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Does arterial blood always flow away from the heart?

Does arterial blood always flow away from the heart?



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Is there a negative net flow of blood in human arteries at any point of the cardiac cycle? I realise that blood flow can be turbulent, e.g. in the aorta or around stenotic arteries, but then the average is still flowing away from the heart.

The question was provoked by this graph, which admittedly I haven't seen in its context:

A similar graph showed negative velocities in the aorta and innominate artery of humans.


Simple answer, yes.

It really is a matter of how technical you wish to be. By defintion, all arterial flow conveys blood away from the heart. The largest arteries exhibit the Windkessel effect which allows for the compression-based effects that bobthejoe inferred. Explicity, as blood is pumped out of the left ventricle, it stretches te aorta, and this tension in turn causes compression on the artery and pushes blood forward. On the flip side, venous vessels (except the pulmonary vein) return blood to the heart, but here you are likely to encounter local areas of reverse flow. This is counteracted to a degree by one-way valves present in the large veins to keep the net movement of blood in the body always toward the heart.


Quick answer, no.

Imagine a balloon; as you compress the balloon, there will be a lot of air that leaves it. But as you let it relax, the balloon pulls in surrounding air even as you fill it from another side. When the aortic and tricuspid valves are closed, there is blood flow forward but due to conservation of mass, the blood goes back to fill in the empty region.


17.4: Blood Vessels

  • Contributed by Suzanne Wakim & Mandeep Grewal
  • Professors (Cell Molecular Biology & Plant Science) at Butte College

Why do bodybuilders have such prominent veins? Bulging muscles push surface veins closer to the skin. Couple that with a virtual lack of subcutaneous fat, and you have bulging veins as well as bulging muscles. Veins are one of three major types of blood vessels in the cardiovascular system.

Figure (PageIndex<1>): 2012 Hong Kong Bodybuilding Championship participant


Blood Vessels

Figure 1. The major human arteries and veins are shown. (credit: modification of work by Mariana Ruiz Villareal)

The blood from the heart is carried through the body by a complex network of blood vessels (Figure 1). Arteries take blood away from the heart. The main artery is the aorta that branches into major arteries that take blood to different limbs and organs. These major arteries include the carotid artery that takes blood to the brain, the brachial arteries that take blood to the arms, and the thoracic artery that takes blood to the thorax and then into the hepatic, renal, and gastric arteries for the liver, kidney, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then smaller vessels called arterioles, to reach more deeply into the muscles and organs of the body.

Arterioles diverge into capillary beds. Capillary beds contain a large number (10 to 100) of capillaries that branch among the cells and tissues of the body. Capillaries are narrow-diameter tubes that can fit red blood cells through in single file and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Fluid also crosses into the interstitial space from the capillaries. The capillaries converge again into venules that connect to minor veins that finally connect to major veins that take blood high in carbon dioxide back to the heart. Veins are blood vessels that bring blood back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart via the lymphatic system.

The structure of the different types of blood vessels reflects their function or layers. There are three distinct layers, or tunics, that form the walls of blood vessels (Figure 2). The first tunic is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. The endothelial tunic is continuous with the endocardium of the heart. In capillaries, this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis. The movement of materials at the site of capillaries is regulated by vasoconstriction, narrowing of the blood vessels, and vasodilation, widening of the blood vessels this is important in the overall regulation of blood pressure.

Figure 2. Arteries and veins consist of three layers: an outer tunica externa, a middle tunica media, and an inner tunica intima. Capillaries consist of a single layer of epithelial cells, the tunica intima. (credit: modification of work by NCI, NIH)

Veins and arteries both have two further tunics that surround the endothelium: the middle tunic is composed of smooth muscle and the outermost layer is connective tissue (collagen and elastic fibers). The elastic connective tissue stretches and supports the blood vessels, and the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the higher pressure and speed of freshly pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different than arteries in that veins have valves to prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart.


Blood Pressure

Blood pressure is the pressure of blood against the blood vessel walls during the cardiac cycle it is influenced by a variety of factors.

Learning Objectives

Describe the process of blood pressure regulation

Key Takeaways

Key Points

  • Normal blood pressure for a healthy adult is 120 mm Hg during systole (peak pressure in the arteries ) and 80 mm Hg during diastole (the resting phase).
  • Blood pressure is regulated in the body by changes to the diameters of blood vessels in response to changes in the cardiac output and stroke volume.
  • Factors such as stress, nutrition, drugs, exercise, or disease can invoke changes in the diameters of the blood vessels, altering blood pressure.

Key Terms

  • cardiac output: the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute
  • hydrostatic: of or relating to fluids, especially to the pressure that they exert or transmit
  • stroke volume: the volume of blood pumped from one ventricle of the heart with each beat

Blood Pressure

Blood pressure is the pressure of the fluid (blood) against the walls of the blood vessels. Fluid will move from areas of high to low hydrostatic pressures. In the arteries, the hydrostatic pressure near the heart is very high. Blood flows to the arterioles (smaller arteries) where the rate of flow is slowed by the narrow openings of the arterioles. The systolic pressure is defined as the peak pressure in the arteries during the cardiac cycle the diastolic pressure is the lowest pressure at the resting phase of the cardiac cycle. During systole, when new blood is entering the arteries, the artery walls stretch to accommodate the increase of pressure of the extra blood. During diastole, the walls return to normal because of their elastic properties.

Blood pressure values are universally stated in millimeters of mercury (mm Hg). The blood pressure of the systole phase and the diastole phase gives the two readings for blood pressure. For example, the typical value for a resting, healthy adult is 120/80, which indicates a reading of 120 mm Hg during the systole and 80 mm Hg during diastole.

Relationship between blood pressure and velocity: Blood pressure is related to the blood velocity in the arteries and arterioles. In the capillaries and veins, the blood pressure continues to decease, but velocity increases.

Blood Pressure Regulation

Throughout the cardiac cycle, the blood continues to empty into the arterioles at a relatively even rate. However, these measures of blood pressure are not static they undergo natural variations from one heartbeat to another and throughout the day. The measures of blood pressure also change in response to stress, nutritional factors, drugs, or disease. The body regulates blood pressure by changes in response to the cardiac output and stroke volume.

Cardiac output is the volume of blood pumped by the heart in one minute. It is calculated by multiplying the number of heart contractions that occur per minute (heart rate) times the stroke volume (the volume of blood pumped into the aorta per contraction of the left ventricle). Therefore, cardiac output can be increased by increasing heart rate, as when exercising. However, cardiac output can also be increased by increasing stroke volume, such as if the heart were to contract with greater strength. Stroke volume can also be increased by speeding blood circulation through the body so that more blood enters the heart between contractions. During heavy exertion, the blood vessels relax and increase in diameter, offsetting the increased heart rate and ensuring adequate oxygenated blood gets to the muscles. Stress triggers a decrease in the diameter of the blood vessels, consequently increasing blood pressure. These changes can also be caused by nerve signals or hormones even standing up or lying down can have a great effect on blood pressure.


BISC 104 Mastering Biology Chapter 20.2 and 11.1

Drag the terms on the left to complete the sentence on the right. Not all terms will be used.

The statements on the left are scientific assertions. The statements on the right are examples from the chapter.

Drag each scientific assertion to the example it best supports.

During a breath, air is drawn into the lungs when the chest cavity enlarges, thanks to contraction of the diaphragm. The brain stem governs this process in response to blood carbon dioxide levels.

Blood vessels can become blocked with cholesterol-containing plaques, which reduce blood flow and increase blood pressure. Plaques can break off and clog a blood vessel, causing tissue death.

Blood moves around the body in the cardiovascular system, powered by the heart's pumping and controlled by arteries and capillary beds.

Gas exchange occurs in the lungs' alveoli. Damage to alveoli reduces the surface area for gas exchange. Chemicals in tobacco smoke can inhibit blood's ability to carry oxygen.

Gases enter the body through the nose and mouth and travel through the respiratory system to the lungs, where materials are exchanged with the bloodstream.

2. During a breath, air is drawn into the lungs when the chest cavity enlarges, thanks to contraction of the diaphragm. The brain stem governs this process in response to blood carbon dioxide levels.

3. Gas exchange occurs in the lungs' alveoli. Damage to alveoli reduces the surface area for gas exchange. Chemicals in tobacco smoke can inhibit blood's ability to carry oxygen.

4. Blood moves around the body in the cardiovascular system, powered by the heart's pumping and controlled by arteries and capillary beds.


The Respiratory System

Take a breath in and hold it. Wait several seconds and then let it out. Humans, when they are not exerting themselves, breathe approximately 15 times per minute on average. This equates to about 900 breaths an hour or 21,600 breaths per day. With every inhalation, air fills the lungs, and with every exhalation, it rushes back out. That air is doing more than just inflating and deflating the lungs in the chest cavity. The air contains oxygen that crosses the lung tissue, enters the bloodstream, and travels to organs and tissues. There, oxygen is exchanged for carbon dioxide, which is a cellular waste material. Carbon dioxide exits the cells, enters the bloodstream, travels back to the lungs, and is expired out of the body during exhalation.

Breathing is both a voluntary and an involuntary event. How often a breath is taken and how much air is inhaled or exhaled is regulated by the respiratory center in the brain in response to signals it receives about the carbon dioxide content of the blood. However, it is possible to override this automatic regulation for activities such as speaking, singing and swimming under water.

During inhalation the diaphragm descends creating a negative pressure around the lungs and they begin to inflate, drawing in air from outside the body. The air enters the body through the nasal cavity located just inside the nose (Figure 1). As the air passes through the nasal cavity, the air is warmed to body temperature and humidified by moisture from mucous membranes. These processes help equilibrate the air to the body conditions, reducing any damage that cold, dry air can cause. Particulate matter that is floating in the air is removed in the nasal passages by hairs, mucus, and cilia. Air is also chemically sampled by the sense of smell.

From the nasal cavity, air passes through the pharynx (throat) and the larynx (voice box) as it makes its way to the trachea (Figure 1). The main function of the trachea is to funnel the inhaled air to the lungs and the exhaled air back out of the body. The human trachea is a cylinder, about 25 to 30 cm (9.8–11.8 in) long, which sits in front of the esophagus and extends from the pharynx into the chest cavity to the lungs. It is made of incomplete rings of cartilage and smooth muscle. The cartilage provides strength and support to the trachea to keep the passage open. The trachea is lined with cells that have cilia and secrete mucus. The mucus catches particles that have been inhaled, and the cilia move the particles toward the pharynx.

The end of the trachea divides into two bronchi that enter the right and left lung. Air enters the lungs through the primary bronchi. The primary bronchus divides, creating smaller and smaller diameter bronchi until the passages are under 1 mm (.03 in) in diameter when they are called bronchioles as they split and spread through the lung. Like the trachea, the bronchus and bronchioles are made of cartilage and smooth muscle. Bronchi are innervated by nerves of both the parasympathetic and sympathetic nervous systems that control muscle contraction (parasympathetic) or relaxation (sympathetic) in the bronchi and bronchioles, depending on the nervous system’s cues. The final bronchioles are the respiratory bronchioles. Alveolar ducts are attached to the end of each respiratory bronchiole. At the end of each duct are alveolar sacs, each containing 20 to 30 alveoli. Gas exchange occurs only in the alveoli. The alveoli are thin-walled and look like tiny bubbles within the sacs. The alveoli are in direct contact with capillaries of the circulatory system. Such intimate contact ensures that oxygen will diffuse from the alveoli into the blood. In addition, carbon dioxide will diffuse from the blood into the alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizes the structural and functional relationship of the respiratory and circulatory systems. Estimates for the surface area of alveoli in the lungs vary around 100 m 2 . This large area is about the area of half a tennis court. This large surface area, combined with the thin-walled nature of the alveolar cells, allows gases to easily diffuse across the cells.

Figure 1. Air enters the respiratory system through the nasal cavity, and then passes through the pharynx and the trachea into the lungs. (credit: modification of work by NCI)

Which of the following statements about the human respiratory system is false?


With the DU in the left ventricular outflow tract blood acceleration is 1430 ± 120 cm/sec 2 , in the sinotubular junction and ascending aorta 2395 ± 195cm/sec 2 , at the aortic arch 1390 ± 225cm/sec 2 , isthmus of aorta 2180 ± 135cm/sec 2 , middle thoracic aorta1260 ± 140m/sec 2 (Figure 1).

Figure 1. Blood acceleration quantification by the duplex US of the heart. A. Investigation of the blood flow at the left ventricular outflow tract. B. Blood flow at the sinotubular junction.

With the MRI, blood acceleration from the left ventricular outflow tract to the sinotubular junction is 3.5 ± 0.3 times higher and to the ascending aorta 2.5 ± 0.2 times higher (Figure 2, Figure 3).

Figure 2. Blood flow quantification in the heart ventricle and aorta. (MRI. True Fisp. Mean curve). Red point - left ventricular outflow tract, yellow - sinotubular junction, green - ascending aorta.

A. Diameter at the ventricular outflow tract is lower, than diameter at the sinotubular junction and the peak flow velocity at the ventricular outflow tract must be higher (Bernoulli&rsquos principle). But the flow velocity increases in the sinotubular junction. Acceleration is formed by the pressure (Newton second law in motion) from the heart muscle and it must be higher in the ventricular outflow tract. Herewith, blood flow velocity in the sinotubular junction must be decreases due to the increasing flow resistivity by the turbulence in the Valsalva sinuses.

B. Mean curve diagrams shows evolution of the flow velocity. Flow acceleration is calculated as the Tg&phi of the angle between the amplitude of the peak velocity and the flow time.

Figure 3. A. Blood velocity graph in the heart ventricle and aorta. (MRI. TrueFisp. Mean curve). Red and yellow points &ndash opposite walls of the left ventricular outflow tract, green and blue - in the sinotubular junction.

B. Blood acceleration at the opposite wall of the vessels is in the different directions, due to the simultaneously: rotational and translational motion of the blood at the boundary layer forming helival motion of the substance (rolling motion in the surface wave). Amplitude of the flow velocity and acceleration increases with the increasing vessel diameter, while it should be decreases due to the Bernoulli&rsquos principle.

Systolic blood pressure from the ascending aorta to the femoral and saphenous elastic arteries enhancing 1.3 ± 0.1 times, increasing energy transmitted to the blood.

Blood flow acceleration is coincident with the ECG-qRs wave. Direction of the negative charge at the heart's ventricles from the circulating erythrocytes and in the fibers of the Purkinje (ECG), mathematically are coincident.


Arteries and arterioles

The arteries, which are strong, flexible, and resilient, carry blood away from the heart and bear the highest blood pressures. Because arteries are elastic, they narrow (recoil) passively when the heart is relaxing between beats and thus help maintain blood pressure. The arteries branch into smaller and smaller vessels, eventually becoming very small vessels called arterioles. Arteries and arterioles have muscular walls that can adjust their diameter to increase or decrease blood flow to a particular part of the body.


The 2 main coronary arteries are the left main and right coronary arteries.

Left main coronary artery (LMCA). The left main coronary artery supplies blood to the left side of the heart muscle (the left ventricle and left atrium). The left main coronary divides into branches:

The left anterior descending artery branches off the left coronary artery and supplies blood to the front of the left side of the heart.

The circumflex artery branches off the left coronary artery and encircles the heart muscle. This artery supplies blood to the outer side and back of the heart.

Right coronary artery (RCA). The right coronary artery supplies blood to the right ventricle, the right atrium, and the SA (sinoatrial) and AV (atrioventricular) nodes, which regulate the heart rhythm. The right coronary artery divides into smaller branches, including the right posterior descending artery and the acute marginal artery. Together with the left anterior descending artery, the right coronary artery helps supply blood to the middle or septum of the heart.

Smaller branches of the coronary arteries include: obtuse marginal (OM), septal perforator (SP), and diagonals.


The "Main artery" is called the aorta, and the "main vein" is called the vena cava.

Also label these blood vessels.

What do the different colours represent?

blue:> deoxygenated blood (blood with extra CO2)


Blood enters and leaves most organs via a pair of blood vessels (artery and vein) which are named according to the organ.

What do you think is the name of the blood vessel bringing blood into the lungs?

The hepatic portal vein is an exception, taking blood from the digestive system to the liver.

In what way do you expect that this blood may differ from other blood in the body?

> extra products of digestion (glucose, amino acids etc.)

Label the following on the diagram above: blood vessels leading to and from the lungs, liver, digestive system, and kidneys.


Blood Flow Positive and Negative Effects

A healthy heart normally beats anywhere from 60 to 70 times per minute when you're at rest. This rate can be higher or lower depending on your health and physical fitness athletes generally have a lower resting heart rate, for example.

Your heart rate rises with physical activity, as your muscles consume oxygen while they work. The heart works harder to bring oxygenated blood where it is needed.

Disrupted or irregular heartbeats can affect blood flow through the heart. This can happen in multiple ways:

  • Electrical impulses that regulate your heartbeat are impacted, causing an arrythmia, or irregular heartbeat. Atrial fibrillation is a common form of this.  
  • Conduction disorders, or heart blocks, affect the cardiac conduction system, which regulates how electrical impulses move through the heart. The type of block—an atrioventricular (AV) block or bundle branch block—depends on where it occurs in the conduction system.  
  • Damaged or diseased valves can become ineffective or leak blood in the wrong direction.
  • A blocked blood vessel, which can happen gradually or suddenly, can disrupt blood flow, such as during a heart attack.

If you experience an irregular heartbeat or cardiac symptoms like chest pain and shortness of breath, seek medical help immediately.