Human Physiology Chapter on Nervous and Circulatory Systems

Homeostasis and ANS Review

• Homeostasis is the maintenance of a relatively stable internal environment.
• The endocrine and autonomic nervous systems influence organ function, but in different ways.
• The endocrine system uses hormones transported through the bloodstream to target cells.
• The autonomic nervous system has two branches:
  • Parasympathetic ("rest and digest"): typically has a single preganglionic axon leading to a ganglion and a postganglionic axon that releases acetylcholine targeting effector cells.
  • Sympathetic ("fight or flight"): typically has two lightly myelinated preganglionic axons leading to a ganglion for release of acetylcholine. A postganglionic axon releases norepinephrine targeting effector cells. An alternate pathway leads to the adrenal medulla and causes the release of epinephrine and norepinephrine into the blood stream that effects distant targets in the body.
• Both systems work together to regulate organ function.
• Most organs receive both sympathetic and parasympathetic input.

Autonomic Transmitters and Receptors

• The parasympathetic system uses acetylcholine (ACh) as its neurotransmitter.
• Acetylcholine binds to two main types of receptors:
  • Nicotinic receptors (found on postganglionic neurons): They respond to nicotine.
  • Muscarinic receptors (found on target cells): They respond to muscarine.
• The sympathetic system uses acetylcholine at the preganglionic synapse and norepinephrine at the postganglionic synapse to target cells.
• Agonists mimic the actions of neurotransmitters.
• Antagonists block neurotransmitter action.

Blood

• Blood functions include transporting gases, nutrients, and waste; defending against pathogens, regulating pH, ion composition, and temperature; and clotting at injury sites.
• Plasma comprises 46-63% of blood volume, consisting of water (91%), proteins (8%), and other substances (1%).
• Red blood cells (erythrocytes): lack nuclei and organelles, carry oxygen, and are produced via erythropoiesis (stimulated by decreased oxygen availability and erythropoietin).
• Hemoglobin (in RBCs): binds oxygen reversibly and comprises 95% of RBC protein. Hemoglobin comprises two alpha subunits and two beta subunits, each with a heme group containing iron, with each hemoglobin molecule capable of binding four oxygen molecules.
• White blood cells (leukocytes): defend the body against pathogens and toxins. Phagocytic leukocytes are capable of engulfing foreign material. Leukocytes include granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).
• Diseases related to blood include jaundice (bilirubin buildup), anemia (decreased oxygen-carrying capacity, including iron deficiency, hemorrhagic, and aplastic types), sickle cell anemia, and leukemia (cancer of white blood cells).
• Platelets: help in blood clot formation from fragments of megakaryocytes.
• Clot formation involves vascular spasm, platelet plug formation, and coagulation (fibrin formation).
• tPA (tissue plasminogen activator): dissolves blood clots.
• Thrombus: blood clot in a vessel.
• Embolus: a detached thrombus in the bloodstream.

Circulation Overview and Vessels

• Systemic circulation: transports blood to body organs except lungs.
• Pulmonary circulation: transports blood to the lungs.
• Blood vessels have three layers: tunica intima, tunica media, and tunica externa.
• Arteries generally have thicker walls than veins, with more smooth muscle and elastic fibers.
• Veins have valves to prevent backflow.
• Blood vessel types include elastic vessels, resistance vessels(arterioles), capacitance vessels (veins), and exchange vessels (capillaries).
• Capillaries allow for the exchange of materials between blood and tissues, and capillary types include continuous, fenestrated, and sinusoidal capillaries.
• Venous valves prevent backflow and maintain one way flow of blood.

Anatomy of the Heart

• The heart's position is in the thoracic cavity, to the left of midline.
• The heart wall has three layers: fibrous pericardium, serous pericardium, and myocardium.
• Coronary arteries supply the heart with oxygenated blood, and include the left and right coronary arteries and their branches including circumflex, left anterior descending, and branch arteries.
• Blood flow through the heart involves the right and left sides of the heart, including the atria, ventricles, and valves.
• AV (atrioventricular) valves prevent backflow from ventricles into atria.
• Semilunar valves prevent backflow from arteries into ventricles.
• Pacemaker cells (SA and AV nodes): have the ability to spontaneously depolarize to action potential.
• The electrical conduction system governs cardiac contraction. Impulses originate from the SA node and spread sequentially to the AV node, then the AV bundle and its branches to the Purkinje fibers, depolarizing cardiac contractile cells resulting in a coordinated contraction.

The Cardiac Cycle

• The cardiac cycle involves atrial and ventricular systole and diastole, with corresponding pressure changes and valve actions.
• ECG (electrocardiogram): records the heart's electrical activity, with waves corresponding to atrial and ventricular depolarization and repolarization.
• The cardiac cycle involves atrial systole, ventricular systole (in two phases), and ventricular diastole.

Cardiodynamics

• Cardiac output (CO) is the volume of blood pumped per minute and is a function of heart rate (HR) and stroke volume (SV), CO= HR X SV,.
• Factors affecting CO include heart rate, end-diastolic volume (EDV), end-systolic volume (ESV).
• Heart rate is regulated by the autonomic nervous system.
• Stroke volume is regulated by preload, afterload, and contractility.
• Ejection fraction (EF): is the percentage of EDV pumped out in one beat, a measure of the heart's pumping efficiency

Regulation of Blood Flow

• Blood flow is proportional to pressure difference and inversely proportional to resistance (F = ΔP/R).
• Blood pressure changes throughout the systemic circulation, and mean arterial pressure (MAP) reflects the average pressure.
• Pulse pressure is the difference between systolic and diastolic pressure.
• Vascular resistance is determined by vessel diameter, blood viscosity, and turbulence.
• Capillary exchange is driven by blood pressure (filtration) and osmosis (reabsorption).

Cardiovascular Adaptation

• Autoregulation adjusts blood flow locally to meet tissue needs.
• Baroreceptors: detect changes in blood pressure and trigger adjustments to maintain homeostasis.
• Hormonal reflexes: regulate longer-term cardiovascular adjustments (e.g., blood volume change).
• Cardiovascular adaptations occur in postural changes, exercise, and hemorrhage.

Disorders of the Cardiovascular System

• Risk factors for heart disease include hypertension, obesity, diabetes, and high cholesterol.
• Atherosclerosis and hypertension alter vessel structure and function.
• Procedures to restore blood flow to the heart wall include coronary artery bypass graft (CABG), balloon angioplasty, and stent insertion.
• Congestive heart failure: occurs when the heart cannot pump enough blood to meet the body's needs.
• Drugs used in cardiovascular treatments include alpha blockers, beta blockers, and calcium channel blockers.
• The cardiovascular system changes with age due to changes in the blood, blood vessels, and the heart.

Endocrine System I

• Neurotransmitters act locally, hormones act systemically.
• Endocrine signaling uses G-protein receptors to amplify signals.
• Cyclic AMP (cAMP) is commonly used in signaling cascades.
• The hypothalamus regulates hormone release.
• Anterior and posterior pituitary glands have different functions and relationships with the hypothalamus.
• Feedback loops are the main regulators of hormone secretion for homeostasis, often using negative feedback.