Chapter 14 Cardiac Output, Blood Flow and Blood Pressure

Cardiac Output, Blood Flow and Blood Pressure

• The pumping ability of the heart is a function of the beats per minute (cardiac rate) and the volume of blood ejected per beat (stroke volume). In addition, autonomic input and mechanisms intrinsic to the cardiovascular system regulate blood flow and blood pressure. 

Cardiac Output

• The cardiac output is the volume of blood pumped per minute. The average resting cardiac rate is about 72 BPM and the average stroke volume is 70-80 ml per beat. The product of these two variables leads to an average cardiac output of about 5.5 liters per minute. What else equals 5.5 liters in volume?

• Cardiac output can be regulated by  divisions as noted in the figure to the right.

The Frank-Starling Law of the Heart

• Two physiologists, Otto Frank and Ernest Starling, determined that as the end diastolic volume increases and thus greater stretch is placed on the heart ventricles that the heart contracts more forcefully leading to increased stroke volume. This is a built in or intrinsic property of heart muscle.

Regulation of Cardiac Output

• Factors that stimulate cardiac output are shown as solid arrows. 

• Factors that inhibit cardiac output are shown as dashed arrows.

• Cardiac output is a function of ANS input as well as the intersive effect provided by the Frank-Starling law.

Distribution of Blood Within the Circulatory System at Rest

• At rest notice that the venous system contains most of the blood. The veins function as a reservoir from which more blood can be added to the circulation under appropriate conditions such as exercise. Although veins contain 70% of the total blood volume the mean venous pressure is only 2mmHg due in part to loss of pressure between arteries and capillaries and in part to the high venous compliance or stretch. 

Exchange of Fluid Between Capillaries and Tissues

• Tissue or interstitial fluid is formed by filtration as a result of blood pressures at the arteriole ends of capillaries. It is returned to the venule ends of capillaries by the colloid osmotic pressure of plasma proteins; primarily albumin. 

Edema

• Excessive accumulation of tissue fluid is known as edema. This condition is normally prevented by proper balance between capillary filtration and osmotic uptake of water and by proper lymphatic drainage. However there are several conditions that can lead to the development of edema. 

• Causes of Edema

1. High arterial blood pressure

2. Venous obstruction

3.  Leakage of plasma proteins into interstitial fluid 

4. Decreased plasma protein concentration 

5. Obstruction of the lymphatic drainage 

6. Elephantiasis resulting from parasitic larvae that block lymphatic drainage 

Regulation of Blood Volume by Kidneys

• Thirst and antidiuretic hormone (ADH) secretion are triggered by a rise in plasma osmolality. Homeostasis is maintained by countermeasures including drinking water and retention of water by the kidneys. 

Renin-Angiotensin-Aldosterone System

• Conditions of salt deprivation, low blood volume and low blood pressure causes increased production of angiotensin II in the blood. Angiotensin II exerts numerous effects that cause blood pressure to increase. 

1. Kidneys sense decrease in blood pressure

2.  Kidney secretes renin (an enzyme)

3. Renin converts angiotensinogen to angiotensin 1 (peptides in the blood)

4. An enzyme in the lung (ACE) converts angiotensin I to angiotensin II

5. Angiotensin II promotes the adrenal gland to secrete aldosterone leading to salt retention by the kidney (water follows salt)

6. At the same time Angiotensin II causes vasoconstriction of arterioles

7.  BP comes back up. 

Blood Pressure in Different Vessels of the Systemic Circulation

• Notice that the pressure generated by the contraction of the ventricles is largely lost by the time the blood gets into the venous system and that this pressure drop occurs primarily as blood goes through the arterioles and capillaries. Average venous pressure is about 2 mmHg.

Blood Flow to the Heart 

• The heart is the hardest working organ in the body. Capillaries are packed in the myocardium to the tune of 2,500 to 4,000 capillaries per cubic millimeter of tissue. The heart muscle cells contain myoglobin which is related to hemoglobin and serves as an oxygen reservoir in heart muscle cells. In addition, the heart can regulate blood flow to itself under conditions of heavy exercise.

• During heavy exercise oxygen consumption can increase in the heart from 4 to 6 times resting values. Blood flow must also increase  to the myocardium to provide for this increased oxygen demand. Although several metabolic factors play a role, nitric oxide (NO) levels go up in the heart resulting in vasodilation of heart blood vessels.

Blood Vessels supplying the heart 

• The heart is composed mostly of heart muscle tissue called the myocardium. This hardworking tissue needs a generous and constant blood supply to function

• The blood vessels supplying the heart are called the coronary arteries and they branch directly off the aorta. 

• If blood flow to the heart is obstructed you have serious problems. Heart disease is the #1  cause of death in the United States

• A contrast medium (like a dye) is injected into the coronary arteries causing them to become visible on an X-ray.

Heart Attacks and Bypass Surgeries

• Bypass surgeries are required to “bypass” blocked coronary arteries in order to restore blood flow to heart tissue

• Double and triple bypass surgery reflects the number of bypassesinstalled

Blood Flow Changes During Exercise

• At rest the cardiac output is 5 L/min. During heavy exercise the output rises to 25 L/min. Notice that even though the skeletal muscles receive more blood the brain (and most other organs) still receives the same amount. 

• So during heavy exercise increased cardiac output provides adequate blood flow to all organs while increasing blood flow to skeletal muscles due to vasodilation and divertal blood flow to the skeletal muscles.

• Compare resting and exercise blood flow between the brain, heart and muscle. 

Cerebral Circulation

• When the brain is deprived of oxygen for just a few minutes a person loses consciousness and irreversible brain damage may occur after only a few minutes. For these reasons the brain gets a constant blood flow of about 750ml/min whether during exercise or at rest.

•  Blood flow is almost exclusively maintained in the brain through metabolic regulation including NO production. 

Blood Pressure

• A constriction increases blood pressure upstream (analogous to the arteriole pressure) and decreases pressure downstream (analogous to capillary and venous pressure). Variations in the  of arterioles as a result of vasoconstriction and vasodilation thus affect blood flow and arterial blood pressure. 

• The most important variable affecting arterial blood pressure are cardiac output and total peripheral resistance. Peripheral resistance is the resistance of the arteries to blood flow. As the arteries constrict, the resistance increases and as they dilate, resistance decreases. 

• Peripheral resistance is primarily determined by the Autonomic Nervous System: Sympathetic activity constricts  peripheral arteries and Parasympathetic activity dilates  them. 

• The most important variable affecting arterial blood pressure are the cardiac output and total peripheral resistance. Can you see now how hardening of the arteries (atherosclerosis) which restricts blood flow leads to increased blood pressure.

Baroreceptors

• Baroreceptors are stretch receptors located in the aortic arch and the carotid sinus. When blood pressure is going up the baroreceptors are stretched which increases their firing rates via sensory nerve fibers. The higher the blood pressure the greater the stretch and the greater the activity from the baroreceptors.

• The baroreceptors regulate blood pressure in both directions because they signal both the sympathetic and parasympathetic divisions of the autonomic nervous system

• A rise in blood pressure will produce a decline in sympathetic nerve activity while the activity of parasympathetic division increases. As a result a rise in blood pressure will evoke a decrease in cardiac output and decrease in total peripheral resistance. A drop in blood pressure will result in an opposite response (next slide.)

The Baroreceptor Reflex

• When a person goes from a lying to standing position there is a shift of blood going from the veins of the thoracic cavity to the veins of the lower extremities due to gravity. This decreases venous return and cardiac output. A decrease in activity from the baroreceptors inhibits parasympathetic activity in the medulla and promotes sympathetic nerve activity. This leads to increased cardiac output and vasoconstriction to maintain adequate blood flow to the brain. 

Measuring Blood Pressure

• An inflatable cuff is wrapped around the upper arm and a stethoscope is placed over the brachial artery. The cuff is inflated until the artery is pinched shut. The first sound heard is the turbulent flow of blood (systole) when there is no longer a sound to be heard laminar flow has been achieved (diastole)

Korotkoff Sounds

• Korotkoff sounds are the sounds that medical personnel listen for when they are taking blood pressure. They are named after Dr. Nikolai Korotkoff, a Russian doctor who pioneered this method in 1905.

Dangers of Hypertension

1. Makes it difficult for the ventricles to eject blood against higher “back pressure” arteriole pressure

2. Heart grows pathologically as a result

3. Arrhythmias and  problems can result leading to heart failure

4. Higher blood pressure can lead to stroke and “blown out” cerebral arteries.