The Mammalian Circulatory System

The Cardiac Cycle

• Cardiac cycle defined:
  • One complete sequence of pumping and filling of the heart
  • Systole: contraction phase of the cycle
  • Diastole: relaxation phase of the cycle
• Duration of the cycle:
  • About 0.8 seconds
  • This results in an average heart rate of 70 beats/minute.
  • An average heart beats for 75 years (about 3 billion beats) without stopping. No human made machine can do this with no maintenance.

Figure 42.6, page 816, Campbell's Biology, 5th Edition

Cardiac Output

• Cardiac output depends on two factors:
  • Heart rate
  • Stroke volume
    • The amount of blood the heart pumps each cycle (i.e. every 0.8 seconds)
    • The average stroke volume is 75 mL (¹∕₃ cup).
• Total cardiac output = 70 ^beats∕_min × 75 ^mL∕_beat = 5 250 ^mL∕_min = 5.25 ^L∕_min.
  • The average heart pumps the equivalent of five 1-L drinks each minute.
  • The average amount of blood in a person is also 5 L—every minute, all the blood has circulated through the body.
  • During heavy exercise this output can increase five fold, i.e. it will circulate much more often.

What Causes the Heart to Beat?

• Sinoatrial node (SA node or pacemaker)
  • A collection of nervous tissue located in the wall of the right atrium that spontaneously creates a periodic electrical signal.
  • This signal travels through both the right and left atria, causing the atria to contract.
  • Word derivation:
    • Sinus: cavity or depression
    • Atrial: in the atrium
    • Node: a collection of tissue
• Atrioventricular node (AV node)
  • A collection of nervous tissue in the wall between the atria and ventricles.
  • Delays the signal so the atria can fully contract before sending the electrical signal into the ventricles (through the Bundle branches and Purkinje fibers), causing the ventricles to contract.

Figure 42.7, page 817, Campbell's Biology, 5th Edition

Things to Remember About the Pacemaker

• No signal from the brain is necessary to tell the pacemaker to produce its regular signal, 70 times every minute.
• The brain and various chemicals in the body can influence the pace of the pacemaker.
  • Much of this is autonomic (not under conscious control).
  • A stressful situation can cause the pace to increase: the heart rate is used in polygraph tests (lie detectors).
  • A dangerous situation can cause the pace to increase: the “fight or flight response.”
  • An attractive member of the opposite sex can cause the pace to increase.

Blood Vessels and Circulation

Five different kinds of vessels exist in the circulatory system:

• Arteries
  • Carry blood away from heart
  • The thickest-walled vessels, since they experience the greatest pressure in the system.
  • Generally not found near the skin surface.
• Arterioles: arteries divide into these smaller vessels that take blood to the capillaries.
• Capillaries
  • Oxygen, carbon dioxide, nutrients, and wastes diffuse through the capillary walls.
  • They touch every cell in the body.
  • The smallest vessels in the circulatory system.
  • The thinnest-walled vessels, since molecules must diffuse across it: they only have an endothelial cell layer and a basement membrane.
• Venules: capillaries merge into these larger vessels
• Veins
  • Return blood to the heart.
  • Venules merge into these even larger vessels.
  • Have valves to prevent backflow of blood because of the low pressure.
  • Thicker-walled than capillaries, but thinner than arteries, since this is a low pressure system.
  • Can be found near skin surface, because of their low pressure.

Via Domenic Denicola's mad Paint skills; Figures 42.8 and 42.9, page 818, Campbell's Biology, 5th Edition

Blood Pressure

The Basics

• Blood pressure: pressure the blood exerts against the vessel walls due to…
  • Force of heart contraction
  • Size of the arterioles which provide peripheral resistance
• Smooth muscles in the wall of the arterioles can relax or tighten and in turn alter blood pressure.
• Arteries are surrounded by walls that are very elastic, which help maintain pressure in the system during the phase of the cardiac cycle in which the heart is not contracting (diastole).
• As the cross-sectional area of the system increases due to the increasing number of vessels (arterioles and capillaries), the pressure drops (P = F∕A).

Figure 42.10, page 819, Campbell's Biology, 5th Edition

Measuring Blood Pressure

• Pulse
  • The bulging of arteries with each heart beat.
  • A way to measure heart rate.
  • Can be detected fairly easily at several places:
    • Radial artery: on the forearm, next to the wrist
    • Carotid artery: in the neck
• Sphygmomanometer
  • Instrument for measuring blood pressure in the artery in the arm.
  • Measures the systolic pressure (highest pressure) and diastolic pressure (lowest pressure) in the artery during a complete cardiac cycle.

Methods box, page 821, Campbell's Biology, 5th Edition

Blood Flow in Capillaries

• At any one time only 5–10 % of capillaries have blood flowing through them.
• This way, blood is used where it is needed.
• Control of blood flow through capillary beds is performed by:
  • Smooth muscle surrounding the arterioles.
  • Precapillary sphincters (shown below).

Figure 42.11, page 820, Campbell's Biology, 5th Edition

Movement of Material in and out of the Circulatory System

• Occurs in the capillaries.
• Blood pressure causes material to leave the capillary.
  • 85 % leaves!
  • By diffusion across the membrane: oxygen
  • By bulk flow through clefts between cells: water, sugar, salts
• Some material doesn't leave the capillary.
  • Red blood cells
  • Large proteins (plasma proteins)
• Osmotic pressure causes material to return to capillary: 85 % of what has left returns this way.

Figure 42.12, page 821, Campbell's Biology, 5th Edition

Blood

• Considered a connective tissue.
• Made of two kinds of materials
  • Plasma (55 %)
    • 90 % water
    • Many ions
    • Proteins
  • Cells (45 %)
    • Red blood cells (erythrocytes)
      • No nuclei or mitochondria
      • Packed with hemoglobin to transport oxygen.
    • White blood cells (leukocytes)
      • Five types
      • Fight infection.
    • Platelets: pieces of cell used in clotting

Figure 42.13, page 822, Campbell's Biology, 5th Edition

Differentiation of Blood Cells

• Blood cells are produced in the bone marrow.
• All blood cells originate from what are known as pluripotent stem cells.
  • Pluripotent: can become any cell of a restricted subset, e.g. blood cells.
• The buzz in the media about stem cells is referring to embryonic stem cells—the original cells from which any cell can be derived.
  • They are totipotent: can become any cell.
  • Since any cell can be derived from them, the implications are enormous.
  • We will talk about these stem cells and the issues surrounding them later in the course.

Figure 42.14, page 824, Campbell's Biology, 5th Edition

Totipotent, Pluripotent, and Multipotent

• Totipotent cell: able to divide and produce all the differentiated cells in an organism.
  • A single cell can become a new organism.
    • Embryonic stem cells are totipotent, and hence, are very controversial.
  • A plant cutting can be used to grow an entire plant.
• Pluripotent cell: able to differentiate into many cell types.
  • In animals, form the endoderm, mesoderm, and ectoderm.
  • Can eventually specialize in any bodily tissue, but they cannot themselves develop into a full organism.
• Multipotent cell: more specialized than a pluripotent cell, and less specialized than an ordinary cell.
  • Multipotent blood stem cells give rise to the red cells, white cells and platelets in the blood.

Blood Clotting

• The blood is an amazing substance that carries with it the ability to automatically fix holes in the pipes that are carrying it—the blood vessels.
• Do not learn the details, just admire the process.

Figure 42.15, page 825, Campbell's Biology, 5th Edition

Cardiovascular Deaths

• The cardiovascular system is a mission critical system. If it fails, the organism dies.
• Aside from cancer, the leading causes of death in the U.S. are:
  • Heart attacks
  • Strokes
• Atherosclerosis: plaque (formed from connective tissue and lipids) is deposited along the artery walls.
  • Sometimes the artery walls even become hardened with calcium.
  • Prevents blood flow which causes:
    • Heart attacks if it happens to arteries that feed the heart.
    • Strokes if it happens to arteries in the brain.

Figure 42.16, page 825, Campbell's Biology, 5th Edition