One essential process that living organisms must be able to do to survive is to provide their cells with the materials they need to function and survive -- e.g. oxygen, food, nutrients, etc. -- they must also be able to remove waste products.
Single-celled organisms (e.g. bacteria, amoebas, etc.) are surrounded by their environment -- they are separated from their environment by their plasma membrane -- can simply utilize diffusion and active transport to move materials.
Multicellularity presents a problem -- multicellular organisms may have cells that are not in direct contact with the environment -- some cells may inches away from the environment -- organisms have evolved the circulatory system to transport materials to and from cells.
Two general types of circulatory systems are found in animals:
1. open circulatory system -- primitive system found in insects and crustaceans -- blood not always contained in blood vessels -- heart pumps hemolymph (blood and cell fluids) into vessels, but vessels empty into large cavities or sinuses containing organs -- blood diffuses through tissues and back into heart.
Figure 33.3.
2. closed circulatory system -- found in higher animals -- system is "closed" because the blood is always contained in vessels.
Figure 33.3.
Vertebrate circulatory system -- a closed system -- blood contained in blood vessels.
1. heart -- muscular pumping organ that drives circulatory system.
2. blood -- transport medium -- carries gases (CO2, O2), food (glucose), hormones and wastes -- blood also functions as defense against "foreign invaders".
3. vessels -- structures through which blood flows.
a. arteries -- carry blood away from heart.
b. arterioles -- small arteries.
c. capillaries -- single cell layer thick -- exchange of materials between cells and circulatory system occurs in capillaries -- every cell in body is within diffusion distance of a capillary -- 50,000 miles of capillaries in human body -- concentration gradient between blood in capillaries and adjacent cells -- O2 and glucose diffuse from blood into cells through capillary wall, while CO2 and other wastes diffuse out of cell into blood.
d. venules -- collect waste-carrying blood from capillaries.
e. veins -- formed from joining of venules -- carry blood back to heart.
Blood -- medium of circulatory transport -- has a number of important functions:
1. transports oxygen, nutrients and other dissolved molecules to the cells.
2. carries away cellular secretions (e.g. hormones) and metabolic waste products (e.g. carbon dioxide)
3. helps stabilize internal pH -- contains proteins (buffers) that release or pickup hydrogen ions as necessary to maintain pH.
4. in warm-blooded animals (mammals and birds), blood helps maintain a constant body temperature -- carries excess heat away from muscles and other areas of high metabolic activity to the skin where heat is dissipated to the environment.
5. involved in fighting infections.
Blood is made up of several components:
1. plasma -- pH = 7.4 -- liquid portion of blood -- made up mostly of water, but also contains many different different dissolved molecules -- glucose, hormones, salts, proteins (buffers and clotting), and wastes.
2. red blood cells (erythrocytes) -- produced in bone marrow by stem cells -- have a life-span of about 120 days -- are anucleate and contain virtually nothing except the iron-containing protein called hemoglobin -- 265 million hemoglobin molecules in a single RBC -- hemoglobin binds reversibly with oxygen -- hemoglobin carries oxygen picked up in the lungs to the cells for the cells to use -- each hemoglobin molecule can carry 4 O2 molecules-- oxyhemoglobin vs. reduced hemoglobin.
3. white blood cells (leukocytes) -- larger and less abundant than RBC's (1 per 1000 RBC) -- five different types of white cells -- important in fighting infections and invasions by foreign substances -- also produced in bone marrow by stem cells.
4. platelets (thrombocytes) -- some stem cells in bone marrow give rise to giant cells called megakaryoctyes -- these cells shed fragments of cytoplasm enclosed in plasma membrane which are called platelets-- have a lifespan of 5-9 days -- responsible for initiating blood clotting -- platelets damaged by an injury release the enzyme thrombokinase which initiates a series of chemical reactions leading to the formation of a blood clot.
Figure 33.7.
Human heart -- made up of cardiac muscle tissue and surrounded by an outer membrane called the pericardium-- like all mammals, the human heart has four chambers:
1. right atrium -- receives blood coming back from tissues via two large veins.
a. superior vena cava -- blood from head, arms, chest.
b. inferior vena cava -- blood from rest of body.
2. right ventricle -- receives blood from rt. atrium and pumps blood to lungs via the pulmonary arteries -- in lungs, blood gets rid of CO2 and picks up O2 (oxygenation: reduced hemoglobin changed to oxyhemoglobin).
3. left atrium -- receives oxygenated blood from lungs via pulmonary veins.
4. left ventricle -- receives blood from left atrium and pumps it out to the body via a large artery called the aorta.
Figure 33.12.
Chambers contract (systole) and relax (diastole) in a precise sequence called the cardiac cycle - heartbeat regulated and synchronized by a node of myocardial tissue located in the rt. atrium called the sinoatrial node or pacemaker -- sound of heart beat ("lub-dub") caused by closing of valves between atria and ventricles (atrioventricular valves) and the valves between ventricles and their arteries (semilunar valves).
Atrial contractions fill ventricals -- ventricular contraction forces blood out of heart to rest of circulatory system.
Nervous system may speed up or slow down rate at which heart is beating -- heart rate center located in the medulla oblongata of brain -- amounts of O2 and CO2 and blood pressure are monitored by nervous system and serve as cues to determine rate of heart beat.
The heart’s pumping action puts pressure on blood flowing through the circulatory system -- pressure forces small amounts of water and some of the dissolved molecules in plasma out through the capillaries -- the lymphatic system reclaims this fluid and its dissolved molecules and returns them to the circulatory system.
Lymphatic system -- series of vessels and organs -- reclaim fluids forced out of capillaries and also help defend body against infection -- several components to this system:
1. lymph capillaries and vessels -- water and plasma proteins leaked out of capillary beds are taken up into lymph capillaries -- lymph capillaries merge with lymph vessels -- these converge into ducts which drain into veins at various places in the body thus returning things to circulatory system.
2. lymph nodes -- chambers located along lymph vessels -- contain two types of white blood cells called lymphocytes and macrophages -- these cells engulf cellular debris and foreign particles such as bacteria that might be in the lymph before it is returned to the bloodstream.
Figure 33.25.
Next time: Respiration and Excretion.