NOTES FOR BIOLOGY 1002

SECTIONS 004, 005, 006

Spring 2006

DR. STEVEN POMARICO

CHAPTER 38

CIRCULATION

The need for circulation (and circulatory systems) arose, as animals got larger.

Small (and thin) animals have all of their cells in close proximity to the outside environment.

This means there is a very short distance that food and oxygen have to travel to get to the center of each cell. Likewise, the distance that waste and CO₂ have to travel to get out of the cell is also very short.

Materials (good or bad) can efficiently move over short distance in an organism by diffusion.

Larger distances require some energy requiring process (active transport, circulation, etc.).

In larger animals since diffusion is not effective enough to get the good things (nutrients and oxygen) in and get the bad things (waste and carbon dioxide) out, circulation is required.

There are two types of circulatory systems (See fig 38.3):

                     Open circulatory system

                     Closed circulatory system

Both types have three major parts:

                     1) A fluid that circulates (blood)

                     2) A channel (vessels) that the fluid circulates through

                     3) A pump (heart) to keep the fluid moving   

In the open circulatory system part of the “channel” that the fluid circulates through is an open space within the body called the hemocoel (blood cavity)

In the closed circulatory system the blood is usually confined (closed) in the vessels.

THE VERTEBRATE CIRCULATORY SYSTEM

In vertebrates, the circulatory system has seven major functions. The first three (that I’ve already mentioned) are common to all circulatory systems.

          1) Transport of oxygen to the tissues

          2) Transport of nutrients to the tissues

          3) Transport of waste from the tissues

Some of the next four are found only in vertebrates:

          4) Distribution of hormones

          5) Regulation of body temperature (homeostasis)

          6) Prevention of blood loss (clotting)

          7) Protection from invaders (immune system)

The vertebrate heart is a muscular pump that has evolved in design over time (See fig 38.4)

EVOLUTIONARY TREND IN HEART DESIGN

In all of these designs the heart chambers are either receiving blood (atrium, plural atria) from the body or sending the blood out (ventricle) into the body.

THE FOUR-CHAMBERED HEART (See fig 38.12)

NOTE THIS IS DIFFERENT THAN THE TEXT

In the four-chambered heart, one pair (right ventricle + left atrium) of chambers is used for pulmonary circulation.

This part of the circulatory system is responsible for pumping blood to and receiving blood back from the lungs.

The remaining pair (left ventricle + right atrium) of chambers is used for systemic circulation.

This part of the circulatory system is responsible for pumping blood to and receiving blood back from the body.

BLOOD >> The mobile part of the circulatory system

Blood has two components:

                     Specialized cells (red cells, white cells, platelets).

                     Fluid plasma (55-60% of blood volume)

Humans have 5-6 liters of blood, about 8% of total body weight

Plasma is a straw colored fluid composed of 90% water with many dissolved substances.

The dissolved substances include:

          Proteins and hormones.

          Nutrients (glucose, vitamins, amino acids, lipids).

          Gases (CO₂ and O₂)

          Ions (Na⁺, Cl^-, Ca⁺², K⁺, Mg⁺²)

          Wastes (urea)

The Cells of the Blood

The Red Blood Cells (RBCs) are the erythrocytes (See fig 38.6).

These cells lose nuclei as they mature and cannot divide.

They comprise 99% of all blood cells.

The RBCs make up 40% of total blood volume in females and 45% in males.

I ml of blood has about 5 million RBCs.

The RBCs contain the red pigment molecule hemoglobin (Hb)

This molecule is made of 4 protein chains, and each chain contains a heme group with an iron atom.

The function of oxygen delivery is carried out by Hb and 97% of the

blood's O₂ is bound to the iron atoms in Hb.

Hb is also involved in the movement of CO₂ out of the body.

How this works:

Hb picks UP O₂ where the concentration of oxygen is high (lungs) and releases it where the concentration is low (body cells).

After releasing O₂, some Hb picks UP CO₂ from body cells for transport back to the lungs

RBCs have a life span of about 120 days with 2 million RBCs replaced each second.

The iron from dead RBCs is recycled from the liver and spleen to the bone marrow of chest, upper arms, legs, and hips where RBCs are formed

Small amounts of iron are excreted daily and must be replenished by the diet. In addition, iron (and RBCs) lost due to injury or menstruation must also be replaced.

The number of RBCs is maintained through a negative feedback system involving the hormone erythropoietin.

The kidneys in response to oxygen deficiency produce this hormone.

The hormone stimulates rapid production of new RBCs by the bone marrow.

The new RBCs get rid of the oxygen deficiency.

White Blood Cells (WBCs) or leukocytes

There are five types WBCs that comprise less than 1% of blood cells.

The WBCs are distinguished by their size, shape of nucleus, and staining characteristics.

The main function of the WBCs is to fight infections.

Platelets and blood clotting

Platelets are fragments that break off large cells, called megakaryocytes. (See fig 38.7)

The megakaryocyte remains in the bone marrow and the platelets become part of the blood.

BLOOD VESSELS

Sequence of blood flow:

          heart > arteries > arterioles > capillaries > venules > veins > heart

Arteries and arterioles carry blood away from the heart. These vessels are thick and muscular with elastic walls. (See fig 38.15)

CAPILLARIES

          This is where the exchange of wastes, nutrients, gases, and hormones between blood and body cells occurs

The capillaries are thin tubes (See fig 38.25), with walls only one cell thick so that dissolved material can diffuse in and out. WE’RE BACK TO MOLECULES TRAVELING SHORT DISTANCES BY DIFFUSION!

The pressure within capillaries causes loss of interstitial fluid (water with dissolved nutrients, hormones, gases wastes, small proteins) into spaces surrounding spaces and tissues.

The pumping action of the four-chambered heart make-up a cardiac cycle. This cycle is the alternating contraction and relaxation of the heart chambers.

How this works (See fig 38.13):

          1) The two atria contract pumping blood into ventricles

          2) A fraction of a second later, the two ventricles contract forcing blood into                                 arteries - Systole

          3) Both chambers relax briefly - Diastole

When “blood pressure” is measured, you get two numbers. They are the systolic pressure (during the contraction of the ventricles) over the diastolic pressure (during the relaxation).

The heart has four simple one-way valves to ensure unidirectional blood flow.

                     2 - Atrioventricular or AV valves

                     2 - Semilunar valves

These valves are “one-way” because pressure in one direction opens the valve, while pressure in the reverse direction closes them tightly

The atrioventricular valves separate each atria from the ventricle below it.

These valves are slightly different from each other. A tricuspid valve separates the right ventricle and atrium, while a bicuspid valve is between the left ventricle and atrium.

The semilunar valves keep the blood that has just been pumped out of the heart, into the pulmonary artery and the aorta, from flowing backwards into the heart.

CONTROLLING THE PUMP

The heart muscles must beat in smooth coordinated contractions to keep blood flow moving and to avoid random contractions known as fibrillation

This coordinated contraction requires a pacemaker.

Pacemaker is an area of muscle that sets the pace for other muscle cells.

The sinoatrial node (SA node) is the heart's primary pacemaker (See fig 38.14).

The SA node is a small mass of muscle cells in the wall of the right atrium.

This SA node spontaneously generates an electrical impulse that spreads to both atria, causing them to contract.

The atria must contract first to push blood into the ventricles and then refill as the ventricles contract.

There is a very brief delay (about 1/10 second) between contraction of the atria and the contraction of the ventricles caused by a barrier of unexcitable tissue between the atria and the ventricles that blocks the SA signal from reaching the ventricles

The SA signal is channeled around the barrier to a second mass of cells, the atrioventricular node (AV node) that then signals the ventricular contraction.

From the AV node, the signal to contract travels to the base of both ventricles through excitable fibers.

The signal then spreads to the muscles of both ventricles causing them to contract in unison.

Outside influences on heart rate:

The SA node pacemaker maintains a steady beat of 60-100 beats/minute.

Nervous impulses and hormones can both have influences on heart rate.

At rest, when nerve impulses are low, the heart rate slows to about 50-70 beats/minute.

During exercise or stress, the sympathetic nervous system accelerates the heart rate.

The hormone epinephrine (adrenaline) also increases heart rate. This prepares the body for fight or flight.

If blood pressure falls, the sympathetic nervous system stimulates contraction of smooth muscles in the vein walls decreasing their diameter (blood pressure rises).

The muscular walls of arterioles are influenced by nerves, hormones, and local chemical changes so that they contract or relax in response to changing needs.

Venules and Veins

These vessels provide low-pressure pathways for blood back to the heart.

The walls of the veins and venules much thinner. (See fig 38.15)

The return of blood to the heart is aided by muscle contractions during exercise and breathing which squeeze the veins (See fig 38.19).

Controlling the flow of blood

One-way valves in the veins (See fig 38.19) prevent back flow of blood.

Hemostasis or Blood clotting: How it works (See fig 38.23).

          1) Vascular spasms slow the blood flow

          1a) Platelets stick to damaged blood vessels forming a plug

          2) Blood clotting (coagulation) occurs by a complex series of chemical events                      that result in the activation of the enzyme thrombin

          3) Thrombin converts fibrinogen protein (soluble) into fibrin fibers (insoluble)

          4) Platelets and RBC’s adhere to the fibrous mass

          5) Within 30 minutes, the platelets contract, forcing fluid out and making the                      clot denser. This also constricts the wound and promotes healing.