CHAPTER 37 – THE CIRCULATORY AND RESPIRATORY SYSTEMS



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CHAPTER 37 – THE CIRCULATORY AND RESPIRATORY SYSTEMS

THE CIRCULATORY SYSTEM

All organisms move substances internally from one place to another. Some organisms rely on diffusion for this movement; humans cannot because we are too large & complex. We require a circulatory system for the movement of substances (O2, CO2, nutrients, & wastes) through the body. Humans have a closed circulatory system, which means blood is enclosed in vessels. The human circulatory system has three components: the heart, the blood vessels, and the blood.

I. THE HEART

The function of the heart is to keep the blood moving constantly in the body. The heart is a large organ made of cardiac muscle

cells that have a high number of mitochondria. Humans (plus other mammals and birds) have a 4-chambered heart. The heart is

divided into a right and left side with two chambers making up each side. The two sides are divided by a septum.

A. Structure

1. Membranes – There are 3 layers of tissue that surround the heart.

a. Pericardium – Outermost layer; membrane composed of connective tissue that surrounds and protects the heart.

b. Myocardium – Middle layer; muscle tissue that forms the four chambers of the heart. Cardiac muscle cells are striated

and interconnected to produce strong, powerful contractions.

c. Endocardium – Innermost membrane that lines the chambers, covers the valves, and continues into the vessels.

Composed of smooth, epithelial tissue to prevent blood clotting.

2. Chambers

a. Atria (atrium =singular) Upper chambers of the heart that receive blood. Walls are thinner and less elastic than ventricles.

b. Ventricles – Lower chambers of the heart that pump blood to the lungs and body. Have thick, elastic walls, especially

the left ventricle because it pumps blood out to the body.

3. Valves – Flaps of epithelial tissue reinforced with connective tissue that keep blood flowing in one direction and increase the pumping efficiency of the heart.

a. Atrio-Ventricular Valves: between the atria & ventricles; attached by chordae tendineae to papillary muscles; these valves prevent the backflow of blood into the atria when the ventricles pump blood away from the heart (systole).

• Tricuspid Valve – between the RA & RV

• Bicuspid (Mitral) Valve – between the LA & LV

b. Semi-Lunar Valves: between the ventricles and the vessels that lead away from the heart (aorta & pulmonary arteries);

prevent the reflux of blood into the ventricles when the ventricles relax and the atria contract (diastole).

• Pulmonary Valve – between the RV and the pulmonary artery

• Aortic Valve – between the LV and the aorta

B. Types of Circulation

1. Systemic – The flow of blood from the left side of the heart to the body and back to the heart. The main artery involved is the

aorta; main veins involved are the superior and inferior vena cava.

2. Pulmonary – The flow of blood from the right side of the heart to the lungs and back to the heart. Pulmonary arteries carry

O2 poor blood from the heart to the lungs; pulmonary veins carry O2 rich blood from the lungs to the heart.

3. Coronary – since the heart is a major organ, it needs its own supply of O2 rich blood. This is coronary circulation. The

coronary arteries are the first two branches of the aorta. These arteries branch into capillaries that circulate oxygenated

blood to the myocardium for the heart to use, then the coronary veins return the deoxygenated blood to the right atrium.

C. The Heartbeat

1. Contraction

Although the rate and force of contraction can be changed by the medulla oblongata, the heart generates and maintains

its own beat in an area known as the pacemaker. The heartbeat is divided into systole, which is when the ventricles are

contracting to push blood away from the heart; and diastole, which is when the ventricles are relaxed & the atria are

contracting to push blood from the atria into the ventricles.

2. The Pulse

The powerful contraction in the left ventricle causes a surge of blood to leave through the aorta. From there it travels to the

arteries leading to the rest of the body. This is known as a pulse.

THE HEART

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II. THE BLOOD VESSELS

Blood vessels are composed of connective tissue for strength, smooth muscle tissue to facilitate blood flow, and they are lined

with epithelial tissue for smoothness.

A. Types – There are three types of blood vessels:

1. Arteries – Carry blood Away from the heart. Arteries have thick, elastic walls to withstand the pressure of spurts of blood

from contraction of the heart. Arteries are deeply buried in skin to protect them from injury; it is hard to staunch blood flow

due to pressure of blood leaving left ventricle. Arteries branch into arterioles and then to capillaries. Arteries carry blood

that is high in oxygen and low in carbon dioxide, except for the pulmonary artery.

2. Veins – Carry blood to the heart. Veins have thinner, less elastic walls. They contain valves to keep blood flowing in one

direction. Valves are especially important in the legs because blood flows against gravity. Veins branch into venules which

lead to the other side of the capillary bed. Veins carry blood low in oxygen and high in carbon dioxide except for the

pulmonary vein.

3. Capillaries – Network in which O2, glucose, & other nutrients are exchanged with CO2 and other wastes. Capillaries

are only one-cell thick, so red blood cells must flow through one-by-one. This allows time for diffusion.

B. Blood Pressure – Measurement of pressure exerted on the walls of arteries. Systolic pressure is the pressure on the

artery walls when the ventricles contract and diastolic pressure is the pressure when the ventricles relax (atria contract).

Written as a fraction (systolic pressure / diastolic pressure. Average Range = 100 – 140 / 60 – 90.

III. THE BLOOD

Blood is classified as connective tissue with a matrix of plasma. Plasma is the liquid that makes up approximately 55% of

blood. Plasma is 90% water, the other 10% is made up of proteins, ions, vitamin K, lipids, hormones, etc.

A. Erythrocytes (RBC’s)

Function is to deliver O2 to all body cells. Most numerous cells in blood, they are doughnut-shaped cells that lose their

nuclei as they mature in the red marrow. This adaptation allows them to squeeze through tiny capillaries and provides more

area for oxygen transport. Red blood cells contain hemoglobin, and iron-containing protein that binds oxygen.

Erythrocytes circulate every 120 days & are destroyed in the spleen.

B. Leukocytes (WBC’s)

Larger and fewer in number than RBC’s. Their primary function is to fight infection by engulfing pathogens or marking them

for destruction. Some WBC’s are able to leave the blood vessels.

C. Thrombocytes (platelets)

Cell fragments that break off from large cells produced in the red marrow. Like RBC’s, they lack a nucleus, but platelets

are smaller than red blood cells. Platelets begin the clotting process by sticking together to form a plug when they come in

contact with a rough surface. The platelets that make up the plug release a series of proteins called fibrinogens (clotting

factors) which trigger a series of chemical reactions to produce a clot. Thrombocytes live about 10 days.

D. Blood Types – There are ID tags on the surface of RBC’s that are composed of carbohydrates and proteins called antigens.

The two antigens that determine human blood types are A and B. The presence or absence of these antigens is an inherited

trait that has 3 alleles and shows co-dominance. The A & B antigens on RBC’s are very important because an individual is

also born with antibodies dissolved in the plasma to any foreign antigen(s). An antibody attaches to a foreign particle and

marks it for destruction.

III. DISEASES OF THE CIRCULATORY SYSTEM

A. Atherosclerosis – Plaques of cholesterol build up on the wall of arteries (most commonly, coronary arteries). Restricts blood

flow and creates a rough surface that may trigger clot formation.

B. Arteriosclerosis – Also known as “hardening of the arteries”. Artery walls lose their elasticity, become weakened and more

susceptible to rupture.

C. Heart Attack (Myocardial Infarction)– Caused by blockage of coronary artery which cuts off blood flow to the myocardium

of the heart. Cells die due to lack of O2.

D. Anemia – Caused by either a deficiency of red blood cells or hemoglobin in RBC’s. Both result in less O2 available for cells

and a lack of energy due to decreased ATP production.

E. Hypertension – High blood pressure. Called the “silent killer” because it may be fatal before noticeable symptoms appear.

Risk of hypertension is increased by smoking, obesity, high salt intake, lack of exercise.

THE RESPIRATORY SYSTEM

The function of the respiratory system is to bring about the exchange of O2 and CO2 . This is called external respiration. Cellular respiration is the production of ATP from glucose and O2 in the mitochondria of a cell. CO2 and H2O are released as waste products. External respiration provides the O2 needed for cellular respiration to occur!!!

I. THE RESPIRATORY PASSAGEWAY

A. Airway to the Lungs

Air normally enters the respiratory system through the mouth and nasal passages, which begin the process of filtering,

warming, and moistening the air. From there, the air passes through the pharynx (upper throat) and the larynx. The larynx

contains the vocal cords, bands of connective tissue that tighten and vibrate to create sound when air passes through.

Attached to the larynx is a flap of tissue called the epiglottis. The epiglottis closes when food is swallowed to prevent food from

entering the trachea. After air passes over the larynx, it enters the trachea, a tube supported by bands of cartilage to prevent it

from collapsing when air passes in and out. The trachea divides at its lower end into two tubes called bronchi (singular =

bronchus). The trachea and bronchi are lined with cilia and cells that secrete mucus. The mucus traps dirt and dust and the

cilia beat upward to push the dirt and dust toward the nasal passages so that you can either cough or sneeze to get them out of

the respiratory tract.

B. Inside the Lungs

Inside each lung, the bronchi narrow as they branch into smaller passageways called bronchioles. The bronchioles end in

millions of tiny sacs called alveoli. The alveoli are the site for the exchange of O2 and CO2. Each alveolus is surrounded by a

capillary to allow O2 to diffuse from the lung to the blood to be delivered to cells for cellular respiration, and CO2 to diffuse from

the blood to the lung to be exhaled.

II. BREATHING

A. Inhalation & Exhalation

The lungs are surrounded by the pleural membrane and hang freely in the thoracic cavity. There are no muscles attached to

the lungs. The muscles involved in breathing are the intercostal muscles, located between the costas and the diaphragm, a

dome-shaped muscle located below (but not attached to!) the lungs.

Breathing occurs as a result of a change in pressure. When the diaphragm contracts, it flattens which increases the volume of

the chest cavity and decreases the pressure. In addition, the intercostal muscles contract, further increasing the volume of the

chest cavity. Air rushes into the lungs. As the diaphragm and intercostal muscles relax, volume of the chest cavity decreases,

and the increased pressure of the lungs help force the air back out.

B. Control of Breathing

Breathing is controlled by the medulla oblongata. Motor neurons stimulate the skeletal muscle of the diaphragm to contract.

The stimulus for breathing is the concentration of CO2 in the blood, which is monitored by the hypothalamus.

III. RESPIRATORY SYSTEM DISEASES

A. Emphysema – progressive degenerative disease in which alveoli lose their elasticity. Affected individuals cannot fully exhale

which reduces ability to inhale. O2 and CO2 do not diffuse properly. All smokers eventually develop emphysema.

B. Asthma – allergic reaction that affects smooth muscles of the bronchioles. They constrict which prevents airflow into the alveoli.

Gas exchange is reduced.

C. Smoking – three dangerous substances found in cigarette smoke are nicotine, carbon monoxide, and tar. Nicotine is a

stimulant that increases heart rate and constricts blood vessels, therefore increasing blood pressure. The carbon monoxide

binds to hemoglobin, which reduces the oxygen-carrying capacity of RBC’s. Tar is a carcinogenic giving smokers a much higher

incidence of lung cancer. In addition, inhaled nicotine and CO paralyze the cilia lining the digestive tract resulting in frequent

respiratory infections.

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