45 Cardiovascular Integration of Systems
Blood Flow to Organ Systems
Our cardiac output varies with our level of activity, decreasing when resting and increasing when we are more active. For example, cardiac output can increase from the typical 5 L/min at rest up to 15-20 L/min during maximal exercise. When we experience such changes in cardiac output it is important to realize that not all organs and tissues will experience similar changes in blood flow. So when your cardiac output triples, you will not experience a tripling of flow to each organ. Instead, flow is directed to those tissues that need it most, typically based on their metabolic state. If you are sprinting, your cardiac output might triple, but the flow to the muscles in your legs might go up six or eight fold, while flow to your digestive organs can actually decrease. Thus flow is directed to those capillary beds surrounded by cells with the greatest requirement for oxygen and nutrients.
There are some organs in the body where their level of blood flow is not as dependent on their metabolic need, but because they are important in processing and/or interacting with blood in other ways. Examples include the kidneys, which process the blood by removing metabolic wastes, the lungs, which participate in gas exchange, the liver, which processes the substances absorbed in the GI tract, glands, which release hormones into the bloodstream, and the skin, which helps regulate body temperature. In the case of the liver and the pituitary gland, they interact with the blood through a portal system, where the vascular network in these tissues contains two capillary beds along the path of blood flow. The kidney blood flow also involves two capillary beds, but these are connected by an arteriole.
The portal system involving the liver plays an important role in processing substances absorbed by the capillaries along the intestines. These capillary beds in the intestines absorb nutrients and other chemicals. This same blood then travels to the liver where it enters the liver sinusoids (a second capillary bed) before returning to the venous circulation. Specific functions occurring in the liver include storing glucose as glycogen, repackaging lipids into lipoproteins and detoxification of harmful, and even useful, chemicals, such as drugs. In fact, when discussing the ability of orally administered drugs to distribute throughout the body, the effect of the liver is known as the “first pass” effect.
The kidney actually contains two capillary beds in the same organ, where the first capillary bed (the glomerular capillaries) filters the blood and the second (either the peritubular or vasa recta capillaries) reabsorbs most of the original filtrate.
The cells of the anterior pituitary gland respond to chemical signals secreted into capillaries of the hypothalamus. Once the blood from the hypothalamus enters the anterior pituitary, it enters a second capillary bed so that the cells of the anterior pituitary can sense these signals and respond by secreting their own hormones if necessary.
Neural Regulation of the Cardiovascular System
As you have learned, homeostasis relies on communication between body systems, which is primarily carried out by the nervous and endocrine systems. To convey information, the nervous system uses neural impulses, and the endocrine system uses hormones released into the blood. Most homeostatic controls involve negative feedback mechanisms, in which the output of a process either terminates or lessens the original change.
Neural Regulation of Cardiac Output
Both the parasympathetic and sympathetic branches of the autonomic nervous system (ANS) alter the cardiac output as necessary. The ANS modulates the heart rate and force of contraction (contractility) of the myocardium.
The parasympathetic nervous system decreases the heart rate. The vagus nerve (the 10th cranial nerve) innervates the SA and AV nodes of the heart. It is the parasympathetic branch of the ANS that sets the heart rate under normal resting condition. This is commonly referred to as the “vagal tone” as it is the vagus nerve that is responsible for this basal cardiac function. The effect of the parasympathetic system is to decrease the rate at which action potentials are generated at the SA and AV nodes and also to slow down the conduction speed between the SA and the AV nodes. Increasing parasympathetic activity increases the P-R interval of the ECG. Because the parasympathetic nervous system does not innervate the ventricular cells, it does not significantly affect the level of contractility of the ventricular myocardium. Conversely, the heart rate is increased through the actions of the sympathetic nervous system. The sympathetic nerves run from the sympathetic chain ganglia, found along the thoracic region of the spinal cord to the SA node, the AV node, and the myocytes of the ventricles. The sympathetic nerves increase the heart rate by increasing the spontaneous depolarization rate of the SA and AV nodes, and increases the rate of conduction through the AV node. Sympathetic innervation of the ventricular myocytes can be used to increase the contractility of the heart.
Hormonal Regulation of the Cardiovascular System
A number of hormones are involved in the regulation of blood flow and BP. They work by changing cardiac output, systemic vascular resistance, or total blood volume.
The Renin-Angiotensin-Aldosterone System
The renin-angiotensin-aldosterone system is set in motion when decreases in blood pressure or renal blood flow stimulate cells in the kidney to secrete renin into the blood. This leads to a pathway that ultimately produces the hormone angiotensin II, which increases BP in two ways. First, as a vasoconstrictor, angiotensin II increases BP by increasing peripheral vascular resistance. Second, it causes the kidney tubules to reabsorb sodium. High plasma angiotensin is also a trigger for higher levels of aldosterone synthesis by the adrenal cortex. This hormone also acts on the kidneys to increase the reabsorption of sodium ions. The increased reabsorption of Na+ leads to enhanced water reabsorption at the kidneys, and this causes the total blood volume to rise, elevating the BP and leading to an increase in venous return. This increase in the amount of blood flowing into the heart also increases cardiac output.
Epinephrine and Norepinephrine
Sympathetic stimulation promotes the release of epinephrine and norepinephrine by the adrenal medulla. As described above, these hormones (or neurotransmitters) increase the heart rate (chronotropy) and force of contraction (inotropy). As the force of contraction increases the stroke volume also increases. Because cardiac output (CO) is the product of HR and SV, CO increases. The increase is CO will also lead to an increase in arterial pressure, as described earlier. Epinephrine (or norepinephrine) also has the effect of constricting arterioles and veins in the abdominal organs and skin, while dilating vessels in the heart and skeletal muscle. This results in increased blood flow to muscles during physical activity.
Antidiuretic Hormone
Decreased blood volume or dehydration prompts an increased release of antidiuretic hormone (ADH) from the posterior pituitary. This hormone, which is produced by the hypothalamus and stored in the posterior pituitary, causes the kidney to retain water. This retention can lead to an increase in blood volume and increasing preload similarly to the renin-angiotensin system. At very high levels (such as those released during circulatory shock) it also causes peripheral blood vessels to constrict, helping to raise the BP. This is why antidiuretic hormone is also known as vasopressin.
Atrial Natriuretic Peptide (ANP)
Atrial natriuretic peptide (ANP), which is secreted by the myocytes of the atria of the heart, decreases blood pressure in two ways. First, it dilates blood vessels. Second, it reduces blood volume by increasing the excretion of salt. This is a peptide hormone that is secreted in response to higher blood volume that stretches the atrial walls. The effect of ANP is to ultimately lower blood pressure. To achieve that, ANP facilitates glomerular filtration pressure, increases excretion of sodium and urea, inhibits aldosterone secretion, and inhibits the renin-angiotensin system.
