Biol 115 Lecture 5
T.Irving 01/07/98;
Revised A.Howard 02/01/00
Circulatory System

Blood vessels: Three main types:

• Arteries & arterioles: carry blood away from the heart
• capillaries:
  join arteries and veins, deliver nutrients to cells, carry wastes away
• veins and venules: carry blood to the heart

Two circuits in circulatory system:

• Pulmonary Circuit
  • Pulmonary arteries carry deoxygenated blood
  • Pulmonary veins carry oxygenated blood
• Systemic Circuit
  • Systemic arteries carry oxygenated blood
  • Systemic veins carry deoxygenated blood

Heart is responsible for moving blood through both pulmonary and systemic circuits.

Path of Blood through the Heart

• Deoxygenated blood from the body enters the right atrium by way of the superior and inferior vena cava.
• The right atrium sends blood through an atrioventricular valve (a.k.a. tricuspid valve) to the right ventricle.
• The right ventricle sends blood into the pulmonary trunk and hence to the pulmonary arteries to the lungs by way of pulmonary semilunar valves.
• Now - oxygenated blood returns to the left atrium of the heart by way of the pulmonary veins.

• The left atrium sends blood through an atrioventricular valve (a.k.a the bicuspid or mitral valve) to the left ventricle.
• The left ventricle sends blood through the aortic semilunar valve into the aorta and hence to the body

Heart beat

Heart beat under control of two nodal clusters:

1. the SA node (near the right atrium) starts the beat causing the atria to contract
2. the AV node picks up the stimulus and initiates contraction of the ventricles.

• Nodal tissue combines characteristics of muscle and nerve.
• Impulses transmitted partly by way of Perkinje fibers
• The characteristic "beat" heard is due first to the opening and closing of the atrioventricular valves and the accompanying ventricular contractions followed by closing of the semilunar valves.
• Portions of beat:
  • Systole = period when heart is contracting
  • Diastole = period when heart muscles are relaxed.
• Heart beats about 70/minute

Blood pressure

• Blood pressure generated by the heart is responsible for flow of blood in the arteries
• This presssure is lost in the capillary beds so veins need another mechanism:
• Veins have "one-way" valves which allow blood to flow in the heart-ward direction but not back.
• Contraction of skeletal muscles is necessary to push blood along.
• Velocity of blood is slowest in the capillaries:
  maximize exchange of nutrients and wastes.

Blood

Blood is composed of "formed elements" and plasma

Blood functions:

• Transport
• Regulate body temperature and pH
• Form clots in wounds
• Fight infections

Types of cells in "formed elements":

• Red Blood Cells (RBC's); (Erythrocytes)
  • Red blood cells "biconcave" disks that lack a nucleus
  • Produced in bone marrow
  • Destroyed in liver and spleen when too old;
    erythrocytes turned over relatively fast
  • Number of RBC’s in circulation depends on O₂concentration in blood:
    Low [O₂] more RBC’s produced
  • RBC's specialized for carrying hemoglobin, the oxygen carrier molecule; gives cells their red color
• White Blood Cells (WBC's);(Leukocytes)
  • White blood cells have a nucleus
  • produced bone marrow like RBC’s
  • Types of leukocytes: 5 in two groups
    • Granular leukocytes: prominent granules
      • eosinophils (pick up lots of eosin stain)
      • basophils (rare)
      • neutrophils (multilobed nucleus);
        most common: phagocytize invading microbes
      Names come from histology: science of using dyes to identify cells & cell functions
    • Agranular leukocytes: (not really without granules)
      • lymphocytes: specialized for immunity
      • monocytes
        become large phagocytotic cells (macrophages)
        consume worn out RBC’s & microbes
• Platelets
  • derived from larger cells
  • responsible in part for clotting activity

Plasma

• composed of 10% plasma proteins 90% water
• Plasma albumin most abundant plasma protein:
  • maintains osmotic balance
  • regulates pH
  • transporting molecules, including bilirubin
• Fibrinogen & prothrombin inactive precursors to proteins that form clots
• Antibodies also circulate in blood, members of general family of proteins called gamma globulins
• Glucose and amino acids circulate in blood as nutrients to cells

Capillaries: what they do and how

• Blood pressure at the arterial end of capillary forces water into tissues carrying small nutrient molecules with it.
• Along the middle part, O₂ and nutrients diffuse along their concentration gradients away from capillaries, CO₂ and wastes diffuse in.
• Water is recovered at the venous end because blood pressure is low and the osmotic pressure exerted by the plasma proteins causes water to re-enter the capillary.

Clotting

• Damaged blood vessels are plugged by the clumping of platelets.
• Clot formation require the enzymatic conversion of prothrombin to thrombin
• Thrombin activates fibrinogen
• Fibrinogen can then form long fibrin threads which bind the platelets together into a solid clump.

Blood types

• Red blood cells may have one of a number of antigens on their surface
• Types are A, B, AB, or none.
• Type O blood is name given to blood displaying neither A nor B
• Plasma may contains one of two antibodies called A or B depending on the person's blood type.
• Anitbodies react with antigens to cause clumping.
• Testing in vitro allows blood typing; Clumping in vivo can be disastrous

Rh factor:

• Another blood antigen is the Rh antigen located on RBC's
• 85% have this antigen & are called Rh⁺
• 15% do not and are called Rh^-
• If a father is Rh⁺ and the mother Rh^-, is it is possible for an unborn child to be Rh⁺
• Blood may leak across the placenta from the fetus to the mother.
• Rh⁺ antigens cause the mother to produce anti-Rh antibodies
• These antibodies could cross back across the placenta in either this pregnancy or another causing the babies' blood cells to be destroyed
• HDN - hemolytic disease of the newborn:
  • RBC breakdown causes a rise in bilirubin in child's bloodstream, leading to mental retardation or death.
  • Possibility can be avoided by giving women anti-Rh antibody injections either midway in pregnancy or within 72 h of giving birth to an Rh⁺ child.
  • Idea is to destroy any circulating blood cells before they have a chance to cause the mother to develop her own antibodies.