General Biology/Tissues and Systems/Circulatory System
Circulatory system functions
a. Respiration: gas exchange (O2 and CO2), overcomes limited rate of
c. Excretory: (remove metabolic wastes)
a. Transport hormones
b. Regulate body temperature
i. Blood clotting
ii. Immune system (carries white blood cells)
Vasodilation: allows heat loss across epidermis, as seen in elephant ears, takes more blood to surface of body, sweating may accompany
Countercurrent heat exchange: used by dolphins in fins to conserve heat in cold water. Veins surround an artery, and blood returning to body absorbs heat from blood traveling out from body to fin, minimizing heat loss. Used by dogs in feet, etc.
Blood made of
1. plasma and
2. formed cellular elements (red and white blood cells, and platelets).
Plasma makes up 55% of blood volume. Cellular elements make up the other 45%.
Plasma makeup: 90% water, 7-8% soluble proteins (albumin maintains blood osmotic integrity, others clot, etc.) 1% electrolytes 1% elements in transit
Red blood cell (erythrocyte): contains hemoglobin, functions in oxygen transport. In mammals, red blood cells lose nuclei on maturation, and take on biconcave, dimpled, shape. No self repair, live 120 days. About 1000x more red blood cells than white blood cells. About 7-8 micrometers in diameter.
Hematocrit: proportion of blood volume that is occupied by cells, about 43% in humans on average. 48% for men and 38% for women.
White blood cells (leukocytes): Nucleated, about 10-14 micrometers in diameter, commonly amoeboid, escape circulatory system in capillary beds. Include basophils, eosinophils, neutrophils, monocytes, B- and T-cell lymphocytes.
Platelets (thrombocytes) Membrane bound cell fragments in mammals, no nucleus. In non-mammals, platelet role replaced by nucleated cells. Accumulate at site of broken blood vessels, form clots. Bud off special cells in bone marrow. 1-2 micrometers in diameter. 7-8 day life span, 1/10 or 1/20 as abundant as white blood cells.
Arteries: carry blood away from heart. Smallest tubes called arterioles, feed blood to capillaries.
Veins: return blood to heart. Smallest veins called venules.
Structure of arteries and veins, listed from inside (lumen) out:
1. epithelium (endothelium),
2. elastic connective tissue fibers,
3. smooth muscle,
4. connective tissue. Arteries have thicker elastic layer than do veins.
Capillaries, where exchange of materials occurs, are very thin and narrow, and red blood cells pass through single file. Capillaries are tiny but numerous, and their total volume is greater than that of supplying arteries.
Blood velocity drops in capillaries, picks back up in veins. Pressure highest in arteries, lower in capillaries and arteries.
Osmotic pressure draws interstitial fluid from blood in arterioles, but replaces it in venules.
One-way valves mean that blood can flow only one way, works with residual blood pressure and compression by skeletal muscles. Low pressure in thoracic cavity caused by breathing also helps move blood.