1. The Circulatory System
Chapter 42

2. Why have a circulatory system?
Diffusion is too slow a process to transport chemicals through the body of an animal.
The presence of a circulatory system reduces the distance a substance must diffuse to enter or leave a cell.
The circulatory system does more than move gases, it is a critical component to maintaining homeostasis of the body.

3. Components of the cardiovascular system
Heart
Four chambers in humans
Arteries
Carry blood away from heart
Branch into smaller arterioles
Veins
Carry blood toward the heart
Capillaries
The site of chemical exchange between blood and interstitial fluid
Connect arteries to veins

4. Blood flow pattern in humans
‘Double-loop’ circulation
body  vena cava  right atrium  right ventricle  pulmonary artery  lung  pulmonary vein  left atrium  left ventricle  aorta  arteries  arterioles  capillaries  venules  veins  vena cava
The blood that is low in oxygen is completely separated from the blood that is rich in oxygen

5. The heart Located beneath the sternum
Cone-shaped and about the size of a clenched fist
Surrounded by a double layered sac
Comprised mostly of cardiac muscle tissue
The 2 atria are thin-walled and collect blood
The ventricles are thick-walled and pump blood
There are 4 valves which prevent backflow of blood during contraction

6. The heartbeat
A complete sequence of contraction and relaxation is the cardiac cycle
Systole – the heart muscle contracts and the chambers pump blood
Diastole – the heart muscle relaxes and the chambers fill with blood
The tempo of contraction is controlled by the sinoatrial (SA) node, sometimes called the pacemaker
The SA node can be influenced by hormones, body temperature, exercise

7. Blood pressure The force that blood exerts against a vessel wall
Pressure is greatest in arteries during ventricular systole – this is the main force propelling blood from the heart through the vessels
Stress may trigger responses that cause the smooth muscles of vessel walls to contract, increasing blood flow and pressure
Blood in the veins is under very little pressure – it is aided in returning to the heart by skeletal muscles
Veins have valves that prevent backflow of blood

8. Blood flow to different body parts is regulated
All tissues and organs receive enough blood even though only 5% - 10% of the capillaries are carrying blood at any given time
The brain, heart, kidneys, and liver usually carry a full load of blood
Blood flow to capillaries in other areas is controlled by smooth muscles in the artery walls
After a meal  increased flow to digestive tract
Exercise  increased flow to skeletal muscles

9. Exchange of materials Occurs across the thin walls of the capillaries
The capillary wall is a single ‘leaky’ layer of endothelial cells
About 85% of the fluid that leaves the blood at the arteriole end of the capillary re-enters from the interstitial fluid at the venous end
The remaining 15% of the fluid is lymph

10. The lymphatic system
Lymph is similar in composition to interstitial fluid
It enters the lymphatic system by diffusing into lymph capillaries
It then drains back into the circulatory system at two locations near the shoulders
Lymph nodes are specialized swellings along the system that filter the lymph and attack viruses and bacteria

11. Blood Connective tissue – cells suspended in liquid plasma
Average person has 4-6 L of whole blood
Plasma
90% water
Also contains electrolytes and plasma proteins
Erythrocytes (RBCs = red blood cells)
Transport oxygen
No nuclei
Contain hemoglobin
Leukocytes (white blood cells)
Function in defense and immunity
Number will increase during times of infection
Platelets
Function in blood clotting

12. Blood clotting Platelets clump together and form a temporary plug
Clotting factors are released that convert fibrinogen to fibrin
Fibrin forms threads to form the clot
Hemophilia – caused by a defect in the genes that control this process

13. Cardiovascular disease
Accounts for more than 50% of all U.S. deaths
Heart attack – blockage of coronary arteries which leads to cardiac muscle damage
Stroke – blockage of arteries in the brain
Atherosclerosis – artery walls develop plaques which narrow the space and increase chance of clot formation and heart attack
Hypertension – chronic high blood pressure which can promote atherosclerosis