Lecture 7: Cardiovascular and Respiratory System

The circulatory path of the blood is:

Beginning with the left ventricle, blood is pumped through the aorta. From the aorta, branch many smaller arteries, which themselves branch into still smaller arterioles, which branch into smaller capillaries. Blood from the capillaries is collected into venules, which themselves collect into larger veins, which collect again into the superior and inferior vena cava. The vena cava empty into the right atrium of the heart. This first half of the circulation as just described is called the systemic circulation.

From the right atrium, blood is squeezed into the right ventricle. The right ventricle pumps blood through the pulmonary arteries, to arterioles, to the capillaries of the lungs. From the capillaries of the lungs, blood collects in venules, then in veins, and finally in the pulmonary veins leading to the heart. The pulmonary veins empty into the left atrium, which fills the left ventricle. This second half of the circulation is called the pulmonary circulation.

Since there are no opening for the blood to leave the vessels, the entire system is said to be a closed circulatory system.

The left ventricle contracts with the most force to propel the blood through the systemic circulation.

Systole occurs when the ventricles contract; distole occurs during relaxation of the entire heart and then contraction of the atria.

The blood is propelled by hydrostatic pressure created by the contraction of the heart. The rate of these contractions is controlled by the autonomic nervous system, but the autonomic nervous system does not initiate the contractions. The heart contracts automatically, paced by a group of specialized cardiac muscle cells called the sinoatrial node (SA node) located in the right atrium. The SA node contrats, spreading its contractions to the surrounding cardiac muscles via electrical synapses made from gap junctions.

The vagus nerve innervates the SA node, slowing the contractions while also increasing digestive activity in the intestines.

The action potential branches out through the ventricular walls via conductive fibers called Purkinje fibers. The Purkinje fibers in the ventricles allow for a more unified, stronger contraction.

• Arteries are elastic, and stretch as they fill with blood. Arteries are wrapped in smooth muscle. Epinephrine is a powerful vasopressin causing arteries to narrow.
• Arterioles are very small. Constriction and dilation of arterioles can be used to regulate blood pressure as well as rerouting blood.

Capillariesare microscopic blood vessels. Capillary walls are only one cell thick. Nurtient and gas exchange with any tissue other than a vascular tissue takes place only across capillary walls, and not across arterioles or venules.

There are four ways for materials to cross capillary walls: 1) pinocytosis 2) diffusion or transport through capillary cell membranes 3) movement through pores in the cells called fenestrations 4) movement through the space between the cells.

As blood flows into a capillary, hydrostatic pressure is greater than osmotic pressure, and net fluid flow is out of hte capillary. Osmotic pressure will overcome hydrostatic pressure near the venule end of a capillary, and net fluid flow is into the capillary and out of the interstitum.

• Blood velocity is inversely proportional to the cross-sectional area. The blood moves the slowest through the capillaries. To compensate for low blood pressure, veins have a valve system that prevents the back flow of blood. Pulmonary arteries contain the most deoxygenated blood in the body.
• Blood pressure increases near the heart and decreases to its lowest in capillaries.

The respiratory system provides a path for gas exchange between the external environment and the blood. The alveoli is where oxygen is exchanged for carbon dioxide. The diaphragm is skeletal muscle, and innervated by the phrenic nerve.

The nasal cavity is the space inside the nose. It filters, moistens, and warms incoming air. Coarse hair at the front of the cavity traps large dust particles. Mucus secreted by goblet cells traps smaller dust particles and moistens the air. Capillaries warm the air. Cilia moves the mucus and dust back toward the pharynx. The pharynx (throat) functions as a passageway for food and air.

The larynx is the voice box. It sits behind the epiglottis, which is the cartilaginous member that prevents food from entering the trachea during swallowing.

The trachea (windpipe) lies in front of the espohagus. It is composed of ringed cartilage covered by ciliated mucous cells. Before entering the lungs, the trachea splits into the right and left bronchi. Each bronchus brances many more times to become tiny bronchioles. Bronchioles terminate in tiny sacs called alveoli.

The job of the respiratory system is to deliver oxygen to the blood and expel carbon dioxide. A problem in microtubule production might result in a problem in breathing.

98% of the oxygen in the blood binds rapidly and reversibly with the protein hemoglobin inside the erythrocytes forming oxyhemoglobin. Each of the four iron atoms in hemoglobin can bind with one O2 molecule.

As O2 pressure increases, the O2 saturation of hemoglobin increases signmoidally. The oxygen saturation of hemoglobin also depends upon carbon dioxide pressure, pH, and temperature of the blood. The oxygen dissociation curve is shifted to the right by an increase in carbon dioxide pressure, hydrogen ion concentration, or temperature. A shift to the right indicates a lowering of hemoglobin's affinity for oxygen.

As blood moves through the systemic capillaries, oxygen diffuses to the tissues, and carbon dioxide diffuses to the blood. Carbon dioxide is carried by the blood in three forms: 1) in physical solution 2) as bicarbonate ion 3) in carbamino compounds

Ten times is carried as bicarbonate than as either of the other forms. The bicarbonate ion formation is governed by the enzyme carbonic anhydrase in the reverisble reaction CO2 + H2O --> HCO3- + H+

Because carbonic anhydrase is inside the red blood cell and not in the plasma, when carbon dioxide is absorbed in the lungs, bicarbonate ion diffuses into the cell. To balance the electrostatic forces, chlorine moves out of the cell in a phenomenon called the chloride shift.

Carbon dioxide has its own dissociation curve, which related blood content of carbon dioxide with carbon dioxide pressure. The greater the pressure of carbon dioxide, the greater the blood content of carbon dioxide.

The lymphatic system collects excess interstitial fluid and returns it to the blood. The lymph system is an open system; fluid enters at one end and leave as theother.

Throughout the lymphatic system are many lymph nodes, containing large quantities of lymphocytes.

The blood is a connective tissue. It contains cells and a matrix. Blood regulates the extracellular environment of the body by transporting nutrients, waste products, hormones, and even heat. Blood is made up of 1) plasma 2) buffy coat (white blood cells) 3) red blood cells

Plasma contains the matrix of the blood. Important proteins contained in plasma are albumin, immunoglobulins, and clotting factors. Albumins transport fatty acids and steroids, as well as acting to regulate the osmotic pressure of the blood.

Erythrocytes (red blood cells) have no organelles, not even a nucleus, which means they do not reproduce nor undergo mitosis. Their main function is to transport O2 and CO2.

Leukocytes (white blood cells) do contain organelles, but do not contain hemoglobin. They function to protect the body from foreign invaders.

All blood cells differentiate from the same type of pecursor, a stem cell, residing in bone marrow.

Platelets are small portions of membrane-bound cytoplasm torn from megakaryocytes.

Granulocytes live a short time and function nonspecifically against all infective agents, whereas most agranulocytes live a long time fighting specific agents of infection.

Coagulation occurs in 3 steps: 1) a dozen or so coagulation factors form a complex called a protombin activator. 2) protrombin activator catalyzes the conversion of prothrombin, a plasma protein, into thrombin, 3) thrombin is an enzyme that governs the polymerization of the plasma protein fibrinogen to fibrin threads that attach to platelets and form a tight plug.

The human body protects itself in two ways: innate immunity and acquired immunity.

Innate immunitiy includes: the skin as a barrier, stomach and digestive acids, phagocytotic cells, and chemicals in the blood.

Injury to tissue results in inflammation, which includes dilation of blood vessels, increased permeability of capillaries, swelling of tissue cels, and migration of granulocytes and macrophages to the inflamed area. Part of the effect of inflammation is to wall-off the effected tissue and local lymph vessels from the rest of the body, stopping the spread of the infection.

Infectious agents are first attacked by local macrophages. Neutrophils are next on the scene and can phagocytize 5-20 bacteria.

There are two types of acquired immunity: humoral or B-cell immunity; cell-mediated or T-cell immunity.

• Humoral immunity is promoted by B lymphocyte. B lymphocytes differentiate and mature in the bone marrow and the liver. Each B lymphocyte is capable of making a single type of antibody or immunoglobulin, which it displays on its membrane. An antibody recognizes a foreign particle, called an antigen.
• If the B lymphocyte antibody contacts a matching antigen (presented by the macrophage), the B lymphocyte, assisted by the a helper-T cell, differentiates into plasma cells and memory B cells. Plasma cells begin synthesizing free antibodies, and releasing them into the blood.

The antibodies may cause the antigenic substances to agglutinate or even precipitate, or, in the case of a toxin, the antibodies may block its chemically active protein. The first time the immune system is exposed to an antigen is known as the primary response.

Memory B cells proliferate, and remain in the body. In the case of reinfection, each of these cells can be called upon to synthesize antibodies, resulting in a faster acting and more potent affect called the secondary response.

Cell-mediated immunity involves T-lymphocytes. T-lymphocytes have an antibody-like protein at their surface that recognizes antigens. T-lymphocytes never make free antibodies. T-lymphocytes differentiate into helper T cells, memory T cells, suppressor T cells, and killer T cells.

• Memory T cells are similar to memory B cells
• Supressor T cells play a negative feedback role in the immune system
• Helper T cells are attacked in HIV
• Killer T cells bind to antigen carrying cell and release a protein that punctures the cell.

Blood types are identified by the A and B antigense. Type A blood means that the red blood cell membrane has A antigense and does not have B antigens. If the erythrocytes have A antigense, the immune system does not make A antibodies.

The genes which produce the A and B antigens are co-dominant. Thus an individual having type A or B blood may be heterozygous or homozygous. An individual with type O blood has to recessive alleles.