Human Circulatory System



The Circulatory System


   Function includes transport of:

            1. oxygen

            2. carbon dioxide,

            3  nutrients,

            4. water,

            5. ions,

            6. hormones,

            7. antibodies,

            8. metabolic wastes. 


            The circulatory system Plus lymphatic system, moves water, ions, and proteins around the body, thus maintaining and regulating proper homeostatic conditions; these conditions are dependent on three fluid regions of the body:


            1) the fluid of blood and lymph,

            2) interstitial fluid - the watery fluid between and among cells; all substances pass

                 from capillaries to cells through this fluid, 

            3) intracellular fluid



The Human Circulatory System- 4 chambered Heart, arteries, capillaries, and veins.


            Arteries - muscular vessels carrying blood away from  heart; all but the pulmonary artery (to lungs) carry oxygenated blood.


            Capillaries - thin walled (one cell layer) vessels that arise from arterioles (tiny arteries) to form capillary beds; all exchange between blood and cells occurs here.


            Veins - receive blood from the capillaries (in tiny veins called venules) and carry blood to the heart; all but pulmonary vein carry deoxygenated blood; they tend to be thin walled and flattened and lie nearer the body surface than arteries.



Circulation Through the Heart


---->  BODY BLOOD ---->---->---->

            Superior Vena Cava and the Inferior Vena Cava carries blood from body to RIGHT SIDE of HEART at the right atrium ----> blood flows into the ----> right ventricle through the tricuspid valve, (checks the backward flow of blood. This three flapped valve is connected by chordae tendinae (tissue strands) which prevent the flaps from collapsing backward when closed). Blood leaves ventricle  and is pumped ----> into the Pulmonary artery and then to ----> lungs (Pulmonary semilunar valve prevents backward flow from pulmonary artery).----> ---->


                        (Left side): Blood in pulmonary arteries flows to ----> alveoli of lungs, (gases are exchanged) ----> blood flows to Pulmonary Veins and into ----> the Left Atrium.  This oxygenated blood passes through ----> the Mitrial Valve (Chordae tendinae prevent this valve from collapsing also) ----> into the Left Ventricle ----> Aortic Semilunar Valve ----> Aorta (first artery) which then branches many times, which then branches many times, ----> e.g. coronary artery to (heart tissue itself), ----> carotid arteries (to head), ----> subclavians (to arms, iliacs to legs).


            Walls of the left ventricle are much thicker than the right because they must force the blood through all the body tissues and back to the heart. Blood leaving the left ventricle passes through the aortic semilunar valve into the first artery - the aorta.


Control of the Heart

            Heart muscle is different, the contraction of heart muscle cells is inherent, i.e. the heart can beat w/o outside regulation but regulated it is - the body's constantly changing demands require heartbeat to change.


            1. Extrinsic (outside) control - autonomic nervous system and adrenal hormone epinephrine can regulate heartbeat; OR the heart itself can secrete hormones when changes in blood pressure in the atria are detected, these hormones can regulate heart.


            2. Intrinsic (within) control - origin of heartbeat is in a region called the sinoatrial (SA) node OR pacemaker; it can be influenced by autonomic nerve fibers; 


            SA node transmits impulses ---->---->

                        a) across atrial walls causing two atria to                   

                           contract simultaneously ---->

                        b) to the atrioventricular (AV) node.  There is a delay between the two impulses which allows the atria to complete their contraction before the ventricles begin theirs.  The impulse at the AV node passes down ----> ----> the septum along a strand of muscle called the Bundle of His - this branches into two ventricular walls along fibers called Purkinje fibers ----> which initiate ventricular contraction. 

            *** The contraction goes from the bottom up - a twisting motion of the contraction forces the blood out and into arteries.



            If you listen to your heartbeat, you hear two sounds, a lub and a dup. 


            The lub is the sudden closing of the tricuspid and bicuspid valves under the force of contraction of the ventricle. 


The dup is the aortic and pulmonary semilunar valves snapping shut by arterial backflow.  These sounds are followed by a pause and hopefully another lub, dup.


             Systole - period of ventricular contraction and 

             Diastole - the period of filling up


            The heart rate at rest is 72 beats/minute and each contraction forces 80 ml (2.7 fluid ounces) of blood into the aorta. 

            stroke volume - The amount of blood pumped per contraction is called the


            Blood Pressure - is the force of blood against the arterial walls - the force that moves blood through the body.


 It is measured with a sphygmomanometer - a cuff is inflated around the arm where the brachial artery is, the artery collapses and no sound is heard through the stethoscope attached.  The pressure in the cuff is slowly released and when a beating sound is first noted, the pressure in the cuff is noted (this is systolic pressure).  When the sound can no longer be heard, the pressure 

is again noted, this is diastolic pressure.  A "normal" resting reading is 120 (systolic) over 80 (diastolic).


            Blood pressure in arteries are maintained because the walls are elastic; small pressure changes are short-lived as artery walls return to their original shape during diastole.


            Arteries maintain and direct the flow of blood to capillaries by vasodilatation (opening of arterioles) which reduces blood pressure and vasoconstriction (closing) which decreases blood pressure - these two mechanisms are under control of autonomic nervous system and hormones.


            Some Circuits in the Human Circulatory System


            1. Hepatic Portal Circuit - some arteries from aorta branch across intestinal membranes to form capillaries with villi, digested food picked up, capillaries merge to form a hepatic (refers to liver) portal vein and travel to a second capillary bed in the liver where many nutrients are removed.  Portal veins are veins that connect two capillary beds.


            2. Renal Circuit - renal arteries from aorta take blood to the kidneys where a complex filtering takes place (more later); renal veins then return the blood to the inferior vena cava.


            3. Cardiac Circuit - the first branch off the aorta is the coronary artery which supplies oxygen to the heart itself; artery forms an anastomoses, a network of small arteries around heart; if a clot (called a coronary thrombosis) forms in one, blood has many other pathways to travel.


            4. Systemic Circuit - refers to all other circuits and capillary beds in the body, although pulmonary (to lungs) circuit discussed separately.



            These are made of interlocking cells a single cell thick; as blood passes through a capillary from an artery it begins to lose nutrients, ions, water, and oxygen; as it passes out of a capillary to a venule, it picks up ions, some of the water lost, carbon dioxide, metabolic wastes, and, around the villi, nutrients.  These materials pass out of capillaries by three mechanisms:


            1. Diffusion - at artery end, materials travel down their concentration gradient; reverse at vein end; most material passes through spaces between cells of capillary wall.


            2. Hydrostatic pressure - greater at artery end; forces material out; pressure reduced at vein end so some materials can pass back in (especially water).


            3. Pinocytosis - materials adhere to plasma membrane of capillary cells (both cell side and blood side), membrane invaginates and pinches inward around substance, it then moves across cell to other side and is released into blood or space around cells; process is a form of active transport and requires energy.



            Blood volume entering veins is equal to that leaving arteries, but blood pressure is severely reduced.  Movement through veins is assisted by 1) one way flap like valves that allow blood to move in one direction only (toward heart), 2) some smooth muscle around larger veins that contracts and moves blood, and 3) limb and breathing movements literally massages veins and squeezes blood along



            Blood is a connective tissue whose matrix is plasma and it contains three basic kinds of cells.  Plasma is 90% water and 10% plasma solids.  These solids are many substances including urea, free amino acids, glucose, hormones, ions, fats, and, mostly, proteins. 


The 3 major proteins are:

                        a. albumins - large proteins that bind impurities and some toxins, aid in    

                            transport of hormones, fatty acids and ions; important in maintaining       

                            osmotic conditions.

                        b. globins - include antibodies (immunoglobins)

                        c. fibrinogen - important in blood clotting


Blood Cells

            A) Erythrocytes (red blood cells) - small, disklike in shape, lack a cell nucleus therefore cannot reproduce, must be continually renewed, last 4 months then rupture, produced by red bone marrow; contain hemoglobin which binds with oxygen for transport.


            B) Leukocytes (white blood cells) - possess nuclei, active in immune system (more later); most are neutrophils which engulf microorganisms; also include basophils, eosinophils, and lymphocytes, all involved in immune response.


            C) Platelets (or thrombocytes) - tiny and very numerous, essentially just cell fragments, important in blood clotting; clotting is important to prevent or reduce fluid leakage from blood vessels; it is a complex and only partly understood process and two plasma proteins, prothrombin and fibrinogen, are important.  Basic process as follows:


            A. vessel damaged


            B. platelets attach at wound site and form a plug by adhering to collagen fibers


            C. platelets rupture and release 1) vasoconstrictors which constrict nearby vessels   

                 to decrease blood flow and 2) release thromboplastins

            D. Thromboplastins are enzymes which convert prothrombin to thrombin

            E. Thrombin breaks apart fibrinogen into a fibrous sticky protein called fibrin

            F. Fibrin forms a network with platelets, red blood cells and white blood cells that

                 becomes a clot.

            G. Clot contract, pulling wound together


            The absence of any of these factors is serious, resulting in one or more types of hemophilia, a hereditary disorder. A deficiency in vitamin K, which is used to synthesize prothrombin, can cause slow clotting.


Lymphatic System

            Another circulatory system consisting of lymph vessels and lymph nodes; the vessels collect fluids, solutes, and foreign materials from tissue spaces and return it to the blood; muscle action moves the lymphatic fluid through the vessels.  Most lymph filter through the lymph nodes which contain lymphocytes; these aid in immune response (nodes become swollen when inflamed)


Roles of the Lymphatic system include:

            1. maintain fluid and ion (electrolyte) balance in body

            2. transport certain fatty acids from villi to blood

            3. assist in immune response

            4. provide a route where interstitial fluid can return   

               to the circulatory system



            The combined efforts of the blood and lymphatic system aid in the defense of an organism against disease.