david sadava h. craig heller gordon h. orians william k. purves david m. hillis biologia.blu c –...

David Sadava H. Craig Heller Gordon H. Orians William K. Purves David M. Hillis

Biologia.bluC – Il corpo umano

Circulatory System and Blood

• Why do humans need a circulatory system?

• How have vertebrate circulatory systems evolved?

• How does the mammalian heart function?

• What are the properties of blood and blood vessels?

• How is the circulatory system controlled and regulated?

Circulatory System and Blood

A circulatory system consists of:

• a muscular pump - the heart;

• a fluid - blood;

• a series of conduits - blood vessels.

Together these are called the cardiovascular system.

Circulatory System and Blood - Why do humans need a circulatory system?

Two circulatory circuits have evolved:

• pulmonary circuit - blood is pumped from the heart to the lungs and back again;

• systemic circuit - blood travels from the heart to the rest of the body and back to the heart.

Circulatory System and Blood - How have vertebrate circulatory systems evolved?

The closed vascular system contains:

• arteries carry blood away from the heart and branch into arterioles that feed the capillary beds;

• capillaries are the site of exchange between blood and tissue fluid;

• venules drain the capillary beds and form veins, which deliver blood back to the heart.

Circulatory System and Blood - How have vertebrate circulatory systems evolved?

The human heart has four chambers: two atria and two ventricles.

The right heart pumps blood through the pulmonary circuit.

The left heart pumps blood through the systemic circuit.

Circulatory System and Blood - How does the mammalian heart function?

Valves prevent backflow of blood:

• atrioventricular valves between the atria and ventricles and prevent backflow when ventricles contract;

• the pulmonary valve and aortic valve lie between the ventricles and the major arteries and prevent backflow when ventricles relax.



The human heart and circulation (part 1)


The human heart and circulation (part 2)

The right atrium receives deoxygenated blood from the body through large veins:

• superior vena cava - blood from upper body;

• inferior vena cava - blood from lower body.


Blood passes from the right atrium through an AV valve into the right ventricle.

The atrium contracts, then the ventricle—the AV valve closes and blood is pumped through the pulmonary artery to the lungs.

Oxygenated blood returns to the left atrium of the heart through the pulmonary veins.

The ventricle fills as blood enters through AV valve.

The left atrium contracts, then the ventricle—the aortic valve opens and blood circulates through the aorta.


In the cardiac cycle both sides of the heart contract at the same time: first the two atria contract, then the two ventricles.

Two phases:

• systole - when ventricles contract;

• diastole - when ventricles relax.



The cardiac cycle

Blood pressure changes are measured with a sphygmomanometer and a stethoscope.

• Systolic value: pressure needed to compress an artery so blood does not flow.

• Diastolic value: pressure needed to allow intermittent flow though the artery.



Measuring blood pressure

Cardiac muscle functions as a pump:

• cells are in electrical contact with each other through gap junctions, spread of action potentials stimulates contraction in unison;

• some cells are pacemaker cells and can initiate action potentials without input from the nervous system.



The heart beat (part 1)


The heart beat (part 2)

An electrocardiogram (ECG) uses electrodes to record events in the cardiac cycle.

Large action potentials in the heart cause electrical current to flow outward to all parts of the body.

Electrodes register the voltage difference at different times.


Arteries and arterioles are called resistance vessels because their resistance can vary:

• walls have elastin and collagen that allow them to stretch and recoil;

• smooth muscle cells in the walls allow them to dilate or constrict.

Circulatory System and Blood - What are the properties of blood and blood vessels?

When the diameter of an artery changes so does its resistance—blood flow changes as a result.

Neuronal and hormonal mechanisms control the resistance by influencing the smooth muscle cells.



Anatomy of blood vessels


Blood pressure and flow through large arteries are high, and are lower through the capillaries.

Pressure is reduced in smaller vessels because:

• arterioles are highly branched;

• capillaries contribute an enormous surface area.

Capillary walls are a single layer of endothelial cells and have tiny holes called fenestrations.

Capillary beds are permeable to water, ions, and small molecules, but not to large proteins.


One-way flow

The cardiovascular control center in the medulla controls heart rate and vessel constriction.

In the carotid arteries and the aorta:

• baroreceptors (stretch receptors) monitor blood pressure changes;

• chemoreceptors send information about blood composition.

Circulatory System and Blood – How is the circulatory system controlled and regulated?

Circulatory System and Blood – How is the circulatory system controlled and regulated?

Regulating blood pressure

Red blood cells are generated in the bone marrow.

Erythropoietin, a hormone released in the kidney in response to hypoxia, controls red blood cell production.

Red and white blood cells originate from pluripotent stem cells in the bone marrow.

These cells constantly divide and can differentiate into a variety of blood cells.



Blood cells (part 1)

Immature red blood cells divide and produce hemoglobin while in the bone marrow.

When cells are 30 percent hemoglobin the organelles break down and the cells enter the circulation.

Cells circulate about 120 days before rupturing as they pass through narrow capillaries, as in the spleen.


Bone marrow produces megakaryocytes that break off cell fragments called platelets.

Platelets initiate blood clotting when activated by collagen exposed in damaged blood vessels.

They release chemical clotting factors which activate other platelets.


Steps in blood clotting:

• cell damage and platelet activation;

• inactive enzyme prothrombin converts to active form, thrombin;

• thrombin cleaves fibrinogen and forms fibrin;

• fibrin threads form mesh that clots blood and seals vessel.



Blood clotting

Plasma contains:

• Gases

• Ions

• Nutrients

• Proteins

• Other molecules, hormones and vitamins


Atherosclerosis: “hardening of the arteries”.

• The endothelial lining of arteries is damaged by high blood pressure, smoking, diet, or microorganisms.

• Plaque forms at sites of damage.

• Damaged cells attract migration of smooth muscle cells.


• Smooth muscle cells have cholesterol deposits that make the plaque fatty.

• Connective tissue and calcium deposits make the artery wall less elastic, or “hardened”.

A thrombus, or blood clot, may form if platelets stick to the plaque.


The coronary arteries supply blood to the heart muscle.

Atherosclerosis in these arteries reduces blood flow; marked by chest pain and shortness of breath.

Coronary thrombosis: if a thrombus forms in a coronary artery it can lead to a heart attack, or myocardial infarction.


An embolus is a piece of a thrombus:

• it may cause an embolism if it lodges in a blood vessel.

• if the embolism is in the brain the cells fed by that artery will die - a stroke.


Causes of atherosclerosis:

• genetic predisposition;

• age;

• environmental risk factors (high-fat diet, smoking, sedentary lifestyle);

• medical conditions (hypertension, obesity, diabetes).



Atherosclerotic plaque

david sadava h. craig heller gordon h. orians william k. purves david m. hillis biologia.blu c –...

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