Digestive And Excretory System

Structure and Function of Life

Digestive, Excretory, Respiratory, and Circulatory Systems

No matter what you eat, everything that goes into your mouth is processed by your body’s digestive system. Three main processes occur within the digestive system: digestion, absorption, and elimination.

• Digestion is the breakdown of food into nutrients, which are molecules that your body’s cells can use.
• Absorption is the movement of nutrients from the digestive system into the bloodstream, where they can be carried to all parts of the body.
• Elimination is the removal of undigested material from the body.

These processes occur in a series of organs called the digestive tract. The organs of the digestive system help the process by moving food around or by producing chemicals used in digestion.

The Esophagus and Stomach

Digestion starts as soon as you put food in your mouth. Chewing breaks food into smaller pieces and enzymes in your saliva help to chemically break down food. Smooth muscle in the digestive tract moves food through the digestive system. In this way, the digestive system interacts with the muscular system. Food moves from your mouth to your stomach through a muscular tube called the esophagus. Your stomach is the body’s main organ of digestion. Food is broken down by the stomach’s digestive juices.

The Small and Large Intestines

Partially digested food moves from the stomach to the small intestine, where absorption occurs. Digested nutrients and water pass through the walls of the small intestine and into the bloodstream. Waste materials continue through the small intestine and move into the large intestine. In the large intestine, waste materials are prepared for elimination, which is the passage of undigested material out of the body through a:n opening called the anus.

The Liver, Gallbladder, and Pancreas

Even though food does not move through them, the liver, gallbladder, and pancreas are important parts of the digestive system. These three organs aid digestion in the small intestine. The liver is the largest internal organ of the body and has many functions. Its job in digestion is to make bile, which breaks down fat. Excess bile made by the liver is stored in a small organ called the gallbladder. The gallbladder releases bile into the small intestine as needed. The pancreas is a leaf-shaped organ that produces digestive enzymes. Enzymes from the pancreas are proteins that speed up biological reactions. The enzymes the pancreas makes help break down proteins, carbohydrates, and fats in the small intestine.

The Excretory System

Processes within your cells result in waste products. These waste products must be removed from your body to keep your body systems in balance. The excretory system is the system that removes liquid, solid, and gas wastes from the body.

Liquid Waste

Your skin is one of the organs of the excretory system, and sweating is one process of excretion. Your skin sweats in conditions such as warm weather and during exercise. The sweat helps your body with temperature regulation. As sweat evaporates from your skin, this helps your body cool down. When your skin sweats, this also allows your body to get rid of excess water and salts. The lungs, which are part of the respiratory system, are also important structures in the excretory system. Carbon dioxide is a gas produced as a waste product from cell processes. Most of it is removed from your body through the lungs when you exhale.

Urine is produced by the kidneys, which are organs that are separate from the digestive tract. Kidneys are the main organs of the excretory system. People usually have two kidneys. The kidneys use millions of tiny filters to separate waste products in the blood from the water, glucose, and minerals the body needs.

Liquid waste produced by the kidneys is carried in urine, which passes through a tube called the ureter and is stored in the bladder. When the bladder is full, it contracts and pushes urine out of the body through the urethra. In most instances, the emptying of the bladder is under the control of voluntary muscles.

Blood enters each kidney through a large artery. Inside the kidney, the artery divides into many networks of capillaries that surround the filtering units of the kidney, which are called nephrons. Each kidney has about one million nephrons. Each nephron looks like a long coiled tube with a cup at one end. The cups of the nephrons are found in the outer rim of the kidney. Fluid from the blood is pushed through the walls of the capillaries and into the nephron. Some of the material that is moved into the nephron is waste material, and some is material the body needs. The material the body needs is returned to the blood through a process called re-absorption. The material that is waste, along with water, leaves the body as urine.

Solid Waste

Solid waste from the digestive system is prepared in the large intestine for elimination. Water is removed from the waste material, which eventually leaves the body through the anus as feces.

The Respiratory System

Did you know that your digestive system is closely connected to your respiratory system? Oxygen, which enters your body through the respiratory system, is required for the process your cells use to release energy from food molecules. Specifically, oxygen is the gas that cells use for cellular respiration. Through cellular respiration, organic molecules from the food you eat are broken down, and energy and carbon-dioxide gas are released. The released energy is used by body cells for all the cells’ activities.

Gas Exchange Within the Lungs

When you breathe, you pull air into a pair of organs inside the chest called lungs. The inside of a lung is divided into many small air sacs, called alveoli, which are surrounded by tiny blood vessels called pulmonary capillaries. Oxygen in the lungs enters the body by diffusing across the alveoli and into the blood vessels. Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. Inside the alveoli, oxygen moves into the bloodstream because there is more oxygen in the alveoli than there is in the blood. Meanwhile, carbon dioxide leaves the bloodstream because there is more carbon dioxide in the blood than there is in the alveoli. The oxygen travels within the bloodstream to other parts of the body, and most of the carbon dioxide is exhaled.

The Circulatory System

The circulatory system transports blood through the human body. Blood delivers water·and nutrients from the digestive system, and oxygen from the respiratory system, to all cells in the body. Blood also carries wastes from body cells to the organs that remove wastes.

The Heart

The center of the circulatory system is the heart, which pumps blood. The heart is a fist-sized muscle divided into two upper chambers called atria and two lower chambers called ventricles.

Oxygen-poor blood from the body enters the right side of the heart. The right atrium and right ventricle pump oxygen-poor blood to the lungs. While in the lungs, the blood takes up oxygen gas and releases waste carbon-dioxide gas it has carried away from body cells. Oxygen-rich blood returns to the left side of the heart from the lungs. The left atrium and left ventricle pump oxygen rich blood to the rest of the body. When it reaches body cells, the oxygen-rich blood gives up its oxygen and nutrients and picks up carbon-dioxide gas. The blood returns back to the right atrium, and the cycle continues.

Blood Vessels

There are three kinds of blood vessels inside the body: arteries, capillaries, and veins. In general, arteries carry oxygen-rich blood away from the heart. As blood moves away from the heart, the arteries get smaller and narrower. Capillaries are microscopic vessels that connect arteries and veins. Capillaries are only one cell thick. Veins carry blood back to the heart from the capillaries.

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Digestive and Excretory Systems for the MCAT: Everything You Need to Know

Learn key MCAT concepts about the digestive and excretory systems, plus practice questions and answers

(Note: This guide is part of our MCAT Biology series.)

Table of Contents

Part 1: Introduction to the digestive and excretory systems

Part 2: Digestive system

a) Gastrointestinal anatomy

b) Accessory organs

c) Enzymatic digestion and absorption

Part 3: Excretory system

a) Urine production

b) The nephron

c) Endocrine control

Part 4: High-yield terms

Part 5: Passage-based questions and answers

Part 6: Standalone questions and answers

Part 1: Introduction to the digestive and excretory systems

The body requires energy to function and maintain homeostasis. How does our body absorb the nutrients it needs and excrete the waste it produces?

The digestive and excretory systems work together to break down the food we eat into absorbable nutrients and expel waste. A significant amount of biology questions on the MCAT directly cover content described in this guide—including the structure and function of the kidney. Having a thorough understanding of these systems is a definite advantage now and in your future medical career.

Throughout this guide, you will find several bolded terms that are important to understand and recall. At the end of this guide, you will also find several MCAT-style questions for you to test your knowledge.

Let’s jump right in!

Part 2: Digestive system

a) Gastrointestinal anatomy

The digestive tract is a long, tube-like structure that runs through the entire torso and abdomen of the human body. Food that enters the digestive tract at the mouth undergoes a series of digestive steps as useful nutrients are absorbed into the bloodstream, and any unabsorbed nutrients exit as waste.

Believe it or not, digestion starts at the mouth. The mouth is responsible for both the mechanical and chemical digestion of food. Mastication, or chewing, is the mouth’s primary method of mechanical digestion. Mechanical digestion results in the breaking down of large particles of food into smaller pieces, increasing the total surface area. The increased surface area aids in chemical digestion performed by salivary enzymes. The salivary glands of the mouth produce enzymes, known as salivary amylase and salivary lipase, which begin to break down the chemical bonds of sugars and lipids in the food. As food doesn’t stay for very long in the mouth, the degree of digestion is quite limited but will continue further along the digestive tract.

In preparation for swallowing, the tongue rolls food into a ball-shaped mass known as a bolus. The bolus is then moved into the oropharynx, where muscles work to push the bolus out of the mouth.

During swallowing, food passes through the pharynx, a tube-shaped structure that is shared between the respiratory and digestive systems. To prevent food from entering the lungs, a flap-like structure known as the epiglottis automatically closes the opening of the larynx. As a result, food can safely travel down the pharynx and into the esophagus.

The esophagus connects the mouth to the stomach and has two sphincters: the upper and lower esophageal sphincters. (Recall that sphincters are simply ring-shaped muscles that can open and close by contracting.) These sphincters regulate the entrance of food into the esophagus and the stomach, respectively.

Perhaps surprisingly, humans lack voluntary control of the esophagus. Only the upper third of the esophagus is composed of skeletal muscle, which allows us to swallow our food. Our food moves down the rest of the esophagus through wavelike movement created by the smooth muscle occupying the rest of the tube. This movement is known as peristalsis. The movement is assisted by the presence of saliva that has been mixed with the bolus, which serves an additional function as a lubricant.

As our food moves down the esophagus, it eventually reaches the stomach. The stomach is where much of the chemical digestion of food occurs. Several glands in the stomach lining secrete acid chemicals and other enzymes to initiate chemical digestion.

• Hydrogen ions are secreted by parietal cells within the stomach.
• Pepsinogen is secreted by chief cells within the stomach. Pepsinogen is a zymogen, an inactive form of the enzyme pepsin that has to be cleaved with hydrogen ions to become active pepsin.
• Pepsin, the activated form of pepsinogen, is responsible for breaking proteins down into amino acids.
• Gastrin is secreted by G-cells located within pyloric glands. Gastrin stimulates the secretion of hydrochloric acid (HCl). The stomach contains a large amount of hydrochloric acid, producing an acidic environment.
• Mucus is secreted by goblet cells. This mucus protects the lining of the stomach and prevents the highly acidic environment from damaging the stomach itself.

Chief cells and parietal cells are found together in gastric glands. Assisted by the mechanical churning of muscles surrounding the stomach, the enzymes and acid secreted in the stomach digest food into chyme. Chyme is a thick, heterogeneous mixture of food and digestive enzymes that undergoes further digestion and absorption in the small intestine.

The resulting chyme travels to the small intestine. The small intestine is responsible for the digestion and absorption of nutrients. The small intestine consists of three different sequential sections: the duodenum, jejunum, and ileum. Alkaline juices produced by the pancreas, an accessory organ, are secreted into the duodenum and neutralize the highly acidic contents arriving from the stomach. Chyme initially travels through the duodenum, where the presence of chyme in the duodenum initiates the release of many enzymes from several accessory organs that are discussed later in this guide. These enzymes perform the bulk of chemical digestion as the food passes through the digestive tract.

After passing through the duodenum, the chyme makes its way to the jejunum and ileum for absorption. Characteristic of these regions are microvilli, which are small, finger-like structures that increase the surface area of the intestine to promote absorption. Here, the nutrients that are present in the digested mixture are absorbed into epithelial cells and transported into the capillaries for use. Most small molecules that aren’t hydrophobic, such as water, simple sugars, most vitamins, and amino acids, are simply absorbed through facilitated diffusion and secondary active transport into epithelial cells and then into capillaries.

For molecules that are hydrophobic, including the fat-soluble vitamins K, A, D, and E, special structures called lacteals exist to allow these molecules into the lymphatic system, which eventually empties into the left subclavian vein. For more information on the absorption and digestion of fats, be sure to refer to our guide on lipid and amino acid metabolism.

Finally, after passing through the small intestine, the mixture makes its way to the large intestine. The majority of nutrients—such as amino acids, carbohydrates, and vitamins—have been absorbed in the small intestine.

The large intestine is responsible for absorbing water and leftover salts. It also contains gut flora, colonies of beneficial bacteria that reside within the body and assist in the production and absorption of nutrients such as vitamin K. After passing through the large intestine, the only material that remains after absorption is waste, which is consolidated into fecal matter.

Fecal matter is stored in the last segment of the large intestine. When the body is ready to eliminate waste, it is pushed through the rectum and exits the body through the anal sphincters.

The movement and peristalsis of material through the digestive tract is largely governed by the enteric nervous system: a subdivision of the autonomic nervous system. The nerve endings within the enteric nervous system enervate sections of the smooth muscle lining the digestive tract, thus governing the rate of digestion as food passes through the body.

b) Accessory organs

Although these accessory organs do not form any portion of the tubelike digestive tract, the liver, gallbladder, and pancreas are important accessory organs that aid in chemical digestion.

Located in the upper-right quadrant of the abdomen, the liver plays many important roles in aiding digestion. One of the most important functions the liver serves is producing bile. Bile is a mixture composed of different salts, cholesterol, and bilirubin pigment.