heart (the pulmonary vein) is carrying blood that has been oxygenated.
3. b. Blood is a suspension of the fluid we call
plasma and the cells that are suspended throughout it. The cells are the red and white blood cells and the platelets.
4. a. Platelets aid in the clotting of blood to seal off
a wound and prevent blood loss or infection.
5. c. Capillary beds are between the very small
arteries called arterioles and the very small veins called venules. Capillary beds are the site of gas exchange in the circulatory system.
6. b. Red blood cells carry oxygen, but they also
carry carbon dioxide.
I n S h o r t
The cardiovascular system, sometimes also called the circulatory system, has three main parts. From the name, we can see two of those parts mentioned. “Car- dio” refers to the heart, which is the pump in the sys- tem. “Vascular” refers to vessels, specifically the blood vessels that provide a route for fluids in the system. The third component is a very specialized fluid called blood that serves as the means of transport for nutrients and oxygen, as well as wastes. Arteries lead blood away from the heart, and veins return blood toward the heart. Capillary beds are the site of exchange for gases, nutrients, and wastes. Blood is a special fluid with red and white cells and platelets suspended in it.
H o m e o s t a s i s
All organisms try to live in a favorable environment, but even such an environment will have conditions that fluctuate over time. Organisms must be able to respond to these changes and yet still maintain a relatively con- stant internal environment within their bodies. For instance, they must maintain a balance of water, tem- perature, and salt concentration. The series of physical and chemical processes that work to maintain an internal equilibrium is called homeostasis. We first mentioned homeostasis in the circulatory system lesson. The circulatory system and the filtration/excretion system work very closely together to help maintain homeostasis. L E S S O N
Filtration and
Excretion:
The Renal
System
The chemical reactions of metabolism that occur in the cells of organ- isms produce waste substances that must be excreted. One-celled organisms can excrete toxic substances by diffusion through their cell membranes or by using specialized organelles called vacuoles, whereas multicellular organisms have developed special organ sys- tems to accomplish the same task. The circulatory and excretory sys- tems work together to eliminate metabolic wastes.
F i l t r a t i o n a n d E x c r e t i o n
When you eat food, some of it is indigestible and must be eliminated by the digestive tract. However, the digestible part is broken down and eventually absorbed as very small molecules. These molecules are transported to the cells by the blood. The cells then carry out the biochemical reactions of metabolism, which results in wastes such as carbon dioxide. The carbon dioxide is carried by the blood to the lungs where it is exhaled, but other wastes are produced dur- ing cellular metabolism besides carbon dioxide. For the most part, these wastes cannot be eliminated in the lungs. Instead, they must be filtered out of the blood and then excreted.
The Kidneys as Filters
The blood delivers wastes such as ammonia, which comes from the metabolism of amino acids in cells, to the liver where they are converted into urea. The blood then carries the urea to the organs in the lower back called the kidneys. The kidneys are bean-shaped (they look very much like a kidney bean, in fact!) and are about the size of your fist. They are part of the renal system, which also includes the ureters and urethra, two tubes that carry the liquid wastes out of the body. The urinary bladder, a holding place for urine, is also part of this system.
The kidneys are sophisticated filters. They are able to take urea, which is less toxic than ammonia but still not well tolerated in high concentrations, and con- vert it into urine, which is very soluble in water and can be excreted. The kidneys also regulate the amount
of water that is used in this process in order to prevent body dehydration. Kidneys also maintain proper levels of a number of substances in the body (including sodium, potassium, chloride, calcium, glucose sugar, and amino acids) by reabsorbing them from urine before it is excreted. The kidneys also help maintain blood pressure and the acidity (pH) level of the blood. It is obvious that the kidneys are important regulators and maintainers of bodily homeostasis.
How Does a Kidney Work?
In each kidney are at least a million individual units called nephrons. Functionally, nephrons are similar to the alveoli in the lungs. The alveoli are structured to function as gas exchange interfaces, whereas the kidney nephrons are structured to function as fluid inter- change points.
Each nephron consists of a bed of capillaries with thin walls surrounded by a tube structure called Bow- man’s capsule. Filtration of the blood occurs for water, nutrients, and wastes through the capillaries into the Bowman’s capsule. Most of the water and nutrients are reabsorbed right away. This concentrates the wastes in the fluid inside the Bowman’s capsule tubules. We now call this fluid urine.
The tubules leading away from Bowman’s cap- sule eventually arrive at the collecting duct where even more water may be absorbed. The collecting duct leads to the interior of the kidney where urine collects and flows into the ureters, which take it to the urinary blad- der. Urine will collect in the urinary bladder until the urge to urinate is strong enough that the urine is expelled from the body through the urethra.
Part of what the kidneys do is regulate the amount of water that circulates in the bloodstream. If the brain detects low levels of water in the blood, then more antidiuretic hormone (ADH) is released. As its name implies, it causes the kidneys to reabsorb water into the bloodstream, thus concentrating the urine and preserving water for the body.
The brain is very good at keeping a balance on many interrelated factors. Our nervous system responds to sensors in the body that keep track of blood sugar, blood pressure, blood carbon dioxide, blood oxy- gen, blood dissolved salts, and so on. Lack of water affects all these values, which is how the body detects it.
When you drink too much alcohol, you usually urinate more often. This is because alcohol inhibits the action of the ADH signal from the brain. Thus, nothing tells the kidneys to conserve water.